Question

I can’t seem to find a .NET answer to this problem, which I would have thought would be fairly common.
What is the best pattern for unit testing an asynchronous method?

Obviously I need to call the method and then see if the callback fires, but is there a better way than simply sleeping for a bit and then checking for a flag that is set by the callback? Managing the flags gets a bit messy where there are multiple tests running.

Answer 1

I typically use an anonymous delegate and a waithandle. FOr example I have a function in my presenter called SetRemoteTableName. When the name is set, it also raises an event. I want to test that event, which is raised asynchronously. The test looks like this:

[TestMethod] [WorkItem(244)] [Description('Ensures calling SetRemoteTableName with a valid name works                AND raises the RemoteTableNameChange event')] public void SetRemoteTableNamePositive() {   string expected = 'TestRemoteTableName';   string actual = string.Empty;    AutoResetEvent are = new AutoResetEvent(false);   SQLCECollectorPresenter presenter = new SQLCECollectorPresenter();   presenter.RemoteTableNameChange += new EventHandler<GenericEventArg<string>>(     delegate(object o, GenericEventArg<string> a)     {       actual = a.Value;       are.Set();     });    presenter.SetRemoteTableName(expected);   Assert.IsTrue(are.WaitOne(1000, false), 'Event never fired');   Assert.AreEqual(actual, expected); }