how do I catch empty/ null String exception in C #?

Empty input is not an exception. It is a matter of input validation. I suppose your input field is empty by default, but you want to validate the form on a specific event. Let's suppose on a button click.
You have several ways to do that, but start with these articles:
http://www.devx.com/dotnet/Article/33506[^]
Validate user input in Windows Forms[^]
Validators for Windows Forms - ValidationProvider Control[^]

Hi you can use following condition to check if text box is empty,

if(String.IsNullOrEmpty(txtYourTextbox.Text))
{
//if textbox is empty, you can show proper message using MessageBox
MessageBox.Show("Please enter value in text box", "TextboxLabel is Required", MessageBoxButtons.OK, MessageBoxIcon.Information);
}


Hope this solves your problem.

Thanks,
Sanjeev Choudhary