Hi Pierre,

I performed a test based on your description. I set up a WinForms
application, set the MinimumSize of the form to 400 x 400 and the Size of
the form to 500 x 500. I handle the form's ResizeBegin event. Below is the
code of the form's ResizeBegin event handler.

using System.Diagnostics;

public class Form1 : Form
   {
       private void Form1_ResizeBegin(object sender, EventArgs e)
       {
           this.MinimumSize = new Size(100, 100);
          // When the program is running with debugging and the below code
is executed, the information will be written in the Output window.
           Debug.WriteLine("resize begin:" +
this.MinimumSize.Width.ToString());
       }
   }

Build the project and press F5 to run the program. I found the facts as
follows.

1. When I begin to resize the form for the first time, I saw the
information is written in the Output window: resize begin:100, but the form
can only be minimized to 400 x 400.  When I begin to resize the form for
the second time, I could minimize the form to 100 x 100.

2. If I move the form before resize it, the information "resize begin:100"
is written in the Output window as well. Then if I resize the form, I could
minimize the form to 100 x 100 at this time.

Until now, we can see that even if the value of the MinimumSize property of
the form is modified, this will take effect in the next time. So I
recommend you to change the value of the MinimumSize property outside the
form's ResizeBegin, Resize and ResizeEnd event handlers. The detailed place
where you should put this code depends on your real requirement.

Hope this helps.

If you have any questions, please feel free to let me know.

Sincerely,
Linda Liu
Microsoft Online Community Support

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My experiments have shown that the information returned by
the first WM_GETMINMAXINFO message sent while the user is
resizing a window gets indeed cached by Windows. Subsequent
WM_GETMINMAXINFO cannot increase the minimum size.

The only solution is to return a minimum size of [1;1] in the
initial WM_GETMINMAXINFO and then handle the WM_SIZING
message in order to restrict the moves of the window rectangle
based on my own minimum size information.

So this has nothing to do with WinForms and everything to do
with the way Windows "optimizes" the message handling. I'd
have loved to find this better documented on MSDN Library.

Pierre