如何有条件地删除一个.NET集合项 [英] How to conditionally remove items from a .NET collection

问题描述

我试图使用.NET编写的扩展方法，将运行在一个通用的集合，从集合中删除匹配给定条件的所有项目。

I'm trying to write an extension method in .NET that will operate on a generic collection, and remove all items from the collection that match a given criteria.

这是我第一次尝试：

public static void RemoveWhere<T>(this ICollection<T> Coll, Func<T, bool> Criteria){
    foreach (T obj in Coll.Where(Criteria))
        Coll.Remove(obj);
}

然而，这引发InvalidOperationException集合已修改;枚举操作可能不会执行。这有一定道理，所以我做了第二次尝试与第二个集合变量来保存需要​​被删除，遍历，与其相关的项目：

However this throws an InvalidOperationException, "Collection was modified; enumeration operation may not execute". Which does make sense, so I made a second attempt with a second collection variable to hold the items that need to be removed and iterate through that instead:

public static void RemoveWhere<T>(this ICollection<T> Coll, Func<T, bool> Criteria){
    List<T> forRemoval = Coll.Where(Criteria).ToList();

    foreach (T obj in forRemoval)
        Coll.Remove(obj);
}

这将引发相同的异常;我不知道我真正明白，为什么是'科尔'不再被遍历集合，所以为什么不能修改？

This throws the same exception; I'm not sure I really understand why as 'Coll' is no longer the collection being iterated over, so why can't it be modified?

如果任何人有任何建议，我怎么能得到这个工作，或者更好的方法来达到同样的，那简直太好了。

If anyone has any suggestions as to how I can get this to work, or a better way to achieve the same, that'd be great.

感谢。

推荐答案

有关名单，其中，T＆GT; ，这个已经存在，如 removeall过（predicate＆LT; T＆GT;）。因此，我建议您将该名称（允许熟悉，和precedence）。

For List<T>, this exists already, as RemoveAll(Predicate<T>). As such, I'd suggest that you keep the name (allowing familiarity, and precedence).

基本上，你不能删除，而迭代。有两个常用的选项：

Basically, you can't remove while iterating. There are two common options:

• 使用索引器基于迭代（为）和去除
• 缓冲物品取出，之后删除的foreach （因为你已经做了）

• use indexer based iteration (for) and removal
• buffer the items to remove, and remove after the foreach (as you've already done)

所以，现在可能：

public static void RemoveAll<T>(this IList<T> list, Func<T, bool> predicate) {
    for (int i = 0; i < list.Count; i++) {
        if (predicate(list[i])) {
            list.RemoveAt(i--);
        }
    }
}

或者更普遍的任何的ICollection＆LT; T＆GT; ：

public static void RemoveAll<T>(this ICollection<T> collection, Func<T, bool> predicate) {
    T element;

    for (int i = 0; i < collection.Count; i++) {
        element = collection.ElementAt(i);
        if (predicate(element)) {
            collection.Remove(element);
            i--;
        }
    }
}

该方法具有避免大量列表的额外副本的优势。

This approach has the advantage of avoiding lots of extra copies of the list.

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