Mockable DateTime.UtcNow

SystemTime.cs

public static class SystemTime
{
    private static ITimeProvider _timeProvider = new RealTimeProvider();

    public static DateTime UtcNow
    {
        get { return _timeProvider.UtcNow; }
    }

    public static IDisposable MockTime(DateTime utcNow)
    {
        var mockedTimeProvider = new MockedTimeProvider(utcNow, RestoreTimeProvider);
            
        _timeProvider = mockedTimeProvider;

        return mockedTimeProvider;
    }

    private static void RestoreTimeProvider()
    {
        _timeProvider = new RealTimeProvider();
    }

    private interface ITimeProvider
    {
            DateTime UtcNow { get; }
    }

    private class RealTimeProvider : ITimeProvider
    {
        public DateTime UtcNow { get { return DateTime.UtcNow; } }
    }

    private class MockedTimeProvider : ITimeProvider, IDisposable
    {
        private readonly Action _onDispose;
        public DateTime UtcNow { get; private set; }

        public MockedTimeProvider(DateTime utcNow, Action onDispose)
        {
            _onDispose = onDispose;
            UtcNow = utcNow;
        }

        public void Dispose()
        {
            if (_onDispose != null)
            {
                _onDispose();
            }
        }
    }
}