这是asp运行时错误(基类包含字段"btnsubmit"，但其类型与控件类型不兼容)

it's asp runtime error(the base class includes the field 'btnsubmit', but it's type is not compatible with the type of control)

在解决方案资源管理器"窗口中，右键单击您的解决方案，然后选择清洁解决方案"，然后重新生成解决方案

From Solution Explorer window, right click on your solution and choose Clean solution and then rebuild solution

并设置元素格式如下

<input type="button" id="btnsubmit" runat="server" class="btn btn-default" onserverclick="btnsubmit_ServerClick" value="Submit" />