Comments by PrasadKumar (Top 1 by date)

Thanks for replies.

It solved. My logic was wrong. I was trying to use object of MDIForm of DLL into project.

What I did,
a. Created a class library (DLL) and added Form1 (which is inherited by System.Windows.Form class) and MDIForm.
b. Created new project and used above DLL.
c. In this project , added Form2 (which is inherited by Form1 of DLL),
d. In Program.cs, Main() method created object of MDIForm (of DLL)
e. Tried to make MDI parent of Form2 object to MDIForm object. (it was wrong logic).
f. Now, I have created MDIFrom in project (and removed from DLL) and its working fine.

Thanks