Detecting if paste event occurred inside a rich text box

It’s a little bit tricky to detect a paste operation in the RichTextBox.

First solution may be to detect the WM_PASTE message overriding the WndProc but unfortunately the control doesn’t send that message to itself when it performs a paste operation.

Naïve detection

To detect the keyboard events may work (you have to override the OnKeyDown function) then check if the key combinations (CTRL+V and SHIFT+INS). Something like this:

protected override OnKeyDown(KeyEventArgs e)
{
     bool ctrlV = e.Modifiers == Keys.Control && e.KeyCode == Keys.V;
     bool shiftIns = e.Modifiers == Keys.Shift && e.KeyCode == Keys.Insert;

     if (ctrlV || shiftIns)
         DoSomething();
}

It works well but you can’t catch the paste operation made using the mouse (right click to open the context menu) and the paste operations made via drag & drop. If you do not need them you can use this solution (at least it’s simply and straightforward).

Better detection

Assumption: when user types inside the RichTextBox he inserts one character per time. How can you use this? Well, when you detect a bigger change you detected a paste operation because user can’t type more than once character per time (ok, you can argue that it’s not always true because of Unicode surrogates). See also VB.NET version and more details about Unicode stuff.

private int _previousLength = 0;

private void richTextBox_TextChanged(object sender, EventArgs e)
{
   int currentLength = richTextBox.Text.Length;
   if (Math.Abs(currentLength - _previousLength) > 1)
      ProcessAllLines();

   _previousLength = currentLength;
}

Please note that you can’t (because of how different IMEs work) use OnKeyDown (or similar). This works well only for western languages but it has problems with Unicode stuff (because, for example, String.Length property may be increased by two Char when user typed a single character. See also this post for much more details about this (well it’s a strongly suggested reading even, even if – in this case – you don’t care about it). In that post you’ll also find code for a better algorithm to determine string length. In short you have to replace:

   int currentLength = richTextBox.Text.Length;

With this:

   int currentLength = StringInfo.GetTextElementEnumerator(richTextBox.Text)
       .Cast<string>()
       .Count();

After all this effort you may realize that…user can even paste a single character and it may go undetected. You’re right, that’s why this is a better detection instead of a perfect solution.

Perfect solution

The perfect solution (if you’re running on Windows 8) of course exists, the native rich edit control sends an EN_CLIPFORMAT notification message. It’s intended to notify a rich edit control’s parent window that a paste occurred with a particular clipboard format. You can then override the WndProc of its parent to detect the WM_NOTIFY message for this notification. Anyway it’s not few lines of code, check this MSDN article for details.