判断用户重名 asp.net

conn.Open();
        string str1 = "select count (*) from tb_admin where hykh='" + TextBox1.Text.Trim() + "'";
        SqlCommand sqlcmd = new SqlCommand(str1, conn);
        int i = Convert.ToInt32(sqlcmd.ExecuteScalar());
        if (i > 0)
        {
            Response.Write("<script>javascript:alert('该用户已经存在!!!');</script>");

        }
        else
        {
            string str = "insert into tb_admin (hykh,pwd,qx) values('" + TextBox1.Text.Trim() + "','" + TextBox2.Text.Trim() + "','" + DropDownList1.Text.Trim() + "')";
            SqlCommand cmd = new SqlCommand(str, conn);
            cmd.Dispose();
            try
            {
                Session["hykh"] = TextBox1.Text;
                cmd.ExecuteNonQuery();
                Response.Write("<script>javascript:alert('添加成功!!!');</script>");
            }
            catch (SqlException)
            {
                Response.Write("<script>javascript:alert('添加失败!!!');</script>");

            }
            finally
            {
                conn.Close();
                str = "";
            }