Autogenerate ID Concept

Button Click Event   

public void AutogenId()

    {

        Int32 ap = 5001;

        try

        {

            cm = new SqlCommand("Select max(roll)+1 from Table1", cn);

            ap = (Int32)cm.ExecuteScalar();

            Label1.Text = ap.ToString();

        }

        catch (Exception)

        {

            Label1.Text = ap.ToString();

        }