C＃：System.InvalidOperationException错误

Question

我正在构建一个桌面应用程序，其中场景是用户将登录，而另一种形式，它的ID将显示在文本框中。

但是在用户登录后我看到了这样的错误：

这是我的第一个表格（Form1.cs）

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;


namespace EmployeeApp
{
    public partial class login : Form
    {
        public login()
        {
            InitializeComponent();
        }

        private string employeeID;
        private void exitButton_Click(object sender, EventArgs e)
        {
            this.Close();
        }
        private void loginButton_Click(object sender, EventArgs e)
        {
            SqlConnection connection = new SqlConnection(@"Data Source=INCEPSYS-SE\TEST;Initial Catalog=Employee;Integrated Security=True");

            SqlCommand command = new SqlCommand("select * from Employees where Name = '" + nameTextBox.Text + " ' and Password = '" + passwordTextBox.Text + "'", connection);
            SqlDataReader myReader = command.ExecuteReader();
            while (myReader.Read())
            {
                string employeeID = myReader["EmployeeID"].ToString();
            }


            SqlDataAdapter sda = new SqlDataAdapter("select count(*) from Employees where Name = '" + nameTextBox.Text + " ' and Password = '" + passwordTextBox.Text + "'", connection);

            DataTable dt = new DataTable();
            sda.Fill(dt);
            if (dt.Rows[0][0].ToString() == "1")
            {
                this.Hide();
                Entry ss = new Entry(employeeID);
                ss.Show();
            }
            else
            {
                MessageBox.Show("Please Check your Username & password");
            }
        }

    }
}

这是我的第二个表单（Entry.cs），我想打印用户ID：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace EmployeeApp
{
    public partial class Entry : Form
    {      public Entry(string employeeId)
        {
            InitializeComponent();
              idTextBox.Text = employeeId;
        }


       private void reportButton_Click(object sender, EventArgs e)
        {
            Report report = new Report();
            report.Show();
        }
    }
}

如果有人发现问题，请帮我找到它！

Answer 1

首先需要打开sql连接实例以对sql server进行查询。

更改您的代码：

SqlConnection connection = new SqlConnection(@"Data Source=INCEPSYS-SE\TEST;Initial Catalog=Employee;Integrated Security=True");

connection.Open();

SqlCommand command = new SqlCommand("select * from Employees where Name = '" + nameTextBox.Text + " ' and Password = '" + passwordTextBox.Text + "'", connection);

//...