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How to write unit-tests for async methods

All developer that works with .NET heard about Task, async, await – Task Parallel Library (TPL). Great library when we need to write code that runs in parallel.
With TPL, writing code that run in parallel is pretty simple. This is great, but of course, all code that run in parallel need to be tested also – unit tests. Do you know how you need to write unit tests for async calls?
I so pretty strange way of unit tests for async methods. Some of them were ugly and complicated. Why? Because the unit test method is a sync one and there we try to run and wait a response from an async call. This is why we can end up with something like this:
        public void MoveFile_ExistingFile_ResultsFileMovedAndOriginalFileDeleted()
            StorageFolder destinationFolder = null;
            Task.Run(() => destinationFolder = 
            var fileToMove = StorageHelper.CreateFile(_originalFolder,FileName);

            Task.Run(() =>  _fileManipulator.MoveFileAsync(fileToMove, destinationFolder))

            Assert.IsTrue(_fileManipulator.Exist(destinationFolder, FileName));
            Assert.IsFalse(_fileManipulator.Exist(_originalFolder, FileName));
        private void SaveContent(byte[] originalContent)
            Task saveTask = Task.Run(() => _applicationFileManager
                                  .SaveAsync(FileName, originalContent));

What do you thing? Do you like to have in the unit tests calls to Task.Run(). Personal I don’t like this and for me is a big smell. Something we are doing wrong, we are missing something.
What we are missing is the way we are writing the unit test method. By default, when we are wring a unit test we define the unit test method in this way:
public void SomeTest() { }
This is okay for testing a sync call. But when testing async call we have more option. It would be nice to be able to have our test method as an async method. In this way we don’t need to call Task.Run().
The reality is that we can define a test method like this:
public async Task SomeTest() { }
Doing this we can call our async method as a normal method and test accordingly.
        public async Task MoveFile_ExistingFile_ResultsFileMovedAndOriginalFileDeleted()
            StorageFolder destinationFolder = null;
           destinationFolder = await CreateFolderAsync(_originalFolder)
            var fileToMove = StorageHelper.CreateFile(_originalFolder,FileName);

            await _fileManipulator.MoveFileAsync(fileToMove, destinationFolder);

            Assert.IsTrue(_fileManipulator.Exist(destinationFolder, FileName));
            Assert.IsFalse(_fileManipulator.Exist(_originalFolder, FileName));
This feature works only on Visual Studio 2012.
On Visual Studio 2010 we need to install a NuGet package called AsyncUnitTests-MSTest. This will allow us to use async and await in our unit test. We will need to replace the TestClass attribute with AsyncTestClass. This attribute is able to run normal tests also.

In this post we saw how easily we can run unit tests for async code, without having to hack our calls.


  1. Indeed, also NUnit (>= 2.6.2) and (>= 1.9) have support for async tests.


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