防止在不可见的ASP.NET面板中进行数据绑定

问题描述

不显示隐藏的（ visible = false ）面板，但是对包含的元素执行数据绑定。为什么这样做？更重要的是，如何避免呢？

A hidden (visible="false") panel is not rendered, but data binding is executed on contained elements. Why is that done? And more important, how to avoid it?

这里有一个令人讨厌的例子：

Here is an example where it is annoying:

<asp:Panel ID="UserPanel" runat="server" visible="<%# SelectedUser != null %>">
    <%# SelectedUser.Name %>
</asp:Panel>

如果 SelectedUser 是 null ，则不会呈现面板，但会评估 SelectedUser.Name 并生成错误。

If SelectedUser is null, the panel is not rendered but SelectedUser.Name is evaluated and generates an error.

我显然可以写<％＃SelectedUser！= null吗？ SelectedUser.Name：％> ，但会增加混乱。

I could obviously write <%# SelectedUser != null ? SelectedUser.Name : "" %> but it adds clutter.

有没有一种方法可以简单，优雅地防止面板内的数据绑定当我知道不需要它时？

Is there a way to simply and elegantly prevent data binding within a panel when I know it is not needed?

Panel 控件在这里并不重要，它可能是带有 runat = server 的纯HTML元素。

The Panel control is not important here, it could be a Placeholder of a plain HTML element with runat="server".

推荐答案

I可能在这一点上很晚，但我也觉得很烦。

I may be late on this one but I also find this very annoying.

如果我渲染的项目列表中每个项目可能是不同的类，那么我经常需要这样做-在这种情况下，数据绑定表达式中的属性会在用于其他类类型的不可见部分。您会知道是否要这样做。

I need this often if I am rendering a list of items where each item may be a different class - in which case the properties in the data binding expressions will give errors in the invisible sections intended for other class types. You'll know if you want this.

这里介绍了我找到的最佳解决方案：

The best solution I have found is described here:

该解决方案是对标准PlaceHolder控件的简单覆盖，以在Visible为false时禁止绑定子控件：

The solution is a simple override of the standard PlaceHolder control to suppress binding child controls if Visible is false:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI.WebControls;
namespace Website.Controls
{ 
    public class DataPlaceHolder : PlaceHolder 
    { 
        protected override void DataBindChildren() 
        { 
            if (Visible) 
            { 
                base.DataBindChildren(); 
            } 
        } 
    } 
}

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