TimeScheduler

Time Scheduler

A library that wraps common .NET scheduling and time related operations in an abstraction, ITimeScheduler, that enables deterministic control of time during testing using a ForwardTime method.

Currently, the following .NET Task and DateTimeOffset-based APIs are supported:

There are two implementations of ITimeScheduler included in the package, DefaultScheduler which is used in production, and TestScheduler which is used during testing.

During testing, you can move time forward by calling TestScheduler.ForwardTime(TimeSpan). This allows you to write tests that run fast and predictable, even if the system under test pauses execution for multiple minutes using e.g. ITimeScheduler.Delay(TimeSpan), the replacement for Task.Delay(TimeSpan).

Installation

Get the latest release from https://www.nuget.org/packages/TimeScheduler

Set up in production

To use in production, pass in DefaultScheduler to the types that depend on ITimeScheduler. This can be done directly, or via an IoC Container, e.g. .NETs built-in IServiceCollection like so:

services.AddSingleton<ITimeScheduler>(DefaultScheduler.Instance);

If you do not want to register the ITimeScheduler with your IoC container, you can instead create an additional constructor in the types that use it, which allow you to pass in a ITimeScheduler, and in the existing constructor(s) you have, just new up DefaultScheduler directly. For example:

public class MyService
{
    private readonly ITimeScheduler scheduler;

    public MyService() : this(DefaultScheduler.Instance)
    {
    }

    public MyService(ITimeScheduler scheduler)
	{
		this.scheduler = scheduler;
	}
}

This allows you to explicitly pass in an TestScheduler during testing.

Example - control time during tests

If a system under test (SUT) uses things like Task.Delay, DateTimeOffset.UtcNow, Task.WaitAsync, or PeriodicTimer, it becomes hard to create tests that runs fast and predictably.

The idea is to replace the use of e.g. Task.Delay with an abstraction, the ITimeScheduler, that in production is represented by the DefaultScheduler, that just uses the real Task.Delay. During testing it is now possible to pass in TestScheduler, that allows the test to control the progress of time, making it possible to skip ahead, e.g. 10 minutes, and also pause time, leading to fast and predictable tests.

As an example, lets test the "Stuff Service" below that performs a specific tasks every 10 second with an additional 1 second delay. We have two versions, one that uses the standard types in .NET, and one that uses the ITimeScheduler.

// Version of stuff service that uses the built in DateTimeOffset, PeriodicTimer, and Task.Delay
public class StuffServiceSystem
{
  private static readonly TimeSpan doStuffDelay = TimeSpan.FromSeconds(10);
  private readonly List<DateTimeOffset> container;

  public StuffServiceSystem(List<DateTimeOffset> container)
  {
    this.container = container;
  }
  
  public async Task DoStuff(CancellationToken cancelllationToken)
  {
    using var periodicTimer = new PeriodicTimer(doStuffDelay);
    
    while (await periodicTimer.WaitForNextTickAsync(cancellationToken))
    {      
      await Task.Delay(TimeSpan.FromSeconds(1));
      container.Add(DateTimeOffset.UtcNow);
    }
  }
}

// Version of stuff service that uses the built in TimeScheduler
public class StuffServiceUsingTimeScheduler 
{
  private static readonly TimeSpan doStuffDelay = TimeSpan.FromSeconds(10);
  private readonly ITimeScheduler scheduler;
  private readonly List<DateTimeOffset> container;

  public StuffServiceUsingTimeScheduler(ITimeScheduler scheduler, List<DateTimeOffset> container)
  {
    this.scheduler = scheduler;
    this.container = container;
  }
  
  public async Task DoStuff(CancellationToken cancelllationToken)
  {
    using var periodicTimer = scheduler.PeriodicTimer(doStuffDelay);
    
    while (await periodicTimer.WaitForNextTickAsync(cancellationToken))
    {      
      await scheduler.Delay(TimeSpan.FromSeconds(1));
      container.Add(scheduler.UtcNow);
    }
  }
}

The test, using xUnit and FluentAssertions, could look like this:

[Fact]
public void DoStuff_does_stuff_every_11_seconds()
{
  // Arrange
  var scheduler = new TestScheduler();
  var container = new List<DateTimeOffset>();  
  var sut = new StuffServiceUsingTimeScheduler(scheduler, container);
  
  // Act
  _ = sut.DoStuff(CancellationToken.None);
  scheduler.ForwardTime(TimeSpan.FromSeconds(11));
  
  // Assert
  container
    .Should()
    .ContainSingle()
    .Which
    .Should()
    .Be(scheduler.UtcNow);
}

Writing a similar test for StuffServiceSystem is both more simple and runs much slower.

[Fact]
public async Task DoStuff_does_stuff_every_11_seconds()
{
  // Arrange
  var container = new List<DateTimeOffset>();  
  var sut = new StuffServiceSystem(container);
  
  // Act
  _ = sut.DoStuff(CancellationToken.None);
  await Task.Delay(TimeSpan.FromSeconds(11));
  
  // Assert
  container
    .Should()
    .ContainSingle()
    .Which
    .Should()
    .BeCloseTo(DateTimeOffset.UtcNow, precision: TimeSpan.FromMilliseconds(50));
}