How do I change the appearance of some text in a textBox?

I'm building a program similar to Messenger, sort of a chatting software.

I created a textBox to display the conversation log, and I want to make it look like this:

Nick1: Hi.

Nick2: Hello.

How do I make part of the text bold? I also want to allow users to change their own font, font-color and so on...

Thanks in advance.

Use RichTextBox if building a Windows Forms or a WPF app.

The best way is to use a RichTextEdit control, this control can take RTF format codes to easily format the text the way you want it.

eg.

\pard\plain\ltrpar\f0\fs14\sl240\slmult1\b\ Nick1.\par
\pard\plain\ltrpar\f0\fs14\sl240\slmult1\Hi

will show up as Nick1 Hi

Assuming you're using WPF I'd use an ItemsControl to display the conversion. Then for each item I'd have a couple of TextBlocks one for the user name (which would be bold) and the other for the message.

Hope that helps.