Disabling or making it readonly a part of the values in a Text Box (.net)

Question

I have a textbox (multiline) which has initial value, now when user tries to append values to it, he should not be able to modify the initial value. Is it possible in any way?

Answer 1

So if you have "lorem ipsum" in the box you want the user to be able to add to but not remove that text? If so then with a RichTextBox you can do this via a selection's .SelectionProtected property which will mark a region as effectively being read only.

rtBox.Select(0, (rtBox.Text = "I am fixed content").Length);
rtBox.SelectionProtected = true;
rtBox.AppendText(Environment.NewLine);