Combobox trong lập trình C# Winform

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 ComboboxGUI
{
    public partial class Form1 : Form
    {
        List<Food> listItem;
        List<CBClass> ListClass;
        public Form1()
        {
            InitializeComponent();

            listItem = new List<Food>() 
            { 
                new Food(){Name = "Mực một nắng nướng sa tế", Price = 200000}, 
                new Food(){Name = "Dú dê nướng sữa", Price = 75000},
                new Food(){Name = "Ếch núp lùm", Price = 50000}
            };
            comboBox1.DataSource = listItem;
            comboBox1.DisplayMember = "Name";

            AddBinding();


            ListClass = new List<CBClass>();
            ListClass.Add(new CBClass()
            {
                ClassName = "12CK1",
                ListStudent = new List<string>() { "K9", "Kosak"}
            });
            ListClass.Add(new CBClass()
            {
                ClassName = "12CK5",
                ListStudent = new List<string>() { "DG" }
            });

            cbBranch.DataSource = ListClass;
            cbBranch.DisplayMember = "ClassName";

            //AddClassBinding();
        }        
        void AddClassBinding()
        {
            cbClass.DataBindings.Add("DataSource", cbBranch.SelectedItem, "ListStudent", true, DataSourceUpdateMode.OnPropertyChanged);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            
        }

        void AddBinding()
        {
            textBox1.DataBindings.Add(new Binding("Text", comboBox1.DataSource, "Price"));
        }

        private void comboBox1_SelectedValueChanged(object sender, EventArgs e)
        {
            //ComboBox cb = sender as ComboBox;

            //if (cb.SelectedValue != null)
            //{
            //    Food food = cb.SelectedValue as Food;
            //    textBox1.Text = food.Price.ToString();
            //}
        }

        private void cbBranch_SelectedValueChanged(object sender, EventArgs e)
        {
            ComboBox cb = sender as ComboBox;

            if (cb.SelectedValue != null)
            {
                CBClass cl = cb.SelectedValue as CBClass;
                cbClass.DataSource = cl.ListStudent;
            }
        }
    }

    public class Food
    {
        public string Name { get; set; }
        public float Price { get; set; }
    }

    public class CBClass
    {
        public string ClassName { get; set; }
        public List<string> ListStudent { get; set; }
    }
}
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namespace ComboboxGUI
{
    partial class Form1
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.comboBox1 = new System.Windows.Forms.ComboBox();
            this.button1 = new System.Windows.Forms.Button();
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.cbBranch = new System.Windows.Forms.ComboBox();
            this.cbClass = new System.Windows.Forms.ComboBox();
            this.SuspendLayout();
            // 
            // comboBox1
            // 
            this.comboBox1.AutoCompleteMode = System.Windows.Forms.AutoCompleteMode.SuggestAppend;
            this.comboBox1.AutoCompleteSource = System.Windows.Forms.AutoCompleteSource.ListItems;
            this.comboBox1.FormattingEnabled = true;
            this.comboBox1.IntegralHeight = false;
            this.comboBox1.Location = new System.Drawing.Point(12, 41);
            this.comboBox1.Name = "comboBox1";
            this.comboBox1.Size = new System.Drawing.Size(121, 21);
            this.comboBox1.TabIndex = 0;
            this.comboBox1.SelectedValueChanged += new System.EventHandler(this.comboBox1_SelectedValueChanged);
            // 
            // button1
            // 
            this.button1.Location = new System.Drawing.Point(12, 12);
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(93, 23);
            this.button1.TabIndex = 1;
            this.button1.Text = "load list items";
            this.button1.UseVisualStyleBackColor = true;
            this.button1.Click += new System.EventHandler(this.button1_Click);
            // 
            // textBox1
            // 
            this.textBox1.Location = new System.Drawing.Point(152, 42);
            this.textBox1.Name = "textBox1";
            this.textBox1.Size = new System.Drawing.Size(100, 20);
            this.textBox1.TabIndex = 3;
            // 
            // cbBranch
            // 
            this.cbBranch.FormattingEnabled = true;
            this.cbBranch.Location = new System.Drawing.Point(12, 126);
            this.cbBranch.Name = "cbBranch";
            this.cbBranch.Size = new System.Drawing.Size(121, 21);
            this.cbBranch.TabIndex = 4;
            this.cbBranch.SelectedValueChanged += new System.EventHandler(this.cbBranch_SelectedValueChanged);
            // 
            // cbClass
            // 
            this.cbClass.FormattingEnabled = true;
            this.cbClass.Location = new System.Drawing.Point(151, 126);
            this.cbClass.Name = "cbClass";
            this.cbClass.Size = new System.Drawing.Size(121, 21);
            this.cbClass.TabIndex = 5;
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(284, 261);
            this.Controls.Add(this.cbClass);
            this.Controls.Add(this.cbBranch);
            this.Controls.Add(this.textBox1);
            this.Controls.Add(this.button1);
            this.Controls.Add(this.comboBox1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        private System.Windows.Forms.ComboBox comboBox1;
        private System.Windows.Forms.Button button1;
        private System.Windows.Forms.TextBox textBox1;
        private System.Windows.Forms.ComboBox cbBranch;
        private System.Windows.Forms.ComboBox cbClass;
    }
}

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