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

I have a textbox (multiline) which开发者_JAVA百科 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?

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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);

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Another option is to use a Masked Textbox. This way, you can have multiple protected areas if you wish.

For example, you would set the mask to:

"This c\annot be ch\anged. But this c\an\: CCCCCCCCCC"

Which would appear as:

"This cannot be changed. But this can: __________"

Enter as many "C"'s as characters that you would like the user to be able to enter. You can also change the prompt character to a space rather than an "_" if you prefer.

For convenience...

Here is a list and description of masking characters

(taken from http://www.c-sharpcorner.com/uploadfile/mahesh/maskedtextbox-in-C-Sharp/).

0 - Digit, required. Value between 0 and 9.
9 - Digit or space, optional.
# - Digit or space, optional. If this position is blank in the mask, it will be rendered as a space in the Text property.
L - Letter, required. Restricts input to the ASCII letters a-z and A-Z.
? - Letter, optional. Restricts input to the ASCII letters a-z and A-Z.
& - Character, required.
C - Character, optional. Any non-control character.
A - Alphanumeric, required.
a - Alphanumeric, optional.
.  - Decimal placeholder.
, - Thousands placeholder.
: - Time separator.
/ - Date separator.
$ - Currency symbol.
< - Shift down. Converts all characters that follow to lowercase.
> - Shift up. Converts all characters that follow to uppercase.
| - Disable a previous shift up or shift down.
\ - Escape. Escapes a mask character, turning it into a literal. "\\" is the escape sequence for a backslash.

All other characters - Literals. All non-mask elements will appear as themselves within MaskedTextBox. Literals always occupy a static position in the mask at run time, and cannot be moved or deleted by the user.

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You could use RichTextBox SelectionProtected property.