Windows.Forms RichTextBox Control - Avoid inserting large data

I have a Windows Form with a RichTextBox on it. The content of the RichTextBox is written to a database field that ist limited to 64k data. For my purpose that is way more than enough text to store.

I have set the MaxLength property to avoid insertng more data than allowed.

rtcControl.MaxLength = 65536

Howevery, that only restricts the amount of characters that so is allowed to put in the text. But with the formatting overhead from the Rtf I can type more text than I should be allowed to. It even get's worse if I insert a large image, which dosn't increase the TextLength at all but the Rtf Length grows quite a lot.

At the moment I check the Length of the richttextboxes' Rtf property in the FormClosing event and display a message to the user if it's to large. However that is just a workaround because I want to disallow putting more data than allowed into the control (like in a textbox if you exceed the MaxLength property nothing is inserted into the control and you hear the default beep().

Any ideas how to achive this?

I already tried:

using a custom control which extends the richtextbox and shadows th Rtf property to intercept the insertation. But it seems it isn't executed if I add text.
Even the TextChanged Event does not fire if I type smth. in the control.

Solution

What about doing this:

Handle the TextChanged event and compare each time it changes. It fired for text entry and image drag and drops.

private void richTextBox1_TextChanged(object sender, EventArgs e)
{
   if (richTextBox1.Rtf.Length > richTextBox1.MaxLength)
   {
      // do something here - I displayed a label for
      // my example 
      label1.Text = "Text exceeds maximum size";
      label1.ForeColor = Color.Red;
   }
   else
   {
      label1.Text = richTextBox1.Rtf.Length.ToString();
      label1.ForeColor = Color.Red;
   }
}

This seemed to work, though I didn't spend a lot of time on this, admittedly. I suspect you could truncate the contents if it exceeds the max size.

Edit: I thought about this some more and I suspect you could utilize a StringBuilder to store off the contents of the richtextbox each time and if one attempted to exceed that length, restore the data to the previous state. I admit this is a bit hacky but it seems to work. Something like this:

StringBuild sb = new StringBuilder();

private void richTextBox1_TextChanged(object sender, EventArgs e)
{
   if (richTextBox1.Rtf.Length > richTextBox1.MaxLength)
   {
      richTextBox1.Rtf = sb.ToString();
   }
   else
   {
      sb.Insert(0,richTextBox1.Rtf);
   }
}

This seems to work pretty well. There might be a more elegant solution.