Question

How would you prevent the user from adding or removing lines in a TextBox? By that I mean, if I set the text in a textbox to 7 lines with some text, how can I make sure that it will always be 7 lines of text there? The user must be able to edit those lines like usual, but not remove a line entirely, and not add any new ones.

Would have to take into account both keyboard input and also things like cut and paste, etc.

Any good ideas?

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Reason: I want to make a file renamer kind of like Oscar’s Renamer. You give it a folder, and it loads the filenames into a textbox where you can do changes pretty much like you do in a text editor. When you are happy with your changes, you write them back. Reason for constant n lines in textbox is then of course that line n is the name of file n. Adding a new line shouldn’t be allowed since you only have those files in that folder. Removing a line should also not be allowed since you would then be missing a name for a file.

Why go through the trouble of making something like this when it already exists? Well, I am curious to see if I could do it, and thought it could be a nice excercise to learn a few things along the way. Since it has some interesting problems that needs to be solved. Like this one 🙂 There are also some features I think are lacking in that Oscar’s Renamer. So… to sum up: I’m doing it to learn and to try make an even better version of it. I know fully well that I may just as well fail completely though, or just never finish it :p But that is another story. I want to learn

Answer 1

One possible way of doing this is to sub-class the Textbox control and override the winProc method. This method handles all window messages pumped to the windowed control (Textbox in your case). You could monitor the use of the backspace and delete keys and carat position and discard the key strokes that attempt to remove carriage return line feed sequences. And provide the user with an interactive alert that tells them why they cannot remove entire lines.

Doing it this way gives you complete control and is the lowest level way to see all input that is coming into your Textbox control. You can intercept certain messages and discard them, the ones that you want to allow just pass them through into the base class method. Such as if the user highlights all lines and hits the delete key. There is other event handlers that you can use to intercept keyboard input but they have some limitations, the winProc will allow you to check any message directed to the control including delete, backspace copy and paste etc, mouse clicks etc.

Sample:

public class myCustomTextBox : TextBox {     protected override void WndProc(ref Message m)     {         if (m.Msg == 770) // window paste message id         {             string clipBoardData = Clipboard.GetDataObject().GetData(DataFormats.Text).ToString();             handlePasteEvent(clipBoardData);         }         else         {             base.WndProc(ref m);         }     }     private void handlePasteEvent(string pasteData)     {         // process pasted data     } }