[API Proposal]: FakeTimeProvider, option to allow going backwards

[jjxtra]

Background and motivation

I'm using reflection to get around FakeTimeProvider inability to go backwards. Would be nice if a bool could be added to allow setting time earlier.

Thanks for considering.

API Proposal

public sealed class FakeTimeProvider : TimeProvider
{
    public bool AllowBackwardsTimeProgression { get; set; }
}

API Usage

// Fancy the value
FakeTimeProvider t = ...
t.AllowBackwardsTimeProgression = true;
t.SetUtcNow(new DateTimeOffset(1990, 1, 1, 1, 1, 1));
t.SetUtcNow(new DateTimeOffset(1984, 1, 1, 1, 1, 1));

Alternative Designs

No response

Risks

None I can think of. This new property will default to false.

[geeknoid]

@stephentoub I presume it's possible for the normal time provider's idea of current time to move backwards? And this presumably doesn't affect any outstanding timers (since they are created with relative time), and presumably wouldn't affect the value returned by GetTimestamp?

[jjxtra]

The internal property storing the time for FakeTimeProvider is called _now and is simply a DateTimeOffset value.

[KalleOlaviNiemitalo]

If this is for testing how your application would react to a server administrator abruptly setting the clock -- then I think the method should also support setting a future wall-clock time without affecting elapsed time.

var fake = new FakeTimeProvider(new DateTimeOffset(1970, 1, 1, 0, 0, 0, TimeSpan.Zero));
long startStamp = fake.GetTimestamp();

// simulate 1 second passing
fake.Advance(TimeSpan.FromSeconds(1));
Debug.Assert(fake.GetElapsedTime(startStamp) == TimeSpan.FromSeconds(1));

// simulate 59 more seconds passing
fake.SetUtcNow(new DateTimeOffset(1970, 1, 1, 0, 1, 0, TimeSpan.Zero));
Debug.Assert(fake.GetElapsedTime(startStamp) == TimeSpan.FromMinutes(1));

// NEW: simulate an administrator setting the clock forward
// no effect on elapsed time
fake.SetUtcNow(new DateTimeOffset(1970, 1, 1, 0, 2, 0, TimeSpan.Zero), abrupt: true);
Debug.Assert(fake.GetElapsedTime(startStamp) == TimeSpan.FromMinutes(1));

// simulate 60 more seconds passing
fake.SetUtcNow(new DateTimeOffset(1970, 1, 1, 0, 3, 0, TimeSpan.Zero));
Debug.Assert(fake.GetElapsedTime(startStamp) == TimeSpan.FromMinutes(2));

// NEW: simulate an administrator setting the clock backward
// no effect on elapsed time
fake.SetUtcNow(new DateTimeOffset(1970, 1, 1, 0, 2, 50, TimeSpan.Zero), abrupt: true); 
Debug.Assert(fake.GetElapsedTime(startStamp) == TimeSpan.FromMinutes(2));

[stephentoub]

@stephentoub I presume it's possible for the normal time provider's idea of current time to move backwards? And this presumably doesn't affect any outstanding timers (since they are created with relative time), and presumably wouldn't affect the value returned by GetTimestamp?

Should be fine. If we find some place it's not, I'd consider that a bug.