[c#] Color different parts of a RichTextBox string

I'm trying to color parts of a string to be appended to a RichTextBox. I have a string built from different strings.

string temp = "[" + DateTime.Now.ToShortTimeString() + "] " +
              userid + " " + message + Environment.NewLine;

This is what the message would look like once it is constructed.

[9:23pm] User: my message here.

I want everything within and including the brackets [9:23] to be one color, 'user' to be another color and the message to be another color. Then I'd like the string appended to my RichTextBox.

How can I accomplish this?

private void Log(string s , Color? c = null)
        {
            richTextBox.SelectionStart = richTextBox.TextLength;
            richTextBox.SelectionLength = 0;
            richTextBox.SelectionColor = c ?? Color.Black;
            richTextBox.AppendText((richTextBox.Lines.Count() == 0 ? "" : Environment.NewLine) + DateTime.Now + "\t" + s);
            richTextBox.SelectionColor = Color.Black;

        }

I think modifying a "selected text" in a RichTextBox isn't the right way to add colored text. So here a method to add a "color block" :

        Run run = new Run("This is my text");
        run.Foreground = new SolidColorBrush(Colors.Red); // My Color
        Paragraph paragraph = new Paragraph(run);
        MyRichTextBlock.Document.Blocks.Add(paragraph);

From MSDN :

The Blocks property is the content property of RichTextBox. It is a collection of Paragraph elements. Content in each Paragraph element can contain the following elements:

• Inline

• InlineUIContainer

• Run

• Span

• Bold

• Hyperlink

• Italic

• Underline

• LineBreak

So I think you have to split your string depending on parts color, and create as many Run objects as needed.

I have expanded the method with font as a parameter:

public static void AppendText(this RichTextBox box, string text, Color color, Font font)
{
    box.SelectionStart = box.TextLength;
    box.SelectionLength = 0;

    box.SelectionColor = color;
    box.SelectionFont = font;
    box.AppendText(text);
    box.SelectionColor = box.ForeColor;
}

I created this Function after researching on the internet since I wanted to print an XML string when you select a row from a data grid view.

static void HighlightPhrase(RichTextBox box, string StartTag, string EndTag, string ControlTag, Color color1, Color color2)
{
    int pos = box.SelectionStart;
    string s = box.Text;
    for (int ix = 0; ; )
    {
        int jx = s.IndexOf(StartTag, ix, StringComparison.CurrentCultureIgnoreCase);
        if (jx < 0) break;
        int ex = s.IndexOf(EndTag, ix, StringComparison.CurrentCultureIgnoreCase);
        box.SelectionStart = jx;
        box.SelectionLength = ex - jx + 1;
        box.SelectionColor = color1;
        
        int bx = s.IndexOf(ControlTag, ix, StringComparison.CurrentCultureIgnoreCase);
        int bxtest = s.IndexOf(StartTag, (ex + 1), StringComparison.CurrentCultureIgnoreCase);
        if (bx == bxtest)
        {
            box.SelectionStart = ex + 1;
            box.SelectionLength = bx - ex + 1;
            box.SelectionColor = color2;
        }
        
        ix = ex + 1;
    }
    box.SelectionStart = pos;
    box.SelectionLength = 0;
}

and this is how you call it

   HighlightPhrase(richTextBox1, "<", ">","</", Color.Red, Color.Black);

Using Selection in WPF, aggregating from several other answers, no other code is required (except Severity enum and GetSeverityColor function)

 public void Log(string msg, Severity severity = Severity.Info)
    {
        string ts = "[" + DateTime.Now.ToString("HH:mm:ss") + "] ";
        string msg2 = ts + msg + "\n";
        richTextBox.AppendText(msg2);

        if (severity > Severity.Info)
        {
            int nlcount = msg2.ToCharArray().Count(a => a == '\n');
            int len = msg2.Length + 3 * (nlcount)+2; //newlines are longer, this formula works fine
            TextPointer myTextPointer1 = richTextBox.Document.ContentEnd.GetPositionAtOffset(-len);
            TextPointer myTextPointer2 = richTextBox.Document.ContentEnd.GetPositionAtOffset(-1);

            richTextBox.Selection.Select(myTextPointer1,myTextPointer2);
            SolidColorBrush scb = new SolidColorBrush(GetSeverityColor(severity));
            richTextBox.Selection.ApplyPropertyValue(TextElement.BackgroundProperty, scb);

