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Asked: 2026-05-27T01:25:15+00:00

I need to compare a value in a string to what user typed in

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I need to compare a value in a string to what user typed in a richtextbox.

For example: if a richtextbox holds string rtbText = "aaaka" and I compare this to another variable string comparable = "ka"(I want it to compare backwards). I want the last 2 letters from rtbText (comparable has only 2 letters) to be replaced with something that was predetermined(doesn’t really matter what).
So rtbText should look like this:

rtbText = "aaa(something)"

This doesn’t really have to be compared it can just count letters in comparable and based on that it can remove 2 letters from rtbText and replace them with something else.

UPDATE:
Here is what I have:

int coLen = comparable.Length;
           comparable = null;
           TextPointer caretBack = rtb.CaretPosition.GetPositionAtOffset(coLen, LogicalDirection.Backward);
           TextRange rtbText = new TextRange(rtb.CaretPosition, caretBack);
           string text = rtbText.Text;

rtbText returns an empty string or I get an error for everything longer than 3 characters. What am I doing wrong?

Let me elaborate it a little bit further. I have a listbox that holds replacements for values that user types in rtb. The values(replacements) are coming from there, meaning that I don’t really need to go through the whole text to check values. I just need to check the values right before caret. I am comparing these values to what I have stored in another variable (comparable). What needs to be done

Please let me know if you don’t understand something.
I did my best to explain what needs to be done.
Thank you

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1 Answer

  1. Editorial Team

    gumenimeda, I had similar problems few weeks ago. I found my self doing the following (I asume you will have more than one occurance in the RichTextBox that you will need to change), note that I did it for Windows Forms where I have access directly to the Rtf text of the control, not quite sure if it will work well in your scenario:

    1. I find all the occurancies of the string (using IndexOf for example) and store them in a List for example.
    2. Sort the list in descending order (max index goes first, the one before him second, etc)
    3. Start replacing the occurancies directly in the RichTextBox, by removing the characters I don’t need and appending the characters I need.

    The sorting in step 2 is necessary as we always want to start from the last occurance going up to the first. Starting from the first occurance or any other and going down will have an unpleasant surprise – if the length of the chunk you want to remove and the length of the chunk you want to append are different in length, the string will be modified and all other occurancies will be invalid (for example if the second occurance was in at position 12 and your new string is 2 characters longer than the original, it will become 14th). This is not an issue if we go from the last to the first occurance as the change in string will not affect the next occurance in the list).

    Ofcourse I can not be sure that this is the fastest way that can be used to achieve the desired result. It’s just what I came up with and what worked for me.

    Good luck!

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