Preventing duplicate User Names with ASP.NET

I have a registration form.I want to prevent duplicate User Names with ASP.NET.Now,I can do to prevent duplicate User Names.But,I want to give a warning message to User.How can I do that?

Code:

try{

SqlCommand cmd = sqlConnection.command("If not exists (Select user_name from users Where user_name=@userName) Insert into users(userId,user_name,user_su开发者_JAVA百科rname,email) Values (@userID,@userName,@userSurname,@email)");

        SqlParameter p1 = new SqlParameter("@userID", Session["userID"].ToString());
        SqlParameter p2 = new SqlParameter("@userName", txt_userName.Text);
        SqlParameter p3 = new SqlParameter("@userSurname", txt_surName.Text);
        SqlParameter p4 = new SqlParameter("@email", txt_email.Text);

        cmd.Parameters.Add(p1);
        cmd.Parameters.Add(p2);
        cmd.Parameters.Add(p3);
        cmd.Parameters.Add(p4);
        cmd.ExecuteNonQuery();

        myConnect.close();
}
catch (Exception ex)
    {          
    Response.Write("<script>alert("+ex.Message+ ")</script>");

    }
 }

ExecuteNonQuery returns number of rows affected by command. In your case it will be either 0(not inserted) or 1(inserted).

int res = cmd.ExecuteNonQuery();
if (1 != res)
{
    //show warning
}

Seems like your storing user accounts in a database on you own. Why don't you use the ASP.NET Membership framework? you can configure various settings in the membership framework including Unique Email Addresses. I don't remember exactly right now whether it supports unique usernames too or not, but it'd be a better idea to use the framework overall.

ASP.NET 2.0 Membership Framework