Discover how to solve the 40 most frequent C# compile-time errors with our in-depth guide. Introduction The article demonstrates common compile-time errors from missing semicolons to type mismatches and solutions to fix those compile-time errors. Table of Contents Missing Semicolon Missing or Extra Braces Undefined Variable Type Mismatch Incorrect Method Signatures Inconsistent Accessibility Circular Base Class Dependency No Implicit Conversion The method with Conditional Attributes Not Emitting Output Use of Unassigned Local Variable Lock on Non-Reference Type Abstract Class Instantiation Property Lacking Getter/Setter Interface Implementation Missing Member Attribute Misplacement Non-Implemented Exception Interface Accessing Static Member via Instance Constructor in Static Class Overloading by Return Type Sealed Class Inheritance Missing Generic Type Parameters Duplicate Identifier Readonly Field Modification Invalid Array Rank Enum Conversion Enum Underlying Type Mismatch Missing Comma in Enum Field Initialization Dependency Method Without Body Invalid Override Switch on Nullable Type Without Null Check Constraints Are Not Satisfied Type Expected Assignment to Expression Lacking Return in a Non-Void Method Invalid Array Initialization Async Method Without Await Case Label with Invalid Type Constructor Calling Itself Constant must be Intialized 1. Missing Semicolon Error Description CS1002: ; expected happens when a developer misses semicolons (;) at the end of a code line. Example int number = 10 Console.WriteLine(number) Solution The compiler will highlight the line number and a simple solution is to add a semicolon at the end of the statement int number = 10; Console.WriteLine(number); 2. Missing or Extra Braces Error Description Missing braces ({ or }) can cause a set of errors, including Example public class Program { public static void Main() Console.WriteLine("Hello, world!"); } } Solution Make sure to close the brace for each opening brace. public class Program { public static void Main() { Console.WriteLine("Hello, world!"); } } 3. Undefined Variable Error Description CS0103 The name 'number' does not exist in the current context happens when a variable that has not been defined or is out of scope. Example public class Program { public static void Main() { Console.WriteLine(number); } } Solution Define the variable as shown below before printing on the console window. public class Program { public static void Main() { int number = 10; Console.WriteLine(number); } } 4. Type Mismatch Error Description A data type mismatch error, indicated by CS0029 Cannot implicitly convert type ‘string’ to ‘int’, which usually happens when one data type is assigned a different data type value as shown below. Example int number = "123"; Solution Developers need to make sure of the type conversion as shown below using int.Parse for converting a string to an integer variable. int number = int.Parse("123"); 5. Incorrect Method Signatures Error Description CS1503 Argument 1: cannot convert from 'string' to 'int' happens when you pass an incorrect type in comparison to the method definition or forget to create/define a method. Example public class Program { public static void PrintNumber(int num) { Console.WriteLine(num); } public static void Main() { PrintNumber("123"); } } Solution A simple solution is to match the type of value passed with the function parameter definition or use type conversion. public class Program { public static void PrintNumber(int num) { Console.WriteLine(num); } public static void Main() { PrintNumber(int.Parse("123")); } } 6. Inconsistent Accessibility Error Description CS0122 'Helper' is inaccessible due to its protection level happens when a developer provides a type with a higher accessibility level as a method parameter or return type than the method itself Example private class Helper { } public class Program { public static Helper GetHelper() { return new Helper(); } } Solution The simplest solution is to match the access modifier of the class Helper with the method GetHelper(). public class Helper { } public class Program { public static Helper GetHelper() { return new Helper(); } } 7. Circular Base Class Dependency Error Description CS0146 Circular base type dependency involving 'ClassA' and 'ClassB' happens when two classes are inherited from each other. Example public class ClassA : ClassB { } public class ClassB : ClassA { } Solution Reevaluate the design to remove the circular dependency. public class ClassA { } public class ClassB { } 8. No Implicit Conversion Error Description CS0266 Cannot implicitly convert type 'double' to 'int'. An explicit conversion exists (are you missing a cast?) happens when an implicit conversion is required between two types. Example double number = 34.5; int value = number; Solution Explicitly cast the double variable to int. double number = 34.5; int value = (int)number; 9. Method With Conditional Attributes Not Emitting Output Error Description Using conditional attributes like ([Conditional("DEBUG")]) does not result in any output if the conditional symbol is not defined. Example [Conditional("DEBUG")] public static void Log(string message) { Console.WriteLine(message); } public static void Main() { Log("Starting application."); } Solution Add a conditional compilation symbol (DEBUG) is defined or handles the logic accordingly as shown below. #define DEBUG using System.Diagnostics; [Conditional("DEBUG")] public static void Log(string message) { Console.WriteLine(message); } public static void Main() { Log("Starting application."); } 10. Use of Unassigned Local Variable Error Description CS0165 Use of unassigned local variable value happens when the developer uses a local variable before a value is assigned to it. Example int value; Console.WriteLine(value); Solution A simple solution is to assign an initial value of 0 to the integer local variable. int value = 0; Console.WriteLine(value); 11. Lock on Non-Reference Type Error Description CS0185 'int' is not a reference type as required by the lock statement that happens when a developer tries to use a lock on the non-reference type. Example public void LockMethod() { int x = 0; lock (x) { // Error Console.WriteLine("Locked"); } } Solution A simple solution is to make the variable reference type. public void LockMethod() { object lockObj = new object(); lock (lockObj) { Console.WriteLine("Locked"); } } 12. Abstract Class Instantiation Error Description CS0144 Cannot create an instance of the abstract type or interface 'Animal ' that happens when a developer tries to create an instance of an abstract class. Example public abstract class Animal { public abstract void Speak(); } public class Program { public static void Main() { Animal myAnimal = new Animal(); // Error } } Solution A simple solution is to create an instance of a class derived from the abstract class as shown below. public abstract class Animal { public abstract void Speak(); } public class Dog : Animal { public override void Speak() { Console.WriteLine("Bark"); } } public class Program { public static void Main() { Animal myAnimal = new Dog(); myAnimal.Speak(); } } 13. Property Lacking Getter/Setter Error Description CS0200 Property or indexer 'Person.Name' cannot be assigned to -- it is read only happens when you try to use a property’s getter or setter when it’s not defined. Example public class Person { public string Name { get; } } public class Program { public static void Main() { var person = new Person(); person.Name = "John"; // Error } } Solution A simple solution is to a setter to the property as shown below if required to update the Name value. public class Person { public string Name { get; set; } } public class Program { public static void Main() { var person = new Person(); person.Name = "John"; } } 14. Interface Implementation Missing Member Error Description CS0535 'Dog' does not implement interface member 'IAnimal.Speak()' happens when the developer fails to implement the member functions in the derived class. Example public interface IAnimal { void Speak(); } public class Dog : IAnimal { } Solution A simple solution is to implement all member functions of the interface as shown below. public interface IAnimal { void Speak(); } public class Dog : IAnimal { public void Speak() { Console.WriteLine("Bark"); } } 15. Attribute Misplacement Error Description CS0592 Attribute 'Serializable' is not valid on this declaration type. It is only valid on 'class, struct, enum, delegate' declarations happens when a developer tries to apply attributes to member functions. Example [Serializable] public int MyMethod() { // Error: Serializable is not valid on methods return 0; } Solution A simple solution is to apply attributes appropriate to program elements. [Serializable] public class MyClass { // Correct usage } 16. Non-Implemented Exception Interface Error Description CS0535 'FileStorage' does not implement interface member 'IStorage.Load()' happens when a method of interface is missing its implementation. Example interface IStorage { void Save(string data); string Load(); } class FileStorage : IStorage { public void Save(string data) { // Implementation here } } Solution A simple solution is to implement all members defined in the interface. class FileStorage : IStorage { public void Save(string data) { // Implementation here } public string Load() { // Implementation here return "data"; } } 17. Accessing Static Member via Instance Error Description CS0176 Member 'Utility.Number' cannot be accessed with an instance reference; qualify it with a type name instead happens when a developer tries to access a static variable by creating an instance of that class. Example public class Utility { public static int Number = 42; } public class Test { public void Display() { Utility util = new Utility(); Console.WriteLine(util.Number); // Error } } Solution A simple solution is to access the static members using the class name as shown below. Console.WriteLine(Utility.Number); 18. Constructor in Static Class Error Description CS0710 Static classes cannot have instance constructors when the developer tries to create a constructor in a static class. Example public static class ApplicationSettings { public ApplicationSettings() // Error { } } Solution A simple solution is to remove the constructor or add a static constructor. public static class ApplicationSettings { // Correct the design or remove the constructor } 19. Overloading by Return Type Error Description CS0111 Type 'Calculator' already defines a member called 'Add' with the same parameter types when a developer tries to overload the method with different return types which is not possible. Example public class Calculator { public int Add(int a, int b) { return a + b; } public double Add(int a, int b) // Error { return a + b; } } Solution A simple solution is to overload methods based upon as shown below by changing the 2nd method parameter type from int to double The number of parameters The type of parameters The order of parameters public double AddDoubles(double a, double b) { return a + b; } 20. Sealed Class Inheritance Error Description CS0509 'DerivedClass': cannot derive from sealed type 'BaseClass' when a developer tries to inherit from a sealed class. Example public sealed class BaseClass { } public class DerivedClass : BaseClass // Error { } Solution A simple solution is to remove the sealed keyword if inheritance should be allowed. public class BaseClass { } public class DerivedClass : BaseClass { } 21. Missing Generic Type Parameters Error Description CS0305 Using the generic type 'GenericClass<T>' requires 1 type arguments when a developer tries to create an object of Generic Class without specifying the T type Example public class GenericClass<T> { } GenericClass obj = new GenericClass(); // Error Solution A simple solution is to define the type T as shown below. GenericClass<int> obj = new GenericClass<int>(); 22. Duplicate Identifier Error Description CS0102 The type 'Person' already contains a definition for 'age' happens when a developer tries to create two different variables within the same name in the same class. Example public class Person { int age; string age; // Error } Solution A simple solution is to have distinct meaningful names for different variables. public class Person { int age; string name; } 23. Readonly Field Modification Error Description CS0191 A read-only field cannot be assigned to (except in a constructor or init-only setter of the type in which the field is defined or a variable initializer) happens when a developer tries to modify the read-only variable. Example public class Settings { readonly int maxUsers = 100; public void SetMaxUsers(int num) { maxUsers = num; // Error } } Solution A simple solution is to remove read-only if a value update is required post-constructor initialization. public class Settings { int maxUsers = 100; public void SetMaxUsers(int num) { maxUsers = num; } } 24. Invalid Array Rank Error Description CS0022 Wrong number of indices inside []; expected 2 happens when a developer