GridView翻页时保持CheckBox选择状态

// 定义翻页bool 1private bool changed=false; // 定义保存选择项主键集合属性
protected  List < string >  SelectedItems
     ... {
      get ...{ return ViewState["selecteditems"]!=null ? (List<string>)ViewState["selecteditems"] : null; }
       set ...{ ViewState["selecteditems"] = value; }
   }
// 获取选择项主键
private   void  GetSelectedItem()
... {
 List<string> selecteditems = null;
 if (this.SelectedItems == null)
 ...{
    selecteditems = new List<string>();
 }
 else
 ...{
   selecteditems = this.SelectedItems;
 }

//获取选择的记录
     for (int i = 0; i < this.gvwResult.Rows.Count; i++)
    ...{
           CheckBox cbx = (CheckBox)this.gvwResult.Rows[i].FindControl("chkSelect");

           string id = this.gvwResult.DataKeys[i].Values[0].ToString();
            if (selecteditems.Contains(id) && !cbx.Checked)
                selecteditems.Remove(id);
            if (!selecteditems.Contains(id) && cbx.Checked)
                selecteditems.Add(id);
        }
        this.SelectedItems = selecteditems;
}
// 在 DataBinding事件中执行GetSelectedItem()
protected   void  gvwResult_DataBinding( object  sender, EventArgs e)
     ... {
        GetSelectedItem();
    changed=true;
    }
// 在行绑定时确定CheckBox选中状态
protected   void  gvwResult_RowDataBound( object  sender, GridViewRowEventArgs e)
      ... {      
        if (e.Row.RowIndex > -1 && this.SelectedItems!= null)
         ...{           
             CheckBox cbx = (CheckBox)e.Row.FindControl("chkSelect");
            string id = this.gvwResult.DataKeys[e.Row.RowIndex].Values[0].ToString();
             if (SelectedItems.Contains(id))
                 cbx.Checked = true;
             else
                cbx.Checked = false;
        }
    }
// 根据changed确定是否再执行GetSelectedItem(),否则不翻页时选择项获取不到.
protected   void  btnConfirm_Click( object  sender, EventArgs e)
     ... {
      if(!changed)
        GetSelectedItem();
      foreach (string id in (List<string>)this.SelectedItems)
        ...{
           //...
        }
     }