ListBox传值的有关问题，进来看看吧

ListBox传值的问题，进来看看吧
 /// <summary>
        /// 添加
        /// </summary>
        protected void addBtn_Click(object sender, EventArgs e)
        {
            if (lstSelPro.SelectedItem != null)
            {
                if (lstSeledPro.Items.Count>0)
                {
                    foreach (ListItem item in lstSeledPro.Items)
                    {
                        if (item.Text != lstSelPro.SelectedItem.Text)
                        {
                            lstSeledPro.Items.Add(lstSelPro.SelectedItem);
                            lstSelPro.Items.Remove(lstSelPro.SelectedItem);
                            lstSeledPro.SelectedItem.Selected = false;
                        }
                    }
                }
                else
                {
                    lstSeledPro.Items.Add(lstSelPro.SelectedItem);
                    lstSelPro.Items.Remove(lstSelPro.SelectedItem);
                    lstSeledPro.SelectedItem.Selected = false;
                }
                

            }
        }
报错:
错误: Sys.WebForms.PageRequestManagerServerErrorException: 集合已修改；可能无法执行枚举操作。

就是从list1 通过Button传到 list2 有重复就不添加 list button null
[解决办法]
            listBox1.SelectedItems.OfType<ListItem>().ToList().ForEach(t => { listBox1.Items.Remove(t); listBox2.Items.Add(t); });

[解决办法]
//添加 
protected void Button_Mritght_Click(object sender, EventArgs e)
    {
        for (int i = 0; i < this.ListBox1.Items.Count; i++)
        {
            if ((this.ListBox1.Items[i].Selected) && (this.ListBox2.Items.FindByValue(this.ListBox1.Items[i].Value) == null))
            {
                ListItem item = this.ListBox1.Items[i];
                this.ListBox2.Items.Add(item);
            }

        }
    }
移除
    protected void Button_Mleft_Click(object sender, EventArgs e)
    {
        for (int i = 0; i < this.ListBox2.Items.Count; i++)
        {
            if (this.ListBox2.Items[i].Selected)
            {
                ListItem item = this.ListBox2.Items[i];
                this.ListBox2.Items.Remove(item);
            }

        }
    }