tries to access a 2D matrix without specifying the 2nd indices. Example int[,] matrix = new int[3, 2]; int num = matrix[0]; // Error Solution A simple solution is to provide both of the indices. int num = matrix[0, 1]; 25. Enum Conversion Error Description CS0266 Cannot implicitly convert type ‘int’ to ‘StatusCode’. An explicit conversion exists (are you missing a cast?) Example enum StatusCode { Ok, Error } StatusCode code = 1; // Error Solution A simple solution to add type casting from int to Enum. StatusCode code = (StatusCode)1; 26. Enum Underlying Type Mismatch Error Description CS0031 Constant value '256' cannot be converted to a 'byte' happens when a developer tries to define value outside the type range bound. Example enum SmallRange : byte { Min = 0, Max = 256 } // Error: 256 is out of byte range Solution A simple solution is to change the type for a higher range or adjust the max according to the provided type. enum SmallRange : ushort { Min = 0, Max = 256 } 27. Missing Comma in Enum Error Description CS1003: Syntax error, ',' expected happens when a developer forgets to add a comma (,) between Enum values/ Example enum Colors { Red Blue, Green } // Error Solution A simple solution is to organize enum values comma-separated as shown below. enum Colors { Red, Blue, Green } 28. Field Initialization Dependency Error Description CS0236 A field initializer cannot reference the non-static field, method, or property ‘MyStruct.y’ when a developer messes the order variable declaration and tries to assign a value to a variable x but its value is dependent on y Example struct MyStruct { int x = y + 1; int y = 1; // Error } Solution A simple solution is to make an order of variable declaration correct for dependent variables. struct MyStruct { int x, y; MyStruct() { y = 1; x = y + 1; } } 29. Method Without Body Error Description CS0501 'MyClass.MyMethod()' must declare a body because it is not marked abstract, extern, or partial when a developer tries to create a non-abstract method without a body. Example public class MyClass { public void MyMethod(); // Error } Solution A simple solution is to provide a method implementation or declare a method as abstract as an abstract method doesn’t require any implementation. public class MyClass { public void MyMethod() {} } 30. Invalid Override Error Description CS0506 'Derived.DoWork()': cannot override inherited member 'Base.DoWork()' because it is not marked virtual, abstract, or override when a developer tries to override a method that is not marked as virtual in the base class. Example public class Base { public void DoWork() {} } public class Derived : Base { public override void DoWork() {} // Error } Solution A simple solution is to mark the method as virtual. public class Base { public virtual void DoWork() {} } 31. Switch on Nullable Type Without Null Check Error Description Using a nullable type in a switch statement without handling the null case can lead to runtime issues, although it’s a logical error more than a compile-time error. Example int? num = null; switch (num) { case 1: Console.WriteLine("One"); break; // No case for null } Solution A simple solution is to handle null in the switch statements involving nullable types. switch (num) { case 1: Console.WriteLine("One"); break; case null: Console.WriteLine("No number"); break; } 32. Constraints Are Not Satisfied Error Description CS0311 The type ‘MyClass’ cannot be used as type parameter ‘T’ in the generic type or method ‘MyGenericClass<T>’. There is no implicit reference conversion from ‘MyClass’ to ‘IMyInterface’. Example public interface IMyInterface {} public class MyGenericClass<T> where T : IMyInterface {} public class MyClass {} MyGenericClass<MyClass> myClass = new MyGenericClass<MyClass>(); // Error Solution A simple solution is to make sure that type arguments satisfy the generic constraints. public class MyClass : IMyInterface {} MyGenericClass<MyClass> myClass = new MyGenericClass<MyClass>(); 33. Type Expected Error Description CS1526 A new expression requires an argument list or (), [], or {} after type happens a developer forgot to define a type with new the keyword as with var datatype we need to define a type on the right-hand side. Example var x = new; // Error Solution A simple solution is to define the type as shown below. var x = new MyClass(); 34. Assignment to Expression Error Description CS0131 The left-hand side of an assignment must be a variable, property or indexer Example int i = 0; i++ = 5; Solution A simple solution is to assign values only to variables, properties, or indexers. int i; i = 5; 35. Lacking Return in a Non-Void Method Error Description CS0161 'GetValue()': not all code paths return a value when a developer forgets to return a value from a method with a defined return type. Example public int GetValue() { if (DateTime.Now.DayOfWeek == DayOfWeek.Monday) { return 1; } // No return provided for other days } Solution A simple solution is to make sure the function returns some value in all conditional statement checks or returns a common response as shown below. public int GetValue() { if (DateTime.Now.DayOfWeek == DayOfWeek.Monday) { return 1; } return 0; // Default return value } 36. Invalid Array Initialization Error Description CS0029 Cannot implicitly convert type 'string' to 'int ' when a developer tries to add inconsistent value into an array than its defined type. Example int[] array = { 1, "two", 3 }; // Error Solution A simple solution is to add only int elements in the aforementioned array. int[] array = { 1, 2, 3 }; 37. Async Method Without Await Error Description CS1998: This async method lacks 'await' operators and will run synchronously happens when a developer creates an async method without an await the operator will result in a warning. Example public async Task ProcessData() { Console.WriteLine("Processing data"); } Solution A simple solution is to define a least one await statement otherwise the method will run synchronously. public async Task ProcessData() { await Task.Delay(1000); // Simulate asynchronous operation Console.WriteLine("Processing data"); } 38. Case Label With Invalid Type Error Description CS0029 Cannot implicitly convert type 'string' to 'int ' when a developer tries to define a case with an invalid type. Example int x = 10; switch (x) { case "ten": // Error break; } Solution A simple solution is to match the case statement type with the switch expression type. switch (x) { case 10: break; } 39. Constructor Calling Itself Error Description CS0516 