如何将SQL查询结果保存在C#.net中的变量中？

请帮助我解决此错误，我无法将SQL查询结果存储到变量中？？？

仅打开连接是不够的

---

您需要将

con

与

cmd

相关联。这正是所描述的错误，您尚未设置

SQLCommand

的属性

尝试添加：

namespace Hotel
{
    public partial class Billing : Form
    {
        SqlConnection con = new SqlConnection();
        SqlDataAdapter da;
        SqlCommand cmd = new SqlCommand();
        DataTable dt = new DataTable();
        public Billing()
        {
            InitializeComponent();
        }

        private void Billing_Load(object sender, EventArgs e)
        {
            con.ConnectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename=D:\\Projects\\c# assignments\\Hotel Manager\\Hotel\\database\\master.mdf;Integrated Security=True;User Instance=True";
            //loadData();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            con.Open();
            int rno = Int32.Parse(txtRoom.Text);


            cmd.CommandText = "SELECT SUM(ItemRate) FROM logs WHERE RoomNo=" + rno +"";
            int amt = (int)cmd.ExecuteScalar();   //arror is at this part

       //ExecuteScalar: Connection property has not been initialized.

            cmd.CommandText = "INSERT INTO bill VALUES('" + txtBillNo.Text.ToString() + "','" + txtRoom.Text.ToString() + "','" + amt.ToString() + "')";
            con.Close();
            txtBillNo.Text = "";
            txtRoom.Text = "";
            BillView bv = new BillView();
            bv.ShowDialog();
        }
    }
}

---

在调用

ExecuteScalar（）

之前，您已经打开了一个SqlConnection，但还没有告诉SqlCommand对象使用它。尝试添加以下行：

cmd.Connection = con;

在执行查询之前

你是开放的。不要连接字符串以生成查询。而是使用SQL参数

用于连接（以及实现IDisposable的所有其他内容）。Dispose还将关闭连接，即使出现错误，也会使用

using

出现异常的原因是您没有初始化

SqlCommand

的连接，因为您没有指定连接。您可以使用或相应的

下面是一个例子：

cmd.Connection = con;

---

您展示的代码存在一些问题，尤其是一些seroius安全问题，我强烈建议您仔细阅读并准备好语句/参数和

只是一些快速更正/评论：

int amt;  
using (var con = new SqlConnection(ConnectionString)) {
    var sql = "SELECT SUM(ItemRate) FROM logs WHERE RoomNo = @RoomNo";
    using (var cmd = new SqlCommand(sql, con)) {
        cmd.Parameters.AddWithValue("@RoomNo", Int32.Parse(txtRoom.Text));
        con.Open();
        amt = (int)cmd.ExecuteScalar();
    }
}

---

您尚未在按钮1\u单击中提供连接字符串

namespace Hotel
{
    public partial class Billing : Form
    {
        SqlConnection con = new SqlConnection();
        SqlDataAdapter da;
        SqlCommand cmd = new SqlCommand();
        DataTable dt = new DataTable();
        public Billing()
        {
            InitializeComponent();
        }

        private void Billing_Load(object sender, EventArgs e)
        {
            con.ConnectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename=D:\\Projects\\c# assignments\\Hotel Manager\\Hotel\\database\\master.mdf;Integrated Security=True;User Instance=True";
            //loadData();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            con.ConnectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename=D:\\Projects\\c# assignments\\Hotel Manager\\Hotel\\database\\master.mdf;Integrated Security=True;User Instance=True";
            con.Open();
            int rno = Int32.Parse(txtRoom.Text);

            cmd.Connection = con; // This solves the problem you see
            // HERE you SHOULD use a SQL paramter instead of appending strings to build your SQL !!!
            cmd.CommandText = "SELECT SUM(ItemRate) FROM logs WHERE RoomNo=" + rno +"";
            int amt = (int)cmd.ExecuteScalar();   //arror is at this part


            // HERE you SHOULD use a SQL paramter instead of appending strings to build your SQL !!!
            // Another point: you build an INSERT but never execute it ?!?
            cmd.CommandText = "INSERT INTO bill VALUES('" + txtBillNo.Text.ToString() + "','" + txtRoom.Text.ToString() + "','" + amt.ToString() + "')";
            con.Close();
            txtBillNo.Text = "";
            txtRoom.Text = "";
            BillView bv = new BillView();
            bv.ShowDialog();
        }
    }
}

你的代码中也有很多错误。它是这样工作的

con.ConnectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename=D:\\Projects\\c# assignments\\Hotel Manager\\Hotel\\database\\master.mdf;Integrated Security=True;User Instance=True";

---

我认为这会很有帮助，而且更安全

当您在按钮事件中使用空con对象时，您正在另一个方法上设置连接字符串。

{
  // Create Connection Object  
  // Provide connection object with Connection string
  // Create command object
  // Open connection
  // Execute command
  // Close connection
  // Dispose connection
}

using (SqlConnection sqlcon = new SqlConnection("Connection String HERE"))
        {
            using (SqlCommand sqlcmd= new SqlCommand())
            {
                sqlcmd.Connection = sqlcon;            
                sqlcmd.CommandType = CommandType.Text;
                sqlcmd.CommandText = "SELECT SUM(ItemRate) FROM logs WHERE RoomNo=@rno";
                slqcmd.Parameters.AddWithValue("@rno", rno);
                try
                {
                    sqlcon.Open();
                    command.ExecuteNonQuery();
                }
                catch (SqlException)
                {
                    MessageBox.Show("Your Error Here");
                }
                finally
                {
                    connection.Close();
                }
            }