[WPF] Prevent the user from pasting an image in a RichTextBox
WPF’s RichTextBox control is quite powerful, and very handy if you need to accept rich text input. However, one of its features can become an issue: the user can paste an image. Depending on what you intend to do with the text entered by the user, you might not want that.
When I googled for a way to prevent that, the only solutions I found suggested to intercept the Ctrl-V keystroke, and swallow the event if the clipboard contains an image. There are several issues with this approach:
- it doesn’t prevent the user from pasting using the context menu
- it won’t work if the command’s shortcut has been changed
- it doesn’t prevent the user from inserting an image using drag and drop
Since I wasn’t satisfied with this solution, I used the .NET Framework Reference Source website to look for a way to intercept the paste operation itself. I followed the code from the ApplicationCommands.Paste
property, and eventually found the DataObject.Pasting
attached event. It’s not a place where I had thought to look, but when you think about it, it actually makes sense. This event can be used to intercept a paste or drag and drop operation, and lets the hander do a few things:
- cancel the operation completely
- change which data format will be pasted from the clipboard
- replace the
DataObject
used in the paste operation
In my case, I just wanted to prevent an image from being pasted or drag and dropped, so I just cancelled the operation when the FormatToApply
was "Bitmap"
, as shown below.
XAML:
<RichTextBox DataObject.Pasting="RichTextBox1_Pasting" ... />
Code-behind:
private void RichTextBox1_Pasting(object sender, DataObjectPastingEventArgs e)
{
if (e.FormatToApply == "Bitmap")
{
e.CancelCommand();
}
}
Of course, it’s also possible to handle this in a smarter way. For instance, if the DataObject
contains several formats, we could create a new DataObject
with only the acceptable formats. This way the user is still able to paste something, if not the image.