Constructor 'MyClass.MyClass()' cannot call itself when a developer tries to call the constructor within the same class using this. Example public class MyClass { public MyClass() : this() { // Error } } ```= ### Solution A simple solution is to remove the circular constructor call. ```csharp public class MyClass { public MyClass() { // Proper initialization code } } 40. Constant Must Be Initialized CS0145 A const field requires a value to be provided when a developer tries to create a constant variable without assigning an initial value. Example public class MyClass { public const string MyConstant; // Error: A constant must have a value } Solution A simple solution is to remove the circular constructor call. public class MyClass { public const string MyConstant = "Hello World"; // Correctly initialized } More Cheatsheets Cheat Sheets — .Net C# Programming🚀 Thank you for being a part of the C# community! Before you leave: Follow us: Youtube | X | LinkedIn | Dev.to Visit our other platforms: GitHub Discover how to solve the 40 most frequent C# compile-time errors with our in-depth guide. Introduction The article demonstrates common compile-time errors from missing semicolons to type mismatches and solutions to fix those compile-time errors. Table of Contents Missing Semicolon Missing or Extra Braces Undefined Variable Type Mismatch Incorrect Method Signatures Inconsistent Accessibility Circular Base Class Dependency No Implicit Conversion The method with Conditional Attributes Not Emitting Output Use of Unassigned Local Variable Lock on Non-Reference Type Abstract Class Instantiation Property Lacking Getter/Setter Interface Implementation Missing Member Attribute Misplacement Non-Implemented Exception Interface Accessing Static Member via Instance Constructor in Static Class Overloading by Return Type Sealed Class Inheritance Missing Generic Type Parameters Duplicate Identifier Readonly Field Modification Invalid Array Rank Enum Conversion Enum Underlying Type Mismatch Missing Comma in Enum Field Initialization Dependency Method Without Body Invalid Override Switch on Nullable Type Without Null Check Constraints Are Not Satisfied Type Expected Assignment to Expression Lacking Return in a Non-Void Method Invalid Array Initialization Async Method Without Await Case Label with Invalid Type Constructor Calling Itself Constant must be Intialized Missing Semicolon Missing or Extra Braces Undefined Variable Type Mismatch Incorrect Method Signatures Inconsistent Accessibility Circular Base Class Dependency No Implicit Conversion The method with Conditional Attributes Not Emitting Output Use of Unassigned Local Variable Lock on Non-Reference Type Abstract Class Instantiation Property Lacking Getter/Setter Interface Implementation Missing Member Attribute Misplacement Non-Implemented Exception Interface Accessing Static Member via Instance Constructor in Static Class Overloading by Return Type Sealed Class Inheritance Missing Generic Type Parameters Duplicate Identifier Readonly Field Modification Invalid Array Rank Enum Conversion Enum Underlying Type Mismatch Missing Comma in Enum Field Initialization Dependency Method Without Body Invalid Override Switch on Nullable Type Without Null Check Constraints Are Not Satisfied Type Expected Assignment to Expression Lacking Return in a Non-Void Method Invalid Array Initialization Async Method Without Await Case Label with Invalid Type Constructor Calling Itself Constant must be Intialized 1. Missing Semicolon Error Description CS1002: ; expected happens when a developer misses semicolons (;) at the end of a code line. CS1002: ; expected Example int number = 10 Console.WriteLine(number) int number = 10 Console.WriteLine(number) Solution The compiler will highlight the line number and a simple solution is to add a semicolon at the end of the statement int number = 10; Console.WriteLine(number); int number = 10; Console.WriteLine(number); 2. Missing or Extra Braces Error Description Missing braces ({ or }) can cause a set of errors, including Missing braces ({ or }) Example public class Program { public static void Main() Console.WriteLine("Hello, world!"); } } public class Program { public static void Main() Console.WriteLine("Hello, world!"); } } Solution Make sure to close the brace for each opening brace. public class Program { public static void Main() { Console.WriteLine("Hello, world!"); } } public class Program { public static void Main() { Console.WriteLine("Hello, world!"); } } 3. Undefined Variable Error Description CS0103 The name 'number' does not exist in the current context happens when a variable that has not been defined or is out of scope. CS0103 The name 'number' does not exist in the current context Example public class Program { public static void Main() { Console.WriteLine(number); } } public class Program { public static void Main() { Console.WriteLine(number); } } Solution Define the variable as shown below before printing on the console window. public class Program { public static void Main() { int number = 10; Console.WriteLine(number); } } public class Program { public static void Main() { int number = 10; Console.WriteLine(number); } } 4. Type Mismatch Error Description A data type mismatch error, indicated by CS0029 Cannot implicitly convert type ‘string’ to ‘int’ , which usually happens when one data type is assigned a different data type value as shown below. A data type mismatch error, indicated by CS0029 Cannot implicitly convert type ‘string’ to ‘int’ Example int number = "123"; int number = "123"; Solution Developers need to make sure of the type conversion as shown below using int.Parse for converting a string to an integer variable. int number = int.Parse("123"); int number = int.Parse("123"); 5. Incorrect Method Signatures Error Description CS1503 Argument 1: cannot convert from 'string' to 'int' happens when you pass an incorrect type in comparison to the method definition or forget to create/define a method. CS1503 Argument 1: cannot convert from 'string' to 'int' Example public class Program { public static void PrintNumber(int num) { Console.WriteLine(num); } public static void Main() { PrintNumber("123"); } } public class Program { public static void PrintNumber(int num) { Console.WriteLine(num); } public static void Main() { PrintNumber("123"); } } Solution A simple solution is to match the type of value passed with the function parameter definition or use type conversion. public class Program { public static void PrintNumber(int num) { Console.WriteLine(num); } public