windows forms(GUI) (C#) hangs when called from C++ exe

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C++

C++/CLI

.NET

Hi,

I am developing C++/CLI Plugin for Third party software which supports only c++.

My C++/CLI plugin is working as interface between this third party software and my already developed c# code.

C++/CLI calls C# GUI API .

My C# code generates form as follows:

if (mTestUI == null)
 {
   mTestUI = new TestUI(mCFDocument, mClientGraphics);
   Application.Run(mTestUI);
 }

TestUI is inherited from windows form as follows:

public partial class TestUI : Form

when Application.Run(mTestUI) executed both third party exe and generated form get hang.

I am unable to find reason. I can understand third party software gets hang, as control is with my generated form.

But this form is getting hang ?

this same code is working fine with other third party software which supports C# interface for plugin development.