Select text from a Richtextbox in C#

Question

I want to select the text that is between the last '{' and '}' of a richtextbox text. I have the next code, but I have an error on the "LastIndexOf" function and I don't know how to fix it. Can someone give me some help?

    private void highlightText()
    {
        mRtbxOperations.SelectionStart = mRtbxOperations.Text.LastIndexOf(@"{", 1, mRtbxOperations.SelectionStart);
        mRtbxOperations.SelectionLength = mRtbxOperations.Text.IndexOf(@"}", mRtbxOperations.SelectionStart, mRtbxOperations.Text.Length - 1);
        mRtbxOperations.SelectionBackColor = Color.LightBlue;
        mRtbxOperations.SelectionFont = new Font(mRtbxOperations.SelectionFont, FontStyle.Underline);
        mRtbxOperations.SelectionLength = 0;
    }

LastIndexOf Error:

The count must be positive and must refer to a location within the string, array or collection. Parameter name: count

Answer 1

You LastIndexOf parameters are messed up, as well as the Length of the selection, where you need to substract the starting point in order to get the proper length.

Try a simpler version:

int textStart = mRtbxOperations.Text.LastIndexOf(@"{",
                                                 mRtbxOperations.SelectionStart);
if (textStart > -1) {
  int textEnd = mRtbxOperations.Text.IndexOf(@"}", textStart);
  if (textEnd > -1) {
    mRtbxOperations.Select(textStart, textEnd - textStart + 1);
    mRtbxOperations.SelectionBackColor = Color.LightBlue;
  }
}

Answer 2

Seems that you're getting out of the text bounds. When you are getting a substring or an index, you always should use the string bounds, or a substring bounds. Also, you need to check that the selection is valid.

I would rewrite your code as follows:

    private void highlightText()
    {
        Selection selection = GetSelection(mRtbxOperations.Text);
        if (selection == null)
            return;
        mRtbxOperations.SelectionStart = selection.Start;
        mRtbxOperations.SelectionLength = selection.Length;
        mRtbxOperations.SelectionBackColor = Color.LightBlue;
        mRtbxOperations.SelectionFont = new Font(mRtbxOperations.SelectionFont,   
            FontStyle.Underline);
    }

    private static Selection GetSelection(string text)
    {
        int sIndex = text.LastIndexOf(@"{");
        if (sIndex == -1)
            return null;
        int eIndex = text.IndexOf(@"}", sIndex);
        if (eIndex == -1)
            return null;

        return new Selection(sIndex + 1, eIndex);
    }

    public class Selection
    {
        public int Start { get; set; }
        public int End { get; set; }

        public int Length
        {
            get
            {
                return End - Start;
            }
        }

        public Selection(int startIndex, int endIndex)
        {
            this.Start = startIndex;
            this.End = endIndex;
        }
    }