c_sharp_lib/RichTextBoxExtensions.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Extensions
{
	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();
			}
		}
	}
}