Watermark TextBox in WinForms

Question

Can anyone point me to a good implementation of a basic Windows Forms TextBox that will initially show watermark text that disappears when the cursor enters it? I think I ca

Answer 1

I've updated the answer given by @Hans Passant above to introduce constants, make it consistent with pinvoke.net definitions and to let the code pass FxCop validation.

class CueTextBox : TextBox
{
    private static class NativeMethods
    {
        private const uint ECM_FIRST = 0x1500;
        internal const uint EM_SETCUEBANNER = ECM_FIRST + 1;

        [DllImport("user32.dll", CharSet = CharSet.Unicode)]
        public static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, string lParam);
    }

    private string _cue;

    public string Cue
    {
        get
        {
            return _cue;
        }
        set
        {
            _cue = value;
            UpdateCue();
        }
    }

    private void UpdateCue()
    {
        if (IsHandleCreated && _cue != null)
        {
            NativeMethods.SendMessage(Handle, NativeMethods.EM_SETCUEBANNER, (IntPtr)1, _cue);
        }
    }

    protected override void OnHandleCreated(EventArgs e)
    {
        base.OnHandleCreated(e);
        UpdateCue();
    }
}

Edit: update the PInvoke call to set CharSet attribute, to err on the safe side. For more info see the SendMessage page at pinvoke.net.