Unit Testing of async methods .NET 4.0 style

Question

I have library functions in the library that I developed that execute asynchronously. Description by example:

private void Init() { // subscribe to method completion event myLib.CompletionEvent += myLib_CompletionEvent; } private void MyButton1_Click(object sender, EventArgs e) { // start execution of Func1 myLib.Func1(param1, param2); }

the Func1 initiates some processing in the library and immediately returns.

The result is of the processing arrives to the client as an event through a delegate

void myLib_CompletionEvent(ActionResult ar) { // ar object contains all the results: success/failure flag // and error message in the case of failure. Debug.Write(String.Format("Success: {0}", ar.success); }

So here is the question: How do I write Unit test for this?

I recommend you to check this stackoverflow.com/questions/15207631/… — nuclear sweet
– nuclear sweet, Commented Mar 14, 2014 at 7:28

NeddySpaghetti · Accepted Answer · 2014-03-14 08:19:56Z

I'd recommend creating a task based asynchronous method using a TaskCompletionSource and then using the support for async\await by testing frameworks such as MsTest and Nunit to write an asynchronous unit test. The test must be marked as async and is usually required to return a Task

You can write the asynchronous method like this(untested):

public Task Func1Async(object param1, object param2) { var tcs = new TaskCompletionSource<object>(); myLib.CompletionEvent += (r => { Debug.Write(String.Format("Success: {0}", ar.success)); // if failure, get exception from r, which is of type ActionResult and // set the exception on the TaskCompletionSource via tcs.SetException(e) // if success set the result tcs.SetResult(null); }); // call the asynchronous function, which returns immediately myLib.Func1(param1, param2); //return the task associated with the TaskCompletionSource return tcs.Task; }

Then you can write your test like this:

 [TestMethod] public async Task Test() { var param1 = //.. var param2 = //.. await Func1Async(param1, param2); //The rest of the method gets executed once Func1Async completes //Assert on something... }

Although fine approach, I have a few problems with it: 1. It is not what I asked. 2. I doubt if it works: in Test Method aka Test() the "rest of the method gets executed once Func1Async completes" - but the process in the library did not complete yet...(Func1Async returns immediately and so does myLib.Func1)
Func1Async returns immediately but it returns a Task, once the task is complete the rest of the method gets executed. Have a look at this post stackoverflow.com/questions/15736736/how-does-await-async-work

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