基于另一个ComboBox将项添加到Windows窗体ComboBox

Question

我的Windows窗体上有2个ComboBox控件，名为comboBox1和comboBox2。 comboBox1中的项目是从表单的加载事件中的数据库加载的，当用户从comboBox1中选择一个项目并收到此错误时，我试图将项目添加到comboBox2。

  

程序或功能＆＃39; SubjectCodeQuery＆＃39;期望参数＆＃39; @ StudentId＆＃39;，这是未提供的。

这是我的代码：

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

namespace StudentAttendanceSystem
{
    public partial class AttendanceEntry : Form
    {
        private string sasdbConnectionString = @"Data Source=(LocalDB)\v11.0;Initial Catalog=sasdb;Integrated Security=True;Pooling=False";
        public AttendanceEntry()
        {
            InitializeComponent();
        }

        private void AttendanceEntry_Load(object sender, EventArgs e)
        {
            SqlConnection conn = new SqlConnection(sasdbConnectionString);
            SqlCommand cmd = new SqlCommand("SELECT StudentId FROM Students", conn);

            cmd.CommandType = CommandType.Text;

            conn.Open();

            SqlDataReader dr = cmd.ExecuteReader();

            while (dr.Read())
            {
                this.comboBox1.Items.Add(dr.GetInt32(0).ToString());
            }

            dr.Close();

            conn.Close();

            toolStripStatusLabel1.Text = "";
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            SqlConnection conn = new SqlConnection(sasdbConnectionString);
            SqlCommand cmd = new SqlCommand("SubjectCodeQuery", conn);

            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@StudentId", comboBox1.SelectedValue);

            conn.Open();

            SqlDataReader dr = cmd.ExecuteReader();

            while (dr.Read())
            {
                this.comboBox2.Items.Add(dr.GetInt32(0).ToString());
            }

            conn.Close();
        }
    }
}