namespace Hotel
{
    public partial class LoginForm : Form
    {
        public LoginForm()
        {
            InitializeComponent();
        }

        String strConn = string.Empty;
        private void LoginForm_Load(object sender, EventArgs e)
        {
            strConn = "server=.;database=HotelManagement;uid=sa;pwd=123;";
        }

        private void btnOK_Click(object sender, EventArgs e)
        {
            SqlConnection mySqlConnection = new SqlConnection();
            mySqlConnection.ConnectionString = strConn;
            try
            {
                object ReturnValue = null;
                mySqlConnection.Open();
                SqlCommand mySqlCommand = new SqlCommand();
                = "select AdminPwd from Administrator where AdminID='" + textBox1.Text + "'";
                mySqlCommand.Connection = mySqlConnection;
                ReturnValue = mySqlCommand.ExecuteScalar();
                string pwd = ReturnValue.ToString();
                if (pwd == textBox2.Text)
                    MessageBox.Show("OK");
                else
                    MessageBox.Show("NO");
            }
            catch
            {
                throw;
            }
            finally
            {
                mySqlConnection.Close();
            }
        }
    }
}
求高手

求回复丫，好急得！！！真不会！

下断，单步执行看看哪一句出错。

我设置断点运行，运行到if就直接跳过了，

运行到throw就停住了

在string pwd = ReturnValue.ToString();这句下断，然后看ReturnValue的值是不是null

笔记本SQL有问题，无法运行。但是pwd处没有定义。我老师示范的时候是有的。

应该是你通过SQl查询的结果集为Null，所以mySqlCommand.ExecuteScalar()返回值ReturnValue为Null，
那你 string pwd = ReturnValue.ToString();这句将Null的ToString()方法肯定会跳错！

该怎么解决丫？