关于DataGrid中DataKeyField的定位问题，C#语言

我的页面代码如下，我要如何写TitleList_ItemCommand事件才能获取DataKeyDield的值来进行修改和删除呢？？？？？<asp:DataGrid ID="TitleList" runat="server" CssClass="GbText" AutoGenerateColumns="False" CellPadding="4" ForeColor="White" GridLines="None" Width="100%" OnItemCommand="TitleList_ItemCommand" OnItemDataBound="TitleList_ItemDataBound" DataKeyField="ID_">
<Columns>
<asp:TemplateColumn HeaderText="操作" HeaderStyle-HorizontalAlign="Center" ItemStyle-HorizontalAlign="Center" ItemStyle-Width="17%" ItemStyle-BorderWidth="1" ItemStyle-BorderColor="Blue"><ItemTemplate>
<asp:Button ID="UpdateTitle" Runat="server" CommandName="update" CssClass="ButtonCss" Text="修改" Width="30" CommandArgument='<%# DataBinder.Eval(Container.DataItem, "ID_")%>'></asp:Button>
<asp:Button ID="DeleteTitle" Runat="server" CommandName="delete" CssClass="ButtonCss" Text="删除" Width="30" CommandArgument='<%# DataBinder.Eval(Container.DataItem, "ID_")%>'></asp:Button>
</ItemTemplate></asp:TemplateColumn>
</Columns>
</asp:DataGrid>
我的后台代码如下：    protected Guid nPID_ = Guid.NewGuid();
    public Guid nID_ = Guid.NewGuid();
    protected void Page_Load(object sender, EventArgs e)
    {if (Request.Params["PID_"] != null)
        {nPID_ = new Guid(Request.Params["PID_"].ToString());}
        if (!Page.IsPostBack)
        {
            BindTitleData(nPID_);}}
    public void BindTitleData(Guid nPID_)
    {Title title = new Title();
     SqlDataReader rect = title.GetTitleByClass(nPID_);
     if (rect == null) return;
     TitleList.DataSource = rect;
     TitleList.DataBind();
     rect.Close();
    }
    protected void TitleList_ItemCommand(object source, DataGridCommandEventArgs e)
    {string nnID_ = this.TitleList.DataKeyField.ToString();///......这一句要如何写？
        if (e.CommandName.ToLower() == "update")
        {   
            this.Response.Redirect("UpdateKulturTitle.aspx?ID_="+nnID_);
        }
        else
        {
            if (e.CommandName.ToLower() == "delete")
            {
                nID_ = new Guid(nnID_);///.......这一句要如何写呢？
                DeleteTitle(nID_);///这一句的代码我已经知道怎么写，只是在后面没有加上来。
            }
            else
            {
                Response.Write("<script>window.alert('数据错误')</script>");
                return;
            }
        }
    }
    protected void DeleteTitle(Guid nID_)
    {
    }
    protected void TitleList_ItemDataBound(object sender, DataGridItemEventArgs e)
    {
        Button updateButton = (Button)e.Item.FindControl("UpdateTitle");
        Button deleteButton = (Button)e.Item.FindControl("DeleteTitle");
        if (deleteButton != null)
        {deleteButton.Attributes.Add("onclick", "return confirm(;" + ASPNET2System.OPERATIONDELETEMESSAGE + "');");
        }
    }
}