Async Test Sources in NUnit

NUnit has long supported the definition of test cases in numerous forms, including via inline primitive data via the TestCaseAttribute or potentially more complex data returned from a method, property, or other source at runtime via TestCaseSourceAttribute. The latter has typically only supported synchronous methods. This complicated defining data-driven test cases where an internal operation required calling a Task-based API. A common example of this could be a test case which reads from a JSON source file or other stream using a method like JsonSerializer.DeserializeAsync().

In the past this would mean an awkward and unnatural call using something like .GetAwaiter().GetResult():

public class Tests
{
    [TestCaseSource(nameof(MyMethod))]
    public void Test1(MyClass item)
    {
    }

    public static IEnumerable<MyClass> MyMethod()
    {
        using var file = File.OpenRead("Path/To/data.json");
        var t = JsonSerializer.DeserializeAsync<IEnumerable<MyClass>>(file).AsTask();

        return t.GetAwaiter().GetResult();
    }
}

NUnit 3.14 was released a few months ago and included support for “async” or task-based test case sources. Now a TestCaseSource can target a Task-returning method to allow for much more natural code:

public class Tests
{
    [TestCaseSource(nameof(MyMethodAsync))]
    public void Test1Async(MyClass item)
    {
    }

    public static async Task<IEnumerable<MyClass>> MyMethodAsync()
    {
        using var file = File.OpenRead("Path/To/data.json");
        return await JsonSerializer.DeserializeAsync<IEnumerable<MyClass>>(file);
    }
}

The above example focuses on Task, but any awaitable type such as ValueTask or a custom awaitable also works. Other “source” attributes such as TestFixtureSource or ValueSource are supported as well.