hook on default "Paste" event of WinForms TextBox control

Question

I need to "modify" all pasted into TextBox text to be shown in some structured way. I can do it with drag-n-drop, ctrl-v, but how to do it with default context's menu "Paste"?

score 21 · Accepted Answer · edited Dec 23 '13 at 16:08

While I would normally not suggest dropping to low level Windows API, and this may not be the only way of doing this, it does do the trick:

using System;
using System.Windows.Forms;

public class ClipboardEventArgs : EventArgs
{
    public string ClipboardText { get; set; }
    public ClipboardEventArgs(string clipboardText)
    {
        ClipboardText = clipboardText;
    }
}

class MyTextBox : TextBox
{
    public event EventHandler<ClipboardEventArgs> Pasted;

    private const int WM_PASTE = 0x0302;
    protected override void WndProc(ref Message m)
    {
        if (m.Msg == WM_PASTE)
        {
            var evt = Pasted;
            if (evt != null)
            {
                evt(this, new ClipboardEventArgs(Clipboard.GetText()));
                // don't let the base control handle the event again
                return;
            }
        }

        base.WndProc(ref m);
    }
}

static class Program
{
    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);

        var tb = new MyTextBox();
        tb.Pasted += (sender, args) => MessageBox.Show("Pasted: " + args.ClipboardText);

        var form = new Form();
        form.Controls.Add(tb);

        Application.Run(form);
    }
}

Ultimately the WinForms toolkit is not very good. It is a thin-ish wrapper around Win32 and the Common Controls. It exposes the 80% of the API that is most useful. The other 20% is often missing or not exposed in a way that is obvious. I would suggest moving away from WinForms and to WPF if possible as WPF seems to be a better architected framework for .NET GUIs.

thanks, I've just learned sth new (not only how to catch "paste", nut a ne way of doin things) — David, Aug 10 '10 at 11:09
event keyword is missing on Pasted field declaration and why are you using a local variable evt ? — Maxence, Oct 03 '11 at 09:10
You're missing a `return` inside the `if` statement which causes the base control to re-handle this event. I was formatting XML on paste and without the `return`, both the the formatted and unformatted XML show up as the base control handles the paste again. — Mrchief, Dec 23 '13 at 16:07
@Maxence The local variable is to avoid a race condition that can occur between checking if `Pasted == null` and invoking the handler. If the lone subscriber unsubscribes from the event on another thread between the check and invocation, you could end up with a null reference exception. C# 6 allows `Pasted?.Invoke()` as an alternative thead-safe solution. — itsme86, May 02 '17 at 22:39
@itsme86 even in C#6 this capturing the variable is important because not only are we preventing the race condition, but we only want to prevent the default behaviour if the `Pasted` event was registered in the first place. This implementation is therefore still relevant even as we head into C#10. — Chris Schaller, Dec 17 '21 at 04:04

hook on default "Paste" event of WinForms TextBox control

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