this.listBox1.Items.Add("張曉東");

//檢查添加值是否添加過
if(this.listBox1.items.Contains("張曉東")){
  MessageBox.show("集合成員已添加過！");  
}
else{
   //執(zhí)行添加集合成員
}

//判斷所有選中項集合大于0
if(this.listBox1.SelectedItems.Count > 0){
   //獲取選中的值
   this.listBox1.SelectedItem.ToString(); 
}
else{
  MessageBox.Show("未選中l(wèi)istbox集合的值"); 
}

//移除listBox集合的項
this.listBox1.Items.Remove("張曉東");

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 WindowsForms
{
  public partial class Form3 : Form
  {
    public Form3()
    {
      InitializeComponent();
    }

    /// <summary>
    /// 添加人員到采購部門
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void btnInsert_Click(object sender, EventArgs e)
    {
      //獲取添加人的值
      string peopleText = this.txtPeople.Text.Trim().ToString();
      //獲取listbox1的對象
      ListBox list1 = this.listBox1;
      //判斷人員是否已經(jīng)添加過
      if (!list1.Items.Contains(peopleText))
      {
        list1.Items.Add(peopleText);
      }
      else {
        MessageBox.Show("該人員已經(jīng)添加過，無法重復(fù)添加！");
      }
    }

    /// <summary>
    /// 將采購人員轉(zhuǎn)移到銷售部門
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void btnRightMove_Click(object sender, EventArgs e)
    {
      //獲取listbox1的所有選中的項
      if (this.listBox1.SelectedItems.Count > 0)
      {
        string checkPeople = this.listBox1.SelectedItem.ToString();
        //判斷是否添加到listbox2
        if (!this.listBox2.Items.Contains(checkPeople)) {
          //添加人員到listbox2中
          this.listBox2.Items.Add(checkPeople);
          //移除listbox1中
          this.listBox1.Items.Remove(checkPeople);
        }
        else
        {
          MessageBox.Show("該人員已經(jīng)轉(zhuǎn)移過，無法重復(fù)轉(zhuǎn)移！");
        }

      }
      else {
        MessageBox.Show("未選中采購人員，無法轉(zhuǎn)移銷售部門！");
      }
    }

    /// <summary>
    /// 將銷售人員轉(zhuǎn)移到采購部門
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void btnLeftMove_Click(object sender, EventArgs e)
    {
      //獲取listbox2的所有選中的項
      if (this.listBox2.SelectedItems.Count > 0)
      {
        string checkPeople = this.listBox2.SelectedItem.ToString();
        //判斷是否添加到listbox1
        if (!this.listBox1.Items.Contains(checkPeople))
        {
          //添加人員到listbox1中
          this.listBox1.Items.Add(checkPeople);
          //移除listbox1中
          this.listBox2.Items.Remove(checkPeople);
        }
        else
        {
          MessageBox.Show("該人員已經(jīng)轉(zhuǎn)移過，無法重復(fù)轉(zhuǎn)移！");
        }

      }
      else
      {
        MessageBox.Show("未選中銷售人員，無法轉(zhuǎn)移到采購部門！");
      }
    }
  }
}