Question: How can I run and show an exe file (e.g. Telnet) in rich text file in C#? I wanted the rich text file to show what the exe file is showing.

Answer: 
Especially regarding to telnet you have two possibilities:

1. Due to the fact, that telnet is nothing more than sending text over a tcp session you can make your own tcp connection and use it or take some [helper class](https://stackoverflow.com/questions/390188/c-telnet-library).
2. The telnet.exe (command line version) uses the standard input and outputs to send and receive data. So you can start the process by the [Process class](http://msdn.microsoft.com/en-us/library/system.diagnostics.process.aspx) and use a [startInfo](http://msdn.microsoft.com/en-us/library/system.diagnostics.processstartinfo.aspx) to redirect them somewhere into your own app. Here you can take this stream, convert it into a string and put it with any formatting you like into a RichTextBox.
You will have to start the exe using [System.Diagnostics.Process](http://msdn.microsoft.com/en-us/library/system.diagnostics.process.aspx), and then redirect stdout to your rich text file.

[See this article on CodeProject](http://www.codeproject.com/KB/cs/ProcessStartDemo.aspx) for a good example.