Don't allow a specific use of characters

Question

I bumped into a problem, i hope someone can help me out :)
I got a TextBox, and i want to limit users, so that they can\'t write multiple \\ one after another.<

Answer 1

If the textbox contains \\, it is invalid:

if (textBox1.Text.Contains(@"\\"))
{
     MessageBox.Show("Error!");
}