Question

My demo application implements the MVVM pattern in a WPF project. The ViewModel calls a remote webservice via a service agent (proxy) like this:

proxy.GetProjectList((sender, e) => this.ProjectList, username, password);

ProjectList is a property defined in the ViewModel. It holds an array of CProject objects. The view binds to this property to display the project names. Basically this works fine.

However I get a NullReferenceException if I add the following if-statement:

proxy.GetProjectList((sender, e) => this.ProjectList = e.Result, username, password);
if (ProjectList.Length > 0) doSomething();

Debugging the application shows that the ProjectList property is null after the webservice has been called. And I just dont’t know why.

The webservice call above is implemented as follows:

public void GetProjectList(EventHandler<getProjectListCompletedEventArgs> callback, string username, string password) {
  proxy.getProjectListCompleted += callback;
  proxy.getProjectListAsync(username, password);
}

Answer 1

Following the hint from HighCore I changed my implementation to the following.

Calling the operation from the client:

proxy.GetProjectList(GetProjectListCallback, Username, SecurePassword);

Adding the callback method before:

private void GetProjectListCallback(object sender, getProjectListCompletedEventArgs e) {
  ProjectList = e.Result;
  if (ProjectList != null) {
    if (ProjectList.Length > 0) doSomething();
  }
}

Calling the actual webservice operation in a seperate service agent:

public void GetProjectList(getProjectListCompletedEventArgs callback, string username, SecureString password) {
  proxy.getProjectListCompleted += callback;
  proxy.getProjectListAsync(username, password);
}

I don’t know if this is a good programming style but it works 🙂