How to handle sql exception in C#?

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C#

SQL-Server

I am using catch block with SqlException, but I want to show custom messages rather than exception itself. So how do I filter out the messages. Like say I want to catch a Unique Key Constraint Violation. Is there some error code which i can use to determine the error.

protected void Button1_Click(object sender, EventArgs e)
       {
           try
           {

               objuser.username = txt_email.Text;
               objuser.email = txt_username.Text;
               objuser.password = txt_password.Text;
               int i = BusinessUser.BusinessRegisterUser(objuser);
               if (i > 0)
               {
                   Session["user_authenticate"] = "Verified";
                   Session["user_email"] = objuser.email; //  sql exception handle for uniqe key
                   Response.Redirect("user_registration.aspx");
               }
           }
           catch (SqlException ex)
           {
             // Handle Exception here if e-mail already exists
           }
           catch (Exception ex)
           {

           }

       }