Test a function throwing an Exception in a Task in xunit

Question

I want to test using xunit a function that run a task and throw in a that task For example :

public void doSomething(){
     Task.Run(() =>
            {
                throw new ArgumentNullException();
            });
}

When I want to test this function by doing this :

[Fact]
public void TestIfTheMethodThrow()
{
   Assert.Throws<ArgumentNullException>(() => doSomething()); // should return true but return false                                                                   
}

I want that the Task.Run() finish completely then the assert can be done. anyone have a solution ?

Answer 1

Raising and handling exceptions using TPL (the Tasks library) is slightly different, than the "standard" exception handling. It is meaningful to evaluate only a completed task, so you need to wait for the completion, even if in this case it is an exception.

Have a look at this MSDN article Exception handling (Task Parallel Library).

You have two different options:

• add .Wait() to the Task.Run(...)`

  Task.Run(() =>
         {
             throw new ArgumentNullException();
         }).Wait();
  
  
  

• or wait while the task is completed while(!task.IsCompleted) {}

     var task = Task.Run(() =>
            {
                throw new ArgumentNullException();
            });
    while(!task.IsCompleted) {}

The test result should be then as expected - an exception is thrown.

It could be, that before the Wait your test has passed sporadically - don't be irritated - in this case the task execution was faster than the test check - this is a dangerous source of subtle errors.