Unit testing with MVVM Light & DispatcherHelper - silverlight

Question

I have a SL4 app that utilizes the MVVM Light Toolkit. Within a view model, I call a data service that retrieves data from an OData service. Within the VM, I am using the DispatcherHelper utility class (part of MVVM Light) to update the property on the VM from the data in the callback I pass into the data service. For instance, my view model method looks like this:
public string CurrentUserLogin {
get {
if (string.IsNullOrEmpty(_currentUserLogin))
RetrieveCurrentUserLogin();
return string.IsNullOrEmpty(_currentUserLogin) ? _currentUserLogin : _currentUserLogin.Replace(#"\\", #"\");
}
set {
if (_currentUserLogin != value) {
_currentUserLogin = value;
RaisePropertyChanged(CurrentUserLoginPropertyName);
}
}
}
private void RetrieveCurrentUserLogin() {
DataService.GetCurrentUserLogin(result => {
DispatcherHelper.UIDispatcher.BeginInvoke(() => {
CurrentUserLogin = result;
});
});
}
And here's what my data service looks like:
public void GetCurrentUserLogin(Action<string> callback) {
// create query request
var query = OnDemandContext.CreateQuery<string>("GetCurrentUserLogin");
var request = (HttpWebRequest)WebRequest.Create(query.RequestUri);
request.BeginGetResponse(asyncResult => {
var responseStream = request.EndGetResponse(asyncResult).GetResponseStream();
var responseDocument = XDocument.Load(responseStream);
callback(responseDocument.Root.Value);
}, null);
}
Everything works great when I run my SL application. However the problem I have is when I try to write unit tests against it using the SL Unit Testing Framework. I can test my data service without an issue, but it seems the DispatcherHelper is throwing a wrench into all my tests as the DispatcherHelper.UIDispatcher is always null when fired. I'm assuming it has something to do with the initlization (which is in my SL app's Application_Startup()). I tried initializing it in my test app but that isn't helping. I've also tried using DispatcherHelper.CheckBeginInvokeOnUI() but that has no effect on the issue.
Ideas?

Answer 1

AC,
I just created a simple SL UT project and I did this in the App.XAML.CS
private void Application_Startup(object sender, StartupEventArgs e)
{
RootVisual = UnitTestSystem.CreateTestPage();
DispatcherHelper.Initialize();
}
Then I set this as the test (in the tests.cs):
[TestMethod]
public void TestMethod1()
{
Assert.IsNotNull(DispatcherHelper.UIDispatcher, "UI Dispatcher should not be null");
DispatcherHelper.CheckBeginInvokeOnUI(() =>
{
// Do nothing
var x = 1;
});
}
That worked for me. I even set a break point on the "var x = 1;" and it hit the breakpoint. Does this solve your problem? (if so please mark it as the answer).