Visual c#在SQL WHERE子句中使用变量时出现异常

我目前使用c#在visual studio和我使用访问数据库。当客户从列表框中选择时，我试图从数据库中带回数据。当sql硬编码为

时，这可以完美地工作。

command.CommandText = "SELECT * FROM Customers WHERE CustomerID = 2 ";

然而，当我试图使用字符串变量来存储所选的用户ID时，我在

上收到"标准表达式中的数据类型不匹配"。

"OleDbDataReader reader = command.ExecuteReader();".

我已经使用消息框来确认s2变量在选择时包含正确的ID，因此我不确定问题所在。

有谁知道这个问题的解决方法吗?

    private void lst_disp_SelectedIndexChanged(object sender, EventArgs e)
    {
        String s = (String)lst_disp.SelectedItem; // the s string contains the selected customers ID + name,
        string s2 = s.Split(' ').FirstOrDefault(); // by spliting we can gain only the ID and store in s2
        MessageBox.Show("Selected " + s2);
        showCust(s2);
    }
    private void showCust(string s2)
    {
        dataGridView1.AllowUserToAddRows = false;
        dataGridView1.Columns.Add("CustomerID", "Customer ID");
        dataGridView1.Columns.Add("CustomerName", "Customer Name");
        dataGridView1.Columns.Add("Description", "Description");
        dataGridView1.Columns.Add("Email", "Email");
        dataGridView1.Columns.Add("Telephone", "Telephone");
        dataGridView1.Columns.Add("DeliveryAddress", "Delivery Address");
        dataGridView1.Columns.Add("Notes", "Notes");
        OleDbConnection connect = new OleDbConnection();
        connect.ConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=E:'Uni'Year 3'Final Year Project'independent foods'coding'showCustomers'Database1.accdb;Persist Security Info=False";
        connect.Open();
        MessageBox.Show("Connection open");
        OleDbCommand command = new OleDbCommand();
        command.Connection = connect;
        MessageBox.Show("SELECT * FROM Customers WHERE CustomerID = '" + s2 + "' ");
        command.CommandText = "SELECT * FROM Customers WHERE CustomerID = '" + s2 + "' ";

        try
        {
            OleDbDataReader reader = command.ExecuteReader();
            while (reader.Read())
            {
                dataGridView1.Rows[dataGridView1.Rows.Count - 1].Cells["CustomerID"].Value = reader[0].ToString();
                dataGridView1.Rows[dataGridView1.Rows.Count - 1].Cells["CustomerName"].Value = reader[1].ToString();
                dataGridView1.Rows[dataGridView1.Rows.Count - 1].Cells["Description"].Value = reader[2].ToString();
                dataGridView1.Rows[dataGridView1.Rows.Count - 1].Cells["Email"].Value = reader[3].ToString();
                dataGridView1.Rows[dataGridView1.Rows.Count - 1].Cells["Telephone"].Value = reader[4].ToString();
                dataGridView1.Rows[dataGridView1.Rows.Count - 1].Cells["DeliveryAddress"].Value = reader[5].ToString();
                dataGridView1.Rows[dataGridView1.Rows.Count - 1].Cells["Notes"].Value = reader[6].ToString();
            }
        }
        catch(Exception e)
        {
    MessageBox.Show("The File  cann't be read. Error: " + e.Message);
        }
    }

command.CommandText = "SELECT * FROM Customers WHERE CustomerID = " + s2;

SqlParameter custID = new SqlParameter("custID",s2);
command.Parameters.Add(custID);
command.CommandText = "SELECT * FROM Customers WHERE CustomerID = @custID";

SELECT * FROM Customers WHERE CustomerID = 2

SELECT * FROM Customers WHERE CustomerID = '2'

        command.CommandText = "SELECT * FROM Customers WHERE CustomerID = " + s2;