Problem in comparing and highlighting selected text in RichTextBoxes

I am working on a c# add-in project and i want to compare two richtextboxes and highlight the differences.

but my code will highlight the whole content of the richtextbox. But I want to highlight only the differences in the richtextbox. 

this is my code,

public void Compare_HighlightContent()
        {

            txtSourceBefore.Text = SharedObjects.CodeBefore;
            RichTextBox sourceBefore = txtSourceBefore;
            txtSourceAfter.Text = SharedObjects.CodeAfter;
            RichTextBox sourceAfter = txtSourceAfter;
            //int counter=0;
            string[] targetLinesAfter = sourceAfter.Lines;
            string[] destLineBefore = sourceBefore.Lines;
            List<string> result = targetLinesAfter.Except(destLineBefore).ToList<string>();
            foreach (string str in result)
            {

                //get the line Index of 'str' value
                int lineIndex = Array.IndexOf(targetLinesAfter, str);
                int lineCount = 0;
                int index;

                for (index = 0; index < lineIndex; index++)
                {
                    lineCount = lineCount + targetLinesAfter[index].Length;

                }
                sourceAfter.SelectionStart = lineCount;
                sourceAfter.SelectionLength = str.Length;
                // sourceAfter.SelectionColor = Color.Black;
                sourceAfter.SelectionBackColor = Color.Yellow; 
            }
        }