Re: Initialize WPF window throw exception

I have figured it out, thisApp being null really was the problem, the ExternalApplication class was never invoke, and Revit throw the exception at the ExternalCommand level. I just need to make a new ExternalApplication class instance ,call the ShowWindow method and the program does what I expected:

public virtual Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements)
        {
            try
            {
                new ThisApplication().ShowWindow(commandData.Application);

                return Result.Succeeded;
            }
            catch (Exception ex)
            {
                message = ex.Message;
                return Result.Failed;
            }
        }

However, this is exactly how the example in SDK modeless dialog. I wonder why it work there?