Question

I’m trying to make a custom control inherited from TextBox. I’m looking to do some custom drawing of the border of the control. I’ve run into a few issues in my test application:

1) The TextBox.HandleCreated event is never called when I debug the program. The textbox is visible on the form and I can interact with it, so I know the handle was created. I guess this is being called before I subscribe to the event?

2) WM_NCPAINT message is never received when I debug the program. I know this is called early on in the lifetime of the control. I assume I have the same problem here as I do with issue #1.

Is there any way to resolve these issues using Compact Framework 3.5? Am I doing this the preferred way?

Here is the relevant code:

public class ETextBox : TextBox
{
    private IntPtr mOldWndProc;
    private Win32Helper.WndProcDelegate mNewWndProc;

    public ETextBox(Rectangle rc)
    {
        HandleCreated += new EventHandler(ETextBox_HandleCreated);
        HandleDestroyed += new EventHandler(ETextBox_HandleDestroyed);
        Bounds = rc;
    }

    public ETextBox(String s, Rectangle rc)
        : this(rc)
    {
        Text = s;
    }

    private void SubclassWindow()
    {
        mOldWndProc = Win32Helper.GetWindowLong(Handle, Win32Helper.GWL_WNDPROC);
        mNewWndProc = new Win32Helper.WndProcDelegate(WindowProc);
        Win32Helper.SetWindowLong(Handle, Win32Helper.GWL_WNDPROC, Marshal.GetFunctionPointerForDelegate(mNewWndProc));
    }

    private void UnsubclassWindow()
    {
        if (mOldWndProc == IntPtr.Zero)
            throw new InvalidOperationException();
        Win32Helper.SetWindowLong(Handle, Win32Helper.GWL_WNDPROC, mOldWndProc);
    }

    private void ETextBox_HandleDestroyed(object sender, EventArgs e)
    {
        UnsubclassWindow();
    }

    private void ETextBox_HandleCreated(object sender, EventArgs e)
    {
        SubclassWindow();
    }

    private IntPtr WindowProc(IntPtr hwnd, uint msg, IntPtr wParam, IntPtr lParam)
    {
        switch (msg)
        {
            case Win32Helper.WM_ERASEBKGND:
                return IntPtr.Zero;

            case Win32Helper.WM_NCPAINT:
                return IntPtr.Zero;

            default:
                return Win32Helper.CallWindowProc(mOldWndProc, hwnd, msg, wParam, lParam);
        }
    }
}

Answer 1

Is there a way to implement a flat TextBox in C#?

This answer contains code for doing this (it will work on CF). This isn’t technically sub-classing, but it’s a much easier way to achieve this effect.