这是微软的一个Bug,可以去官网下载一个Validators.dll,引用到你的项目下,然后在Web.Config里configuration/system.web/pages节点中添加（或修改）如下的元素:
<pages>
<tagMapping>
<add tagType="System.Web.UI.WebControls.CompareValidator"
mappedTagType="Microsoft.Web.UI.Compatibility.CompareValidator,
Validators, Version=1.0.0.0"/>
<add tagType="System.Web.UI.WebControls.CustomValidator"
mappedTagType="Microsoft.Web.UI.Compatibility.CustomValidator,
Validators, Version=1.0.0.0"/>
<add tagType="System.Web.UI.WebControls.RangeValidator"
mappedTagType="Microsoft.Web.UI.Compatibility.RangeValidator,
Validators, Version=1.0.0.0"/>
<add
tagType="System.Web.UI.WebControls.RegularExpressionValidator"
mappedTagType="Microsoft.Web.UI.Compatibility.RegularExpressionValidator,
Validators, Version=1.0.0.0"/>
<add
tagType="System.Web.UI.WebControls.RequiredFieldValidator"
mappedTagType="Microsoft.Web.UI.Compatibility.RequiredFieldValidator,
Validators, Version=1.0.0.0"/>
<add tagType="System.Web.UI.WebControls.ValidationSummary"
mappedTagType="Microsoft.Web.UI.Compatibility.ValidationSummary,
Validators, Version=1.0.0.0"/>
</tagMapping>
</pages>
</system.web>
</configuration>
重新编译，生成