static void Main() { PrintNumber(int.Parse("123")); } } public class Program { public static void PrintNumber(int num) { Console.WriteLine(num); } public static void Main() { PrintNumber(int.Parse("123")); } } 6. Inconsistent Accessibility Error Description CS0122 'Helper' is inaccessible due to its protection level happens when a developer provides a type with a higher accessibility level as a method parameter or return type than the method itself CS0122 'Helper' is inaccessible due to its protection level Example private class Helper { } public class Program { public static Helper GetHelper() { return new Helper(); } } private class Helper { } public class Program { public static Helper GetHelper() { return new Helper(); } } Solution The simplest solution is to match the access modifier of the class Helper with the method GetHelper(). public class Helper { } public class Program { public static Helper GetHelper() { return new Helper(); } } public class Helper { } public class Program { public static Helper GetHelper() { return new Helper(); } } 7. Circular Base Class Dependency Error Description CS0146 Circular base type dependency involving 'ClassA' and 'ClassB' happens when two classes are inherited from each other. CS0146 Circular base type dependency involving 'ClassA' and 'ClassB' Example public class ClassA : ClassB { } public class ClassB : ClassA { } public class ClassA : ClassB { } public class ClassB : ClassA { } Solution Reevaluate the design to remove the circular dependency. public class ClassA { } public class ClassB { } public class ClassA { } public class ClassB { } 8. No Implicit Conversion Error Description CS0266 Cannot implicitly convert type 'double' to 'int'. An explicit conversion exists (are you missing a cast?) happens when an implicit conversion is required between two types. CS0266 Cannot implicitly convert type 'double' to 'int'. An explicit conversion exists (are you missing a cast?) Example double number = 34.5; int value = number; double number = 34.5; int value = number; Solution Explicitly cast the double variable to int. double number = 34.5; int value = (int)number; double number = 34.5; int value = (int)number; 9. Method With Conditional Attributes Not Emitting Output Error Description Using conditional attributes like ([Conditional("DEBUG")]) does not result in any output if the conditional symbol is not defined. Example [Conditional("DEBUG")] public static void Log(string message) { Console.WriteLine(message); } public static void Main() { Log("Starting application."); } [Conditional("DEBUG")] public static void Log(string message) { Console.WriteLine(message); } public static void Main() { Log("Starting application."); } Solution Add a conditional compilation symbol (DEBUG) is defined or handles the logic accordingly as shown below. #define DEBUG using System.Diagnostics; [Conditional("DEBUG")] public static void Log(string message) { Console.WriteLine(message); } public static void Main() { Log("Starting application."); } #define DEBUG using System.Diagnostics; [Conditional("DEBUG")] public static void Log(string message) { Console.WriteLine(message); } public static void Main() { Log("Starting application."); } 10. Use of Unassigned Local Variable Error Description CS0165 Use of unassigned local variable value happens when the developer uses a local variable before a value is assigned to it. CS0165 Use of unassigned local variable value Example int value; Console.WriteLine(value); int value; Console.WriteLine(value); Solution A simple solution is to assign an initial value of 0 to the integer local variable. int value = 0; Console.WriteLine(value); int value = 0; Console.WriteLine(value); 11. Lock on Non-Reference Type Error Description CS0185 'int' is not a reference type as required by the lock statement that happens when a developer tries to use a lock on the non-reference type. CS0185 'int' is not a reference type as required by the lock statement Example public void LockMethod() { int x = 0; lock (x) { // Error Console.WriteLine("Locked"); } } public void LockMethod() { int x = 0; lock (x) { // Error Console.WriteLine("Locked"); } } Solution A simple solution is to make the variable reference type. public void LockMethod() { object lockObj = new object(); lock (lockObj) { Console.WriteLine("Locked"); } } public void LockMethod() { object lockObj = new object(); lock (lockObj) { Console.WriteLine("Locked"); } } 12. Abstract Class Instantiation Error Description CS0144 Cannot create an instance of the abstract type or interface 'Animal ' that happens when a developer tries to create an instance of an abstract class. CS0144 Cannot create an instance of the abstract type or interface 'Animal ' Example public abstract class Animal { public abstract void Speak(); } public class Program { public static void Main() { Animal myAnimal = new Animal(); // Error } } public abstract class Animal { public abstract void Speak(); } public class Program { public static void Main() { Animal myAnimal = new Animal(); // Error } } Solution A simple solution is to create an instance of a class derived from the abstract class as shown below. public abstract class Animal { public abstract void Speak(); } public class Dog : Animal { public override void Speak() { Console.WriteLine("Bark"); } } public class Program { public static void Main() { Animal myAnimal = new Dog(); myAnimal.Speak(); } } public abstract class Animal { public abstract void Speak(); } public class Dog : Animal { public override void Speak() { Console.WriteLine("Bark"); } } public class Program { public static void Main() { Animal myAnimal = new Dog(); myAnimal.Speak(); } } 13. Property Lacking Getter/Setter Error Description CS0200 Property or indexer 'Person.Name' cannot be assigned to -- it is read only happens when you try to use a property’s getter or setter when it’s not defined. CS0200 Property or indexer 'Person.Name' cannot be assigned to -- it is read only Example public class Person { public string Name { get; } } public class Program { public static void Main() { var person = new Person(); person.Name = "John"; // Error } } public class Person { public string Name { get; } } public class Program { public static void Main() { var person = new Person(); person.Name = "John"; // Error } } Solution A simple solution is to a setter to the property as shown below if required to update the Name value. public class Person { public string Name { get; set; } } public class Program { public static void Main() { var person = new Person(); person.Name = "John"; } } public class Person { public string Name { get; set; } } public class Program { public static void Main() { var person = new Person(); person.Name = "John"; } } 14. Interface Implementation Missing Member Error Description CS0535 'Dog' does not implement interface member 'IAnimal.Speak()' happens when the developer fails to implement the member functions in the derived class. CS0535 'Dog' does not implement interface member 'IAnimal.Speak()' Example public interface IAnimal { void Speak(); } public class Dog : IAnimal { } public interface IAnimal { void Speak(); } public class Dog : IAnimal { } Solution A simple solution is to implement all member functions of the interface as shown below. public interface IAnimal { void Speak(); } public class Dog : IAnimal { public void Speak() { Console.WriteLine("Bark"); } } public interface IAnimal { void Speak(); } public class Dog : IAnimal { public void Speak() { Console.WriteLine("Bark"); } } 15. Attribute Misplacement Error Description CS0592 Attribute 'Serializable' is not valid on this declaration type. It is only valid on 'class, struct, enum, delegate' declarations happens when a developer tries to apply attributes to member functions. CS0592 Attribute 'Serializable' is not valid on this declaration type. It is only valid on 'class, struct, enum, delegate' declarations Example [Serializable] public int MyMethod() { // Error: Serializable is not valid on methods return 0; } [Serializable] public int MyMethod() { // Error: Serializable is not valid on methods return 0; } Solution A simple solution is to apply attributes appropriate to program elements. [Serializable] public class MyClass { // Correct usage } [Serializable] public class MyClass { // Correct usage } 16. Non-Implemented Exception Interface Error Description CS0535 'FileStorage' does not implement interface member 'IStorage.Load()' happens when a method of interface is missing its implementation. CS0535 'FileStorage' does not implement interface member 'IStorage.Load()' Example interface IStorage { void Save(string data); string Load(); } class FileStorage : IStorage { public void Save(string data) { // Implementation here } } interface IStorage { void Save(string data); string Load(); } class FileStorage : IStorage { public void Save(string data) { // Implementation here } } Solution A simple solution is to implement all members defined in the interface. class FileStorage : IStorage { public void Save(string data) { // Implementation here } public string Load() { // Implementation here return "data"; } } class FileStorage : IStorage { public void Save(string data) { // Implementation here } public string Load() { // Implementation here return "data"; } } 17. Accessing Static Member via Instance Error Description CS0176 Member 'Utility.Number' cannot be accessed with an instance reference; qualify it with a type name instead happens when a developer tries to access a static variable by creating an instance of that class. CS0176 Member 'Utility.Number' cannot be accessed with an instance reference; qualify it with a type name instead Example public class Utility { public static int Number = 42; } public class Test { public void Display() { Utility util = new Utility(); Console.WriteLine(util.Number); // Error } } public class Utility { public static int Number = 42; } public class Test { public void Display() { Utility util = new Utility(); Console.WriteLine(util.Number); // Error } } Solution A simple solution is to access the static members using the class name as shown below. Console.WriteLine(Utility.Number); Console.WriteLine(Utility.Number); 18. Constructor in Static Class Error Description CS0710 Static classes cannot have instance constructors when the developer tries to create a constructor in a static class. CS0710 Static classes cannot have instance constructors Example public static class ApplicationSettings { public ApplicationSettings() // Error { } } public static class ApplicationSettings { public ApplicationSettings() // Error { } } Solution A simple solution is to remove the constructor or add a static constructor. public static class ApplicationSettings { // Correct the design or remove the constructor } public static class ApplicationSettings { // Correct the design or remove the constructor } 19. Overloading by Return Type Error Description CS0111 Type 'Calculator' already defines a member called 'Add' with the same parameter types when a developer tries to overload the method with different return types which is not possible. CS0111 Type 'Calculator' already defines a member called 'Add' with the same parameter types Example public class Calculator { public int Add(int a, int b) { return a + b; } public double Add(int a, int b) // Error { return a + b; } } public class Calculator { public int Add(int a, int b) { return a + b; } public double Add(int a, int b) // Error { return a + b; } } Solution A simple solution is to overload methods based upon as shown below by changing the 2nd method parameter type from int to double The number of parameters The type of parameters The order of parameters The number of parameters The type of parameters The order of parameters public double AddDoubles(double a, double b) { return a + b; } public double AddDoubles(double a, double b) { return a + b; } 20. Sealed Class Inheritance Error Description CS0509 'DerivedClass': cannot derive from sealed type 'BaseClass' when a developer tries to inherit from a sealed class. CS0509 'DerivedClass': cannot derive from sealed type 'BaseClass' Example public sealed class BaseClass { } public class DerivedClass : BaseClass // Error { } public sealed class BaseClass { } public class DerivedClass : BaseClass // Error { } Solution A simple solution is to remove the sealed keyword if inheritance should be allowed. public class BaseClass { } public class DerivedClass : BaseClass { } public class BaseClass { } public class DerivedClass : BaseClass { } 21. Missing Generic Type Parameters Error Description CS0305 Using the generic type 'GenericClass<T>' requires 1 type arguments when a developer tries to create an object of Generic Class without specifying the T type CS0305 Using the generic type 'GenericClass<T>' requires 1 type arguments Example public class GenericClass<T> { } GenericClass obj = new GenericClass(); // Error public class GenericClass<T> { } GenericClass obj = new GenericClass(); // Error Solution A simple solution is to define the type T as shown below. GenericClass<int> obj = new