        }

        richTextBox.ScrollToEnd();
    }

This is the modified version that I put in my code (I'm using .Net 4.5) but I think it should work on 4.0 too.

public void AppendText(string text, Color color, bool addNewLine = false)
{
        box.SuspendLayout();
        box.SelectionColor = color;
        box.AppendText(addNewLine
            ? $"{text}{Environment.NewLine}"
            : text);
        box.ScrollToCaret();
        box.ResumeLayout();
}

Differences with original one:

• possibility to add text to a new line or simply append it
• no need to change selection, it works the same
• inserted ScrollToCaret to force autoscroll
• added suspend/resume layout calls

Selecting text as said from somebody, may the selection appear momentarily. In Windows Forms applications there is no other solutions for the problem, but today I found a bad, working, way to solve: you can put a PictureBox in overlapping to the RichtextBox with the screenshot of if, during the selection and the changing color or font, making it after reappear all, when the operation is complete.

Code is here...

//The PictureBox has to be invisible before this, at creation
//tb variable is your RichTextBox
//inputPreview variable is your PictureBox
using (Graphics g = inputPreview.CreateGraphics())
{
    Point loc = tb.PointToScreen(new Point(0, 0));
    g.CopyFromScreen(loc, loc, tb.Size);
    Point pt = tb.GetPositionFromCharIndex(tb.TextLength);
    g.FillRectangle(new SolidBrush(Color.Red), new Rectangle(pt.X, 0, 100, tb.Height));
}
inputPreview.Invalidate();
inputPreview.Show();
//Your code here (example: tb.Select(...); tb.SelectionColor = ...;)
inputPreview.Hide();

Better is to use WPF; this solution isn't perfect, but for Winform it works.

It`s work for me! I hope it will be useful to you!

public static RichTextBox RichTextBoxChangeWordColor(ref RichTextBox rtb, string startWord, string endWord, Color color)
{
    rtb.SuspendLayout();
    Point scroll = rtb.AutoScrollOffset;
    int slct = rtb.SelectionIndent;
    int ss = rtb.SelectionStart;
    List<Point> ls = GetAllWordsIndecesBetween(rtb.Text, startWord, endWord, true);
    foreach (var item in ls)
    {
        rtb.SelectionStart = item.X;
        rtb.SelectionLength = item.Y - item.X;
        rtb.SelectionColor = color;
    }
    rtb.SelectionStart = ss;
    rtb.SelectionIndent = slct;
    rtb.AutoScrollOffset = scroll;
    rtb.ResumeLayout(true);
    return rtb;
}

public static List<Point> GetAllWordsIndecesBetween(string intoText, string fromThis, string toThis,bool withSigns = true)
{
    List<Point> result = new List<Point>();
    Stack<int> stack = new Stack<int>();
    bool start = false;
    for (int i = 0; i < intoText.Length; i++)
    {
        string ssubstr = intoText.Substring(i);
        if (ssubstr.StartsWith(fromThis) && ((fromThis == toThis && !start) || !ssubstr.StartsWith(toThis)))
        {
            if (!withSigns) i += fromThis.Length;
            start = true;
            stack.Push(i);
        }
        else if (ssubstr.StartsWith(toThis) )
        {
            if (withSigns) i += toThis.Length;
            start = false;
            if (stack.Count > 0)
            {
                int startindex = stack.Pop();
                result.Add(new Point(startindex,i));
            }
        }
    }
    return result;
}