Highlighting a line of text in a richtextbox

Question

So I am working on an program that offers the user a 3-D visualization of data structures and sort algorithms. What I would like to do is have a richtextbox on the UI that shows the code for the particular algorithm that is being performed. And then I would like to have each particular line of the code to be highlighted as it is being executed. I just wanted to start with visualizing a stack since it is easier to deal with as I learn and work through this project. Right now I have a text file of c++ push and pop functions and I am saving the text into a list. I am then writing the text to the richtextbox. All of this is working but I don't know how to highlight a line and then highlight the next line. For example when I click "push" I would like it to highlight "list[stackTop] = newItem;" then draw the 3d cube (already done), then highlight the "stackTop++" line. Then the user can do it again or whatever else they want.

class CppFunctionsArray
    {
    List<string> ReadFunctions = new List<string>();
    int Position = 0;

    //Reads input from selected file and stores into ReadFunctions Array;
    public void ReadInput(string fileName)
    {

        using (StreamReader r = new StreamReader(fileName))
        {
            string line;
            while ((line = r.ReadLine()) != null)
            {
                ReadFunctions.Add(line);
            }
        }
    }

    //Writes lines to a RichTextBox.
    public void WriteToRichTextBox(RichTextBox rtb, int startIndex, int endIndex, int  lineNumber)
    {
        Position = 0;
        for (int i = startIndex; i < endIndex; i++)
        {
            rtb.AppendText(ReadFunctions[i]);
            rtb.AppendText(Environment.NewLine);
            rtb.Font = new Font("times new roman", 12, FontStyle.Bold);

            //Temporary
            if (lineNumber == Position)
                  rtb.SelectionBackColor = Color.Red;
            Position++;

        }
    }

These are not topics they are teaching me college. I am just teaching myself here. So if I am approaching this totally wrong, I am open to anything here.

Here is my event handler for "stackPush" button.

    //Adds cube on top of the previous.
    private void StackPush_Click(object sender, EventArgs e) 
    {

        CppFunctionsArray ArrayOfFunctions = new CppFunctionsArray();
        CodeTextBox.Clear();
        ArrayOfFunctions.ReadInput("StackFunctions.txt");

        //The 4 represents the line Number to highlight. TODO FIX THIS.
        ArrayOfFunctions.WriteToRichTextBox(CodeTextBox, 1, 12,4);

        //Draws a new cube of 1 unit length.
        cube = new Visual();
        //Adds cube to list;
        cubeList.Add(cube);
        cube.y = position;
        position++;

    }

Answer 1

If you are looking for an extension method to clear the background color from all lines of a RichTextBox , then color a specific line, the following should suffice:

    public static void HighlightLine(this RichTextBox richTextBox, int index, Color color)
    {
        richTextBox.SelectAll();
        richTextBox.SelectionBackColor = richTextBox.BackColor;
        var lines = richTextBox.Lines;
        if (index < 0 || index >= lines.Length)
            return;
        var start = richTextBox.GetFirstCharIndexFromLine(index);  // Get the 1st char index of the appended text
        var length = lines[index].Length;
        richTextBox.Select(start, length);                 // Select from there to the end
        richTextBox.SelectionBackColor = color;
    }