程序代码：

  private void button1_Click(object sender, EventArgs e)
        {
            SqlConnection scon = new SqlConnection("server=GATEWAY-DING;database=njdl;uid=sa;pwd=sa");
            scon.Open();
            DataSet ds = new DataSet();
            System.Data.SqlClient.SqlDataAdapter da = new System.Data.SqlClient.SqlDataAdapter(
                            "select name,password from nj_user ", scon);
             da.Fill(ds);

            if (textBox1.Text.Trim() == "")
            {
                MessageBox.Show(null, "用户名不能为空", "登陆失败", MessageBoxButtons.OK, MessageBoxIcon.Question);
                return;
            }

            else if (textBox1.Text.Trim() == ds.Tables[0].Rows[0][0].ToString() && textBox2.Text.Trim() == ds.Tables[0].Rows[0][1].ToString())
            {

                this.DialogResult = DialogResult.OK;
            }
            else
            {
                MessageBox.Show("用户名或密码不正确");
            }
        }

程序代码：

   if (textBox1.Text.Trim() == "" || textBox2.Text.Trim() == "")
            {
                MessageBox.Show(null, "用户名和密码不能为空", "登陆失败", MessageBoxButtons.OK, MessageBoxIcon.Question);
                return;
            }

            SqlConnection scon = new SqlConnection("server=GATEWAY-DING;database=njdl;uid=sa;pwd=sa");
            DataSet ds = new DataSet();
            string sql = "select count(*) as count from nj_user where name='" + textBox1.Text + "' and password='" + textBox2.Text + "'";

            SqlDataAdapter da = new SqlDataAdapter(sql, scon);
            da.Fill(ds);

            string fieldvalue = ds.Tables[0].Rows[0]["count"].ToString();
            int count = Convert.ToInt32(fieldvalue);
            if (count != 0)
            {
                this.DialogResult = DialogResult.OK;
            }
            else
            {
                MessageBox.Show("用户名或密码不正确");
            }