What is the best way to unit test an asynchronous method?

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);
}