I have a `RichTextBox` for a simple chat where I add lines programmatically.
I make the usernames bold and the messages in regular style.
After some lines I want to delete the first lines to keep the chat in a acceptably length. But when I do so I lose the text format and everything appears bold. What am I doing wrong and how can I fix this?

**EDIT**

I could solve the problem where I wasn't able to delete the first line.
I had to set the the `ReadOnly` property to false. Even though I was able to add new lines it prevented deleting lines. So the following code works to delete lines. Thanks to @TaW!

```
if (ChatText.Lines.Length >= 10)
{
    int p = 0; int count = 0;
    do
    {
        p = ChatText.Text.IndexOf("\n\r");
        if (p >= 0)
        {
            ChatText.SelectionStart = p;
            ChatText.SelectionLength = 2; // length of "\n\r"
            ChatText.SelectedText = "\n";
            count++;
        }
    }
    while(p >= 0);

    int nll = 1;  // <<===  pick the length of your new line character(s)!!
    int pS = ChatText.Lines.Take(0).Select(x => x.Length + nll).Sum() - nll;
    int pL = ChatText.Lines.Take(1).Select(x => x.Length + nll).Sum() - nll;
    if (pS < 0) { pS = 0; pL++; }
    ChatText.SelectionStart = pS;
    ChatText.SelectionLength = pL - pS;
    ChatText.Cut();
}
//////////////////////////////////
// now add new lines
//////////////////////////////////
string[] chatstr;
// string text is given as method parameter
chatstr = text.Split(new string[] { ": " }, 2, StringSplitOptions.None);
// go to the end of the text
ChatText.SelectionStart = ChatText.Text.Length;
ChatText.SelectionLength = 0;
// make text bold
ChatText.SelectionFont = new Font(ChatText.Font, FontStyle.Bold);
// add username (chatstr[0]) and colon
ChatText.AppendText(chatstr[0] + ": ");
// make text regular
ChatText.SelectionFont = new Font(ChatText.Font, FontStyle.Regular);
// add message (chatstr[1])
ChatText.AppendText(chatstr[1] + "\n");
// and finaly scroll down
ChatText.ScrollToCaret();

```

So deleting lines works and new lines are added as intended. Finaly!

solved :)
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