如何创建扩展方法来处理带有谓词输入的 bindinglist.removeall

发布于 2024-11-18 21:20:27 字数 172 浏览 6 评论 0原文

myGenericList.RemoveAll(x => (x.StudentName == "bad student"));

效果很好，但是绑定列表没有这个方法。如何为绑定列表创建一个扩展方法，该方法将谓词作为输入，并像列表的罐装删除一样发挥作用，

谢谢

原文

myGenericList.RemoveAll(x => (x.StudentName == "bad student"));

Works great, but a bindinglist does not have this method. How can I create an extension method for the bindinglist that takes as input a predicate and does the magic like the canned removeall for List

thankyou

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舂唻埖巳落 2024-11-25 21:20:27

就像我在评论中所说，扩展方法没有什么神奇之处，只需像正常编写代码一样编写代码，只需将其放在静态类中的静态方法中并使用 this 关键字：

public static void RemoveAll<T>(this BindingList<T> list, Func<T, bool> predicate)
{
    foreach (var item in list.Where(predicate).ToArray())
        list.Remove(item);
}

您必须使用 ToArray() （或 ToList()），因为 Where() 是惰性的，仅在以下情况下枚举集合：需要，并且您无法枚举更改的集合。

尽管此解决方案相当慢 (O(N²))，因为每个 Remove() 都必须遍历集合以找到要删除的正确项目。我们可以做得更好：

public static void FastRemoveAll<T>(this BindingList<T> list, Func<T, bool> predicate)
{
    for (int i = list.Count - 1; i >= 0; i--)
        if (predicate(list[i]))
            list.RemoveAt(i);
}

这利用了我们可以在常数时间内到达第 i 项的事实，因此整个方法是 O(N)。迭代更容易向后编写，因此我们尚未考虑的项目的索引不会改变。

编辑：实际上，第二个解决方案仍然是 O(N²)，因为每个 RemoveAt() 都必须移动该项目之后的所有项目被删除。

Like I said in a comment, there is no magic in extension methods, just write the code the same way as if you wrote it normally, just put it in a static method in a static class and use the this keyword:

public static void RemoveAll<T>(this BindingList<T> list, Func<T, bool> predicate)
{
    foreach (var item in list.Where(predicate).ToArray())
        list.Remove(item);
}

You have to use ToArray() (or ToList()), because Where() is lazy and only enumerates the collection when needed and you can't enumerate changing collection.

Although this solution is quite slow (O(N²)), because every Remove() has to look through the collection to find the correct item to remove. We can do better:

public static void FastRemoveAll<T>(this BindingList<T> list, Func<T, bool> predicate)
{
    for (int i = list.Count - 1; i >= 0; i--)
        if (predicate(list[i]))
            list.RemoveAt(i);
}

This uses the fact that we can get to i-th item in constant time, so the whole method is O(N). The iteration is easier to write backwards, so that indexes of items we have yet to consider aren't changing.

EDIT: Actually the second solution is still O(N²), because every RemoveAt() has to move all the items after the one that was removed.

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爱的十字路口 2024-11-25 21:20:27

我想说：

public static class BindingListExtensions
{
    public static void RemoveAll<T>