c_sharp_lib/1/RichTextBoxExtensions.cs

using System.Drawing;
using System.Windows.Forms;

namespace UTIL
{
    #region Расширенные методы RichTextBox
    public static class RichTextBoxExtensions
    {
        public static void AppendText(this RichTextBox box, string text, Color color)
        {
            if (box.InvokeRequired)
            {
                box.Invoke((MethodInvoker)(delegate
                {
                    box.SelectionStart = box.TextLength;
                    box.SelectionLength = 0;
                    box.SelectionColor = color;
                    box.AppendText(text);
                    box.SelectionColor = box.ForeColor;
                    box.ScrollToCaret();
                }));
            }
            else
            {
                box.SelectionStart = box.TextLength;
                box.SelectionLength = 0;
                box.SelectionColor = color;
                box.AppendText(text);
                box.SelectionColor = box.ForeColor;
                box.ScrollToCaret();
            }
        }
        public static void AppendText(this RichTextBox box, string text, Color bgcolor, Color fgcolor)
        {
            if (box.InvokeRequired)
            {
                box.Invoke((MethodInvoker)(delegate
                {
                    box.SelectionStart = box.TextLength;
                    box.SelectionLength = 0;
                    box.SelectionColor = fgcolor;
                    box.SelectionBackColor = bgcolor;
                    box.AppendText(text);
                    box.SelectionColor = box.ForeColor;
                    box.SelectionBackColor = box.BackColor;
                    box.ScrollToCaret();
                }));
            }
            else
            {
                box.SelectionStart = box.TextLength;
                box.SelectionLength = 0;
                box.SelectionColor = fgcolor;
                box.SelectionBackColor = bgcolor;
                box.AppendText(text);
                box.SelectionColor = box.ForeColor;
                box.SelectionBackColor = box.BackColor;
                box.ScrollToCaret();
            }
        }
    }
    #endregion
}