DermFrench DermFrench - 2 months ago 47
C# Question

Application.Current is null when calling from a unittest

I have a method that I'm trying to call from a unit test.
This method will in real life be run from a background thread. It uses some code to kick off in the invoke updates to the UI thread (using

Application.Current.Dispatcher.BeginInvoke
.... ).

However
Application.Current
is
null
when being called from the unit tests.

I don't really what to put an
if (Application.Current !=null)
around everything to fix.

Is there any other way around this?

_statusUpdates is an ObservableCollection

Below is the part of the code in the method I'm looking to test (it is more of an integration test than a unit test to be fair).

Application.Current.Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal, (EventHandler)delegate
{
_statusUpdates.Add(new StatusUpdate
{
DateTime = DateTime.Now,
Message = "Checking For Messages"
});
}, null, null);

Answer

As already stated, you simply won't have an Application class during unit tests.

That said, there's an issue here I think needs addressing - by having code that relies on a defined static property, in your case Application.Current.Dispatch, you are now very tightly coupled to the specific implementation of that class, namely the WPF Application class, where you do not need to be.

Even if you simply wrap the idea of "the current root dispatcher" in a Singleton-style class wrapper, now you have a way of decoupling yourself from the vagaries of the Application class and dealing directly with what you care about, a Dispatcher:

Note, there are MANY MANY ways to write this, I'm just putting up the simplest possible implementation; hence, I will not be doing any multithreaded safety checks, etc.

public class RootDispatcherFetcher
{
     private static Dispatcher _rootDispatcher = null;

     public static Dispatcher RootDispatcher
     {
         get 
         {
             _rootDispatcher = _rootDispatcher ??
                 Application.Current != null 
                     ? Application.Current.Dispatcher
                     : new Dispatcher(...);
             return _rootDispatcher;
         }
         // unit tests can get access to this via InternalsVisibleTo
         internal set
         {
             _rootDispatcher = value;
         }
     }
}

Ok, now this implementation is only slightly better than before, but at least you now have finer control over access to the type and are no longer strictly dependent on the existence of an Application instance.