string sqlselect = "select count(*) from tab where name='"+TextBox1.Text + "'";
            string sqlinsert = "insert into tab (name)values('"+TextBox1.Text+"')";
            SqlConnection  con = new SqlConnection(connstr);
            SqlCommand cmd = new SqlCommand(sqlselect, con);
            con.Open();
            if ((int)cmd.ExecuteScalar() > 0)
            {
                con.Close();
                Label1.Text ="用户名已经存在";
            }
            else
            {
                SqlCommand cmd1 = new SqlCommand(sqlinsert, con);
                cmd1.ExecuteNonQuery();
                con.Close();
                Label1.Text = "用户名添加成功";
            }