Why does my DDE call from Excel freeze after I have a .NET App rendering level using a thread pool?

Question

I found a very strange problem when, if I get the .Net application rendering level using ThreadPool, it will hang a very simple DDE call from Excel. The problem occurred when starting a complex WPF application while calling DDE from Excel. I was able to reproduce the problem in a few lines of code, which can be found below.

C # .Net App

//Need to reference PresentationCore.dll
class Program
{
    private static int _renderTier;
    static void Main(string[] args)
    {
        ThreadPool.QueueUserWorkItem(x =>
                                         {
                                             _renderTier = RenderCapability.Tier;
                                             Console.WriteLine(_renderTier);
                                         });
        Console.ReadLine();
    }
}

Excel DDE Macro

Sub Using_DDE1()

  ' Dimension the variables.
  Dim Chan As Integer
  Dim RequestItems As Variant

  ' Start a channel to Word using the System topic.
  Chan = DDEInitiate("WinWord", "System")

  ' Requesting information from Word using the Formats item
  ' this will return a one dimensional array.
  RequestItems = DDERequest(Chan, "Formats")

  ' Uses a FOR loop to cycle through the array and display in a message box.
  For i = LBound(RequestItems) To 3

      MsgBox RequestItems(i)

  Next i

  ' Terminate the DDE channel.
  DDETerminate Chan

  End Sub 

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Answer 1

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