Skip to main content

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.


Post a Comment

Popular posts from this blog

How to check in AngularJS if a service was register or not

There are cases when you need to check in a service or a controller was register in AngularJS.
For example a valid use case is when you have the same implementation running on multiple application. In this case, you may want to intercept the HTTP provider and add a custom step there. This step don’t needs to run on all the application, only in the one where the service exist and register.
A solution for this case would be to have a flag in the configuration that specify this. In the core you would have an IF that would check the value of this flag.
Another solution is to check if a specific service was register in AngularJS or not. If the service was register that you would execute your own logic.
To check if a service was register or not in AngularJS container you need to call the ‘has’ method of ‘inhector’. It will return TRUE if the service was register.
if ($injector.has('httpInterceptorService')) { $httpProvider.interceptors.push('httpInterceptorService&#…

ADO.NET provider with invariant name 'System.Data.SqlClient' could not be loaded

Today blog post will be started with the following error when running DB tests on the CI machine:
threw exception: System.InvalidOperationException: The Entity Framework provider type 'System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer' registered in the application config file for the ADO.NET provider with invariant name 'System.Data.SqlClient' could not be loaded. Make sure that the assembly-qualified name is used and that the assembly is available to the running application. See for more information. at System.Data.Entity.Infrastructure.DependencyResolution.ProviderServicesFactory.GetInstance(String providerTypeName, String providerInvariantName) This error happened only on the Continuous Integration machine. On the devs machines, everything has fine. The classic problem – on my machine it’s working. The CI has the following configuration:

TeamCity.NET 4.51EF 6.0.2VS2013
It seems that there …

Entity Framework (EF) TransactionScope vs Database.BeginTransaction

In today blog post we will talk a little about a new feature that is available on EF6+ related to Transactions.
Until now, when we had to use transaction we used ‘TransactionScope’. It works great and I would say that is something that is now in our blood.
using (var scope = new TransactionScope(TransactionScopeOption.Required)) { using (SqlConnection conn = new SqlConnection("...")) { conn.Open(); SqlCommand sqlCommand = new SqlCommand(); sqlCommand.Connection = conn; sqlCommand.CommandText = ... sqlCommand.ExecuteNonQuery(); ... } scope.Complete(); } Starting with EF6.0 we have a new way to work with transactions. The new approach is based on Database.BeginTransaction(), Database.Rollback(), Database.Commit(). Yes, no more TransactionScope.
In the followi…