C#字符串截取datalist用Substring(0,8)

<asp:datalist id="DL_commend" runat="server" cssclass="dl_css" repeatdirection="Horizontal">
                    <itemtemplate>
                        <div class="menu_pic"><img src="uploadfiles/<%#Convert.ToString(DataBinder.Eval(Container.DataItem,"menu_pic")).Substring(0,8)%>/<%#DataBinder.Eval(Container.DataItem,"menu_pic")%>" /><br /><%#DataBinder.Eval(Container.DataItem,"menu") %></div>