I'm afraid that there isnt a C# highlighter built-in to the forum software (the VB one is specially made, and causes hassles during upgrades!).
Most people just post in normal Code tags, however if you use FireFox you can use the wonderful VBForumExtension (linked from post #2 here), which includes its own highlighters for several languages including C#.
This is a sample of the output (code taken from a random post):
Code:
private void Button1_Click(object sender, System.EventArgs e)
{MailMessage mail = new MailMessage();
string strto;
strto=TextBox1.Text;
string strfrom;
strfrom = TextBox2.Text;
string strsubject;
strsubject = TextBox3.Text;
string strbody;
strbody = TextBox3.Text;
mail.From=strfrom;
mail.To=strto;
mail.Subject=strsubject;
mail.Body=strbody;
//mail.Cc=strcc;
mail.BodyFormat=MailFormat.Html;
SmtpMail.Send(mail);
}