Okay I'll dial down the hyperbole, over my too many years (read I'm feeling old) as an engineer I have run into a problem where my code has contained a or .  When I write a test, I can't validate the actual time because milliseconds have passed from when the code ran, and then my test goes to validate.  It's not a huge problem but annoying as I like to validate everything to make sure I'm not accidentally manipulating those values somewhere else. DateTime.Now DateTime.UtcNow There is an easy solution to this, and before I detail out the solution that I used, I need to call out the for this.  The TL;DR is that you can consume my to help you solve this.  The implementation of the code for this lies below. inspirations SimpleDateTimeProvider NuGet Package The Solution I have created a consisting of an interface, and two implementations of the interface.  One implementation returns the System values and the other returns Mocked values that are preset by the user. DateTimeProvider The Inteface public interface IDateTimeProvider
{
    DateTime Now { get; }
    DateTime Today { get; }
    DateTime UtcNow { get; }
} The System Implementation public class SystemDateTimeProvider : IDateTimeProvider
{
    public DateTime Now => DateTime.Now;
    public DateTime Today => DateTime.Today;
    public DateTime UtcNow => DateTime.UtcNow;
} The Mock Implementation public class MockDateTimeProvider : IDateTimeProvider
{
    public DateTime Now
    {
        get => this.now.ThrowIfNotSet(DateTimeType.Now);
        set => this.now = value;
    }
    public DateTime Today
    {
        get => this.today.ThrowIfNotSet(DateTimeType.Today);
        set => this.today = value;
    }
    public DateTime UtcNow
    {
        get => this.utcNow.ThrowIfNotSet(DateTimeType.UtcNow);
        set => this.utcNow = value;
    }
} Bending Date and Time This is a simple solution and it's easy to get underway using the providers, simply inject the system provider under the interface in your functional code. If you are using another library, you'll know the syntax but follow the same formula. IDateTimeProvider _ = services.AddSingleton<IDateTimeProvider, SystemDateTimeProvider>(); Next step is to create your class and use that registered that we just created via the interface. Then use the provider to set the values in your class. SystemDateTimeProvider IDateTimeProvider DateTime public class Service
{
    private readonly IDateTimeProvider dateTimeProvider;

    public Service(IDateTimeProvider dateTimeProvider)
    {
        this.dateTimeProvider = dateTimeProvider;
    }

    public string DateTimeNow()
    {
        return $"DateTime.Now is {this.dateTimeProvider.Now}";
    }
} The whole purpose of this was to allow for testable code. So now that you have your class above, you can inject the in its place to control the values in your tests. The following example shows how to write a test in , using for assertion. MockDateTimeProvider DateTime XUnit Shouldly [Fact]
public void Today_ShouldReturn_MockedToday()
{
    // Arrange
    var provider = new MockDateTimeProvider();
    var service = new Service(provider);
    var today = DateTime.Today;

    provider.Today = today;

    // Act
    var result = service.DateTimeToday();

    // Assert
    _ = result.ShouldBeOfType<string>();
    result.ShouldBe($"DateTime.Today is {today}");
} Where Can I Find This? All of this open-sourced.  You can find my work on GitHub at and the published package at . SimpleDateTimeProvider Repository SimpleDateTimeProvider NuGet First published here

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How to Manipulate DateTime in .NET to Help you Code Better

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How to Manipulate DateTime in .NET to Help you Code Better

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Stephen Walsh

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