ITimeProvider Pattern for Testable Time Management in C#

ITimeProvider_Pattern.cs

// ITimeProvider.cs
public interface ITimeProvider
{
    DateTime UtcNow { get; }
}

// SystemTimeProvider.cs
public class SystemTimeProvider : ITimeProvider
{
    public DateTime UtcNow => DateTime.UtcNow;
}

// FakeTimeProvider.cs
public class FakeTimeProvider : ITimeProvider
{
    public DateTime UtcNow { get; set; } = DateTime.UtcNow;
}

// TokenService.cs
public class TokenService
{
    private readonly ITimeProvider _clock;

    public TokenService(ITimeProvider clock)
    {
        _clock = clock;
    }

    public bool IsExpired(DateTime expiry)
    {
        return _clock.UtcNow > expiry;
    }
}

// Program.cs - DI Setup
builder.Services.AddSingleton<ITimeProvider, SystemTimeProvider>();

// Example test
[Fact]
public void Token_ShouldBeExpired_WhenTimePasses()
{
    var clock = new FakeTimeProvider { UtcNow = new DateTime(2025, 1, 1) };
    var service = new TokenService(clock);
    Assert.True(service.IsExpired(new DateTime(2024, 12, 31)));
}