The top-level control cannot be added to the control.

When developing a Windows program using Visual Studio 2005, You need to display a new form in the container control. If an exception occurs in the following code, you cannot add the top-level control to the control:
Myform = new myform ();
Container. Controls. Add (myform );
I checked the relevant information online, and the solution is very simple. Because form is a top-level control by default, an exception occurs when adding a form to a container control.
You only need to set the form to a non-top-level control before adding the control code.
Myform = new myform ();
Myform. toplevel = false;
Container. Controls. Add (myform );