Problem positioning window when using SetParent()

Question

I'm trying to set childForm as the child of the main Excel window using the SetParent API through PInvoke:

Form childForm = new MyForm();
IntPtr excelHandle = (IntPtr) excelApplication.Hwnd;
SetParent(childForm.Handle, excelHandle);
childForm.StartPosition = FormStartPosition.Manual;
childForm.Left = 0;
childForm.Top = 0;

As you can see above, my intention is also to position the child in the top left corner of Excel window. However, for some reason the childForm always ends up at some weird location.

What is it that I am doing wrong?

Answer 1

While all answers here suggest perfectly logical approaches, none of them worked for me. Then I tried MoveWindow. For some reason I don't understand, it did the job.

Here's the code:

[DllImport("user32.dll", SetLastError = true)]
internal static extern bool MoveWindow(IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint);

...

Form childForm = new MyForm();
IntPtr excelHandle = (IntPtr) excelApplication.Hwnd;
SetParent(childForm.Handle, excelHandle);
MoveWindow(childForm.Handle, 0, 0, childForm.Width, childForm.Height, true);