TextRange of a given piece of text from the RichTextBox

You need this code (made on the basis of this post ):

TextPointer FindPointerAtTextOffset(TextPointer from, int offset, bool seekStart) { if (from == null) return null; TextPointer current = from; TextPointer end = from.DocumentEnd; int charsToGo = offset; while (current.CompareTo(end) != 0) { Run currentRun; if (current.GetPointerContext(LogicalDirection.Forward) == TextPointerContext.Text && (currentRun = current.Parent as Run) != null) { var remainingLengthInRun = current.GetOffsetToPosition(currentRun.ContentEnd); if (charsToGo < remainingLengthInRun || (charsToGo == remainingLengthInRun && !seekStart)) return current.GetPositionAtOffset(charsToGo); charsToGo -= remainingLengthInRun; current = currentRun.ElementEnd; } else { current = current.GetNextContextPosition(LogicalDirection.Forward); } } if (charsToGo == 0 && !seekStart) return end; return null; } 

Explanation: In FlowDocument , you have different supporting positions: the beginning of the document, the beginning of a paragraph, the beginning of a Span 'a, etc. For these positions, you can iterate using GetNextContextPosition . Of these, all the text is only inside the Run , and the rest are official. Thus, we iterate and find all Run blocks, for each of them we get the length of the inner text and see if our position is in the current block or not.

Use so. Suppose we want to get characters from the 5th to the 18th. We write:

 TextPointer start = FindPointerAtTextOffset(flowDocument.ContentStart, 5, seekStart: true); if (start == null) { // 5-ая позиция вне документа, выходим } TextPointer end = FindPointerAtTextOffset(start, 18 - 5, seekStart: false); if (end == null) { // 18-ая позиция вне документа, выходим } TextRange range = new TextRange(start, end);