fake-async C#

Simulates passage of time to test asynchronous long-running code in synchronous way.

This library is inspired from Angular's FakeAsync.

bool flag = false;

async Task MethodUnderTest()
{
    await Task.Delay(TimeSpan.FromSeconds(10));

    flag = true;

    _ = Task.Run(async () =>
    {
        await Task.Delay(TimeSpan.FromSeconds(10));

        Console.WriteLine("Hello from faked thread pool");
    });
}

var fakeAsync = new FakeAsync
{
    UtcNow = new DateTime(2020, 10, 20),
};
          
fakeAsync.Isolate(async () =>
{
    var testing = MethodUnderTest();

    // Current time is changed
    Console.WriteLine("{0}, flag={1}", DateTime.UtcNow, flag); // 2020-10-20 00:00:00, false

    // Skip 9s synchronously, no real delay
    fakeAsync.Tick(TimeSpan.FromSeconds(9));

    Console.WriteLine("{0}, flag={1}", DateTime.UtcNow, flag); // 2020-10-20 00:00:09, false

    // Skip 1s more
    fakeAsync.Tick(TimeSpan.FromSeconds(1));

    // Flag changed
    Console.WriteLine("{0}, flag={1}", DateTime.UtcNow, flag); // 2020-10-20 00:00:10, true

    // Now MethodUnderTest() is completed, await it to propagate any exceptions
    await testing;

    // Skip remaining time to display the message
    fakeAsync.Tick(TimeSpan.FromSeconds(10)); // Hello from faked thread pool
});

// Print real system time
Console.WriteLine(DateTime.UtcNow);

Supported calls:

• DateTime.Now
• DateTime.UtcNow
• Task.Run()
• Task.Factory.StartNew() when TaskScheduler is ThreadPoolTaskScheduler (default)
• new Task().Start()
• Task.Delay()
• Thread.Sleep() (TODO)
• Task.Yield()

Stopwatch is not changed, so it is possible to measure time.

Known issues

Tiered compilation in .NET Core 3.0 is enabled by default. It occasionally overrides FakeAsync patches, so mock is not reliable. Currently, there is an open issue in Harmony.Lib. Workaround is to disable tiered compilation in tests project:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TieredCompilation>false</TieredCompilation>
  </PropertyGroup>

</Project>

After changing this setting Clean and Rebuild solution to take effect.

Road map

• Suitable API to test asynchronous long running code
• Implement tick() (like Angular's FakeAsync)
• Allow tests with Polly timeout policies
• Make a Nuget package

Credits

• Harmony - a library for patching, replacing and decorating .NET and Mono methods during runtime