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Primary Constructors in C# 12: What They Are and Why They Are Useful

by Sukhpinder SinghMarch 23rd, 2024

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In C# 12, primary constructors play a crucial role in defining the behaviour of classes and structs. By adding parameters to a struct or class declaration, you can create a primary constructor. Primary constructors can be leveraged in inheritance hierarchies. When a derived class has its primary constructor, it can invoke the base class’s primary constructor using the base keyword.

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As we bid farewell to the older syntax, let’s embrace the newer, more expressive way of creating objects.

Introduction

In C# 12, primary constructors play a crucial role in defining the behavior of classes and structs. By adding parameters to a struct or class declaration, you can create a primary constructor.

Use Cases

Base Constructor Invocation: Pass primary constructor parameters as arguments to a base constructor invocation. This allows for seamless initialization of base class properties.
Member Field Initialization: Primary constructor parameters are often used to initialize member fields or properties. By directly assigning values during construction.

Getting Started

The struct & constructor declaration before C# 12. Please find below the example of the older syntax.

public readonly struct Distance
{
    public readonly double Magnitude { get; }

    public readonly double Direction { get; }

    public Distance(double dx, double dy)
    {
        Magnitude = Math.Sqrt(dx * dx + dy * dy);
        Direction = Math.Atan2(dy, dx);
    }
}

Newer Syntax

Consider the following example, let's create a struct namedDistance with two primary constructor parameters: dx and dy. Later, we can compute two read-only properties—Magnitude and Direction—based on these parameters:

public readonly struct Distance(double dx, double dy)
{
    public readonly double Magnitude { get; } = Math.Sqrt(dx * dx + dy * dy);
    public readonly double Direction { get; } = Math.Atan2(dy, dx);
}

Invoking Base Class Primary Constructors

Primary constructors can be leveraged in inheritance hierarchies. When a derived class has its primary constructor, it can invoke the base class’s primary constructor using the base keyword.

Example: Bank Account Hierarchy

Consider a base class BankAccount with a primary constructor for account number and initial balance:

public class BankAccount(int accountNumber, decimal initialBalance)
{
      // Additional initialization specific to SavingsAccount
      // ...

}

Now, a derived class SavingsAccount can inherit from BankAccount and invoke its primary constructor as shown below

public class SavingsAccount(int accountNumber, decimal initialBalance) : BankAccount (accountNumber, initialBalance)
{
      // Additional initialization specific to SavingsAccount
      // ...
}

Custom Behavior in Derived Classes

Derived classes can further customize their behavior within the constructors. For instance, a CheckingAccount class might inherit from BankAccount but invoke the base constructor using the base keyword

    public class CheckingAccount : BankAccount
    {
        public CheckingAccount(int accountNumber, decimal initialBalance)
            : base(accountNumber, initialBalance)
        {
            // Additional initialization specific to CheckingAccount
            // ...
        }
    }

Conclusion

In the ever-evolving landscape of programming languages, C# 12 introduces a powerful feature: primary constructors. These concise and elegant constructs allow us to define the behavior of classes and structures with utmost clarity.