ASP.NET验证多个FileUploads

Question

我希望用户使用FileUpload上传4张照片，如果其中任何一张为空，我想显示错误消息。如何验证所有这些都已正确填充？

<div id="upload-area">
    <div id="photo1">
        <asp:FileUpload ID="PhotoUpload1" CssClass="photo-upload" ValidationGroup="PhotoUpload" runat="server" size="38" />
    </div>
    <div id="photo2">
        <asp:FileUpload ID="PhotoUpload2" CssClass="photo-upload" ValidationGroup="PhotoUpload" runat="server" size="38" />
    </div>
    <div id="photo3">
        <asp:FileUpload ID="PhotoUpload3" CssClass="photo-upload" ValidationGroup="PhotoUpload" runat="server" size="38" />
    </div>
    <div id="photo4">
        <asp:FileUpload ID="PhotoUpload4" CssClass="photo-upload" ValidationGroup="PhotoUpload" runat="server" size="38" />
    </div>
</div>

ASP.NET验证程序

<div id="upload-warning">
    <asp:RequiredFieldValidator ID="PhotoUploadValidator" runat="server"
    ControlToValidate="PhotoUpload1" CssClass="upload-warning">
    <img src="image/mobile/exclamation-mark.png" />
    <span style="vertical-align: middle;">请上传四幅作品，您还需上传一幅作品。</span>
    </asp:RequiredFieldValidator>
</div>

感谢。