Question

I use a form with border NONE. I need to override WndProc for resize and move form. However, using this code, my app crashes!

 static const int WM_NCHITTEST = 0x0084;
 static const int HTCLIENT = 1;
 static const int HTCAPTION = 2;

 protected: virtual void Form1::WndProc(System::Windows::Forms::Message %m) override 
 {        
     switch (m.Msg)
     {
         case WM_NCHITTEST:
             if (m.Result == IntPtr(HTCLIENT))
             {
                 m.Result = IntPtr(HTCAPTION);
             }
             break;
     }
     Form1::WndProc(m);
 }       


virtual System::Windows::Forms::CreateParams^ get() override
{

    System::Windows::Forms::CreateParams^ cp = __super::CreateParams;
    cp->Style |= 0x40000; 
    return cp;
}

How can I fix my code not to crash but still allow my form to be moved and resized?

Answer 1

You have to call the base class’s WndProc method first, in order for m.Result to be set to the correct value. The code you posted doesn’t call it until the end.

Additionally, you’re trying to call the base class with Form1::WndProc, which actually calls the current class’s implementation of the WndProc method. That’s why your application is “crashing” every time you try to run it with a StackOverflowException—your code within the WndProc method is calling itself recursively. Directly calling the superclass’s implementation (like you did in the CreateParams property) with the __super keyword works just fine.

So, you need to re-write that method like this:

virtual void Form1::WndProc(System::Windows::Forms::Message %m) override 
{
    __super::WndProc(m);

    switch (m.Msg)
    {
    case WM_NCHITTEST:
        if (m.Result == IntPtr(HTCLIENT))
        {
            m.Result = IntPtr(HTCAPTION);
        }
        break;
    }
}