为什么IEnumerable丢失更新数据? [英] Why is IEnumerable losing updated data?

问题描述

您能否解释一下为什么执行以下代码后Selected属性未更新为true?

Could you explain to me why after executing the following code the Selected property is not updated to true?

所使用的ListItem类型来自System.Web.UI.WebControls命名空间，并且是一个类(不是结构.)我相信FirstOrDefault函数返回对实例的引用，我可以对其进行更新并在items中传递枚举.

The ListItem type used comes from System.Web.UI.WebControls namespace and is a class (not a struct.) I believed the FirstOrDefault function returns a reference to an instance which I can update and pass around in the items enumerable.

// produce list items out of the communities
IEnumerable<ListItem> items = communities.Select(community => new ListItem(community.Name, community.Id.ToString()));

// mark the right list item as selected, if needed
if (platform.CommunityId > 0)
{
    string strCommunityId = platform.CommunityId.ToString();
    ListItem selectedItem = items.FirstOrDefault(item => item.Value == strCommunityId);
    if (selectedItem != null) selectedItem.Selected = true;
}

// now items do not store any updated item!

是因为每次调用foreach都会执行枚举器，从而创建新项目，而不是返回包含我更新的项目的集合吗?

Is that because the enumerator is executed each time a foreach is called and thus creating new items instead of returning the set containing the item I updated?

推荐答案

问题是IEnumerable是不可重复的.每次枚举(community => new ListItem)都在执行投影(community => new ListItem)-因此每次都是新的ListItem. Select是无缓冲的延迟投影.

The problem is that IEnumerable is not repeatable. You are performing the projection (community => new ListItem) every time it is enumerated - hence it is a new ListItem each time. Select is a non-buffered deferred projection.

您可以通过简单地添加.ToList()来将此处的内容全部修复，以将数据强制放入单个列表中；

You can fix everything here with the simple addition of a .ToList() to force the data into a single list;

var items = communities.Select(
    community => new ListItem(community.Name, community.Id.ToString())
).ToList();

现在数据在列表中，您可以循环遍历列表多次-它始终是相同的项目，并且更改将被保留.

Now that the data is in the list, you can loop over the list any number of times - it'll always be the same items, and changes will be retained.

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