GridView翻頁時保持CheckBox選擇狀態

//定義翻頁bool 1private bool changed=false;//定義保存選擇項主鍵集合屬性
protected List<string> SelectedItems
    ...{
      get ...{ return ViewState["selecteditems"]!=null ? (List<string>)ViewState["selecteditems"] : null; }
       set ...{ ViewState["selecteditems"] = value; }
   }
//獲取選擇項主鍵
private void GetSelectedItem()
...{
 List<string> selecteditems = null;
 if (this.SelectedItems == null)
 ...{
    selecteditems = new List<string>();
 }
 else
 ...{
   selecteditems = this.SelectedItems;
 }

//獲取選擇的記錄
     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事件中執行GetSelectedItem()
protected void gvwResult_DataBinding(object sender, EventArgs e)
    ...{
        GetSelectedItem();
    changed=true;
    }
//在行綁定時確定CheckBox選中狀態
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;
        }
    }
//根據changed確定是否再執行GetSelectedItem(),否則不翻頁時選擇項獲取不到.
protected void btnConfirm_Click(object sender, EventArgs e)
    ...{
      if(!changed)
        GetSelectedItem();
      foreach (string id in (List<string>)this.SelectedItems)
        ...{
           //...
        }
     }