Question

I have the following function:

[DllImport("user32.dll", CharSet=CharSet.Auto)]
static extern int SendMessage(IntPtr hWnd, int wMsg, int wParam, SPoint lParam);

It keeps complaining about Pinvoke stack imbalance when the following code executes:

SendMessage(EventRichTextBox.Handle, EM_GETSCROLLPOS, 0, OldScrollPoint);

What can cause that issue?

this is my SPoint

  private struct SPoint
    {
        public Int32 x;
        public Int32 y;
    }

and

SPoint OldScrollPoint = default(SPoint);

Answer 1

Can’t say for sure, but one obvious possibility is that you are on a 64 bit machine and int is the wrong type for wParam. It needs to be a 64 bit value in a 64 bit process.

We also have no idea how you have declared SPoint. You are meant to pass a pointer to a POINT struct. It doesn’t look as though you have done that.

The correct signature is:

[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(
    IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);

---

Your edit clarifies that SPoint is a struct. This then is clearly wrong. You could simply pass the SPoint as an out parameter. That would be the simplest solution.

[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(
    IntPtr hWnd, int Msg, IntPtr wParam, out SPoint lParam);

If you wanted a more general SendMessage signature then you should use IntPtr as I stated above and use Marshal.StructureToPtr.