在Repeater中设置HtmlButton属性抛出NullReferenceException

Question

下面是Repeater控件的ItemDataBound事件中的代码：

HtmlButton btnLike = (HtmlButton)e.Item.FindControl("LikeButton");
var itemLiked = new LikesService().GetByPostIdUserId(pollQ.PostId, new Guid(Membership.GetUser(this.Context.User.Identity.Name).ProviderUserKey.ToString()));
if (itemLiked != null)
    btnLike.Attributes["class"] = "btn btn-primary btn-sm";

我调试了代码，但找不到这一行抛出NullReferenceException的原因：

btnLike.Attributes["class"] = "btn btn-primary btn-sm";

我在调试器中验证了对象itemLiked不是null。

下面是Repeater中HTML按钮的标记：

<button type="button" id="LikeButton" value='<%#Eval("PollQuestionID") %>' class='<%#GetCSS(Eval("PollQuestionID")) %>'>
    <span class="glyphicon glyphicon-thumbs-up"></span> <%#LikeText(Eval("PollQuestionID")) %>
</button>

Answer 1

FindControl方法只能找到具有runat="server"属性的服务器控件。将此属性添加到<button>中

<button runat="server" type="button" id="LikeButton" ...