GenericClass<int>(); GenericClass<int> obj = new GenericClass<int>(); 22. Duplicate Identifier Error Description CS0102 The type 'Person' already contains a definition for 'age' happens when a developer tries to create two different variables within the same name in the same class. CS0102 The type 'Person' already contains a definition for 'age' Example public class Person { int age; string age; // Error } public class Person { int age; string age; // Error } Solution A simple solution is to have distinct meaningful names for different variables. public class Person { int age; string name; } public class Person { int age; string name; } 23. Readonly Field Modification Error Description CS0191 A read-only field cannot be assigned to (except in a constructor or init-only setter of the type in which the field is defined or a variable initializer) happens when a developer tries to modify the read-only variable. CS0191 A read-only field cannot be assigned to (except in a constructor or init-only setter of the type in which the field is defined or a variable initializer) Example public class Settings { readonly int maxUsers = 100; public void SetMaxUsers(int num) { maxUsers = num; // Error } } public class Settings { readonly int maxUsers = 100; public void SetMaxUsers(int num) { maxUsers = num; // Error } } Solution A simple solution is to remove read-only if a value update is required post-constructor initialization. public class Settings { int maxUsers = 100; public void SetMaxUsers(int num) { maxUsers = num; } } public class Settings { int maxUsers = 100; public void SetMaxUsers(int num) { maxUsers = num; } } 24. Invalid Array Rank Error Description CS0022 Wrong number of indices inside []; expected 2 happens when a developer tries to access a 2D matrix without specifying the 2nd indices. CS0022 Wrong number of indices inside []; expected 2 Example int[,] matrix = new int[3, 2]; int num = matrix[0]; // Error int[,] matrix = new int[3, 2]; int num = matrix[0]; // Error Solution A simple solution is to provide both of the indices. int num = matrix[0, 1]; int num = matrix[0, 1]; 25. Enum Conversion Error Description CS0266 Cannot implicitly convert type ‘int’ to ‘StatusCode’. An explicit conversion exists (are you missing a cast?) CS0266 Cannot implicitly convert type ‘int’ to ‘StatusCode’. An explicit conversion exists (are you missing a cast?) Example enum StatusCode { Ok, Error } StatusCode code = 1; // Error enum StatusCode { Ok, Error } StatusCode code = 1; // Error Solution A simple solution to add type casting from int to Enum. StatusCode code = (StatusCode)1; StatusCode code = (StatusCode)1; 26. Enum Underlying Type Mismatch Error Description CS0031 Constant value '256' cannot be converted to a 'byte' happens when a developer tries to define value outside the type range bound. CS0031 Constant value '256' cannot be converted to a 'byte' Example enum SmallRange : byte { Min = 0, Max = 256 } // Error: 256 is out of byte range enum SmallRange : byte { Min = 0, Max = 256 } // Error: 256 is out of byte range Solution A simple solution is to change the type for a higher range or adjust the max according to the provided type. enum SmallRange : ushort { Min = 0, Max = 256 } enum SmallRange : ushort { Min = 0, Max = 256 } 27. Missing Comma in Enum Error Description CS1003: Syntax error, ',' expected happens when a developer forgets to add a comma (,) between Enum values/ CS1003: Syntax error, ',' expected happens when a developer forgets to add a comma (,) between Enum values/ Example enum Colors { Red Blue, Green } // Error enum Colors { Red Blue, Green } // Error Solution A simple solution is to organize enum values comma-separated as shown below. enum Colors { Red, Blue, Green } enum Colors { Red, Blue, Green } 28. Field Initialization Dependency Error Description CS0236 A field initializer cannot reference the non-static field, method, or property ‘MyStruct.y’ when a developer messes the order variable declaration and tries to assign a value to a variable x but its value is dependent on y CS0236 A field initializer cannot reference the non-static field, method, or property ‘MyStruct.y’ Example struct MyStruct { int x = y + 1; int y = 1; // Error } struct MyStruct { int x = y + 1; int y = 1; // Error } Solution A simple solution is to make an order of variable declaration correct for dependent variables. struct MyStruct { int x, y; MyStruct() { y = 1; x = y + 1; } } struct MyStruct { int x, y; MyStruct() { y = 1; x = y + 1; } } 29. Method Without Body Error Description CS0501 'MyClass.MyMethod()' must declare a body because it is not marked abstract, extern, or partial when a developer tries to create a non-abstract method without a body. CS0501 'MyClass.MyMethod()' must declare a body because it is not marked abstract, extern, or partial Example public class MyClass { public void MyMethod(); // Error } public class MyClass { public void MyMethod(); // Error } Solution A simple solution is to provide a method implementation or declare a method as abstract as an abstract method doesn’t require any implementation. public class MyClass { public void MyMethod() {} } public class MyClass { public void MyMethod() {} } 30. Invalid Override Error Description CS0506 'Derived.DoWork()': cannot override inherited member 'Base.DoWork()' because it is not marked virtual, abstract, or override when a developer tries to override a method that is not marked as virtual in the base class. CS0506 'Derived.DoWork()': cannot override inherited member 'Base.DoWork()' because it is not marked virtual, abstract, or override Example public class Base { public void DoWork() {} } public class Derived : Base { public override void DoWork() {} // Error } public class Base { public void DoWork() {} } public class Derived : Base { public override void DoWork() {} // Error } Solution A simple solution is to mark the method as virtual. public class Base { public virtual void DoWork() {} } public class Base { public virtual void DoWork() {} } 31. Switch on Nullable Type Without Null Check Error Description Using a nullable type in a switch statement without handling the null case can lead to runtime issues, although it’s a logical error more than a compile-time error. Example int? num = null; switch (num) { case 1: Console.WriteLine("One"); break; // No case for null } int? num = null; switch (num) { case 1: Console.WriteLine("One"); break; // No case for null } Solution A simple solution is to handle null in the switch statements involving nullable types. switch (num) { case 1: Console.WriteLine("One"); break; case null: Console.WriteLine("No number"); break; } switch (num) { case 1: Console.WriteLine("One"); break; case null: Console.WriteLine("No number"); break; } 32. Constraints Are Not Satisfied Error Description CS0311 The type ‘MyClass’ cannot be used as type parameter ‘T’ in the generic type or method ‘MyGenericClass<T>’. There is no implicit reference conversion from ‘MyClass’ to ‘IMyInterface’. CS0311 The type ‘MyClass’ cannot be used as type parameter ‘T’ in the generic type or method ‘MyGenericClass<T>’. There is no implicit reference conversion from ‘MyClass’ to ‘IMyInterface’. Example public interface IMyInterface {} public class MyGenericClass<T> where T : IMyInterface {} public class MyClass {} MyGenericClass<MyClass> myClass = new MyGenericClass<MyClass>(); // Error public interface IMyInterface {} public class MyGenericClass<T> where T : IMyInterface {} public class MyClass {} MyGenericClass<MyClass> myClass = new MyGenericClass<MyClass>(); // Error Solution A simple solution is to make sure that type arguments satisfy the generic constraints. public class MyClass : IMyInterface {} MyGenericClass<MyClass> myClass = new MyGenericClass<MyClass>(); public class MyClass : IMyInterface {} MyGenericClass<MyClass> myClass = new MyGenericClass<MyClass>(); 33. Type Expected Error Description CS1526 A new expression requires an argument list or (), [], or {} after type happens a developer forgot to define a type with new the keyword as with var datatype we need to define a type on the right-hand side. CS1526 A new expression requires an argument list or (), [], or {} after type Example var x = new; // Error var x = new; // Error Solution A simple solution is to define the type as shown below. var x = new MyClass(); var x = new MyClass(); 34. Assignment to Expression Error Description CS0131 The left-hand side of an assignment must be a variable, property or indexer CS0131 The left-hand side of an assignment must be a variable, property or indexer Example int i = 0; i++ = 5; int i = 0; i++ = 5; Solution A simple solution is to assign values only to variables, properties, or indexers. int i; i = 5; int i; i = 5; 35. Lacking Return in a Non-Void Method Error Description CS0161 'GetValue()': not all code paths return a value when a developer forgets to return a value from a method with a defined return type. CS0161 'GetValue()': not all code paths return a value Example public int GetValue() { if (DateTime.Now.DayOfWeek == DayOfWeek.Monday) { return 1; } // No return provided for other days } public int GetValue() { if (DateTime.Now.DayOfWeek == DayOfWeek.Monday) { return 1; } // No return provided for other days } Solution A simple solution is to make sure the function returns some value in all conditional statement checks or returns a common response as shown below. public int GetValue() { if (DateTime.Now.DayOfWeek == DayOfWeek.Monday) { return 1; } return 0; // Default return value } public int GetValue() { if (DateTime.Now.DayOfWeek == DayOfWeek.Monday) { return 1; } return 0; // Default return value } 36. Invalid Array Initialization Error Description CS0029 Cannot implicitly convert type 'string' to 'int ' when a developer tries to add inconsistent value into an array than its defined type. CS0029 Cannot implicitly convert type 'string' to 'int ' Example int[] array = { 1, "two", 3 }; // Error int[] array = { 1, "two", 3 }; // Error Solution A simple solution is to add only int elements in the aforementioned array. int[] array = { 1, 2, 3 }; int[] array = { 1, 2, 3 }; 37. Async Method Without Await Error Description CS1998: This async method lacks 'await' operators and will run synchronously happens when a developer creates an async method without an await the operator will result in a warning. CS1998: This async method lacks 'await' operators and will run synchronously Example public async Task ProcessData() { Console.WriteLine("Processing data"); } public async Task ProcessData() { Console.WriteLine("Processing data"); } Solution A simple solution is to define a least one await statement otherwise the method will run synchronously. public async Task ProcessData() { await Task.Delay(1000); // Simulate asynchronous operation Console.WriteLine("Processing data"); } public async Task ProcessData() { await Task.Delay(1000); // Simulate asynchronous operation Console.WriteLine("Processing data"); } 38. Case Label With Invalid Type Error Description CS0029 Cannot implicitly convert type 'string' to 'int ' when a developer tries to define a case with an invalid type. CS0029 Cannot implicitly convert type 'string' to 'int ' Example int x = 10; switch (x) { case "ten": // Error break; } int x = 10; switch (x) { case "ten": // Error break; } Solution A simple solution is to match the case statement type with the switch expression type. switch (x) { case 10: break; } switch (x) { case 10: break; } 39. Constructor Calling Itself Error Description CS0516 Constructor 'MyClass.MyClass()' cannot call itself when a developer tries to call the constructor within the same class using this. CS0516 Constructor 'MyClass.MyClass()' cannot call itself Example public class MyClass { public MyClass() : this() { // Error } } ```= ### Solution A simple solution is to remove the circular constructor call. ```csharp public class MyClass { public MyClass() { // Proper initialization code } } public class MyClass { public MyClass() : this() { // Error } } ```= ### Solution A simple solution is to remove the circular constructor call. ```csharp public class MyClass { public MyClass() { // Proper initialization code } } 40. Constant Must Be Initialized CS0145 A const field requires a value to be provided when a developer tries to create a constant variable without assigning an initial value. CS0145 A const field requires a value to be provided Example public class MyClass { public const string MyConstant; // Error: A constant must have a value } public class MyClass { public const string MyConstant; // Error: A constant must have a value } Solution A simple solution is to remove the circular constructor call. public class MyClass { public const string MyConstant = "Hello World"; // Correctly initialized } public class MyClass { public const string MyConstant = "Hello World"; // Correctly initialized } More Cheatsheets Cheat Sheets — .Net Cheat Sheets — .Net C# Programming🚀 Thank you for being a part of the C# community! Before you leave: Follow us: Youtube | X | LinkedIn | Dev.to Visit our other platforms: GitHub Youtube X LinkedIn Dev.to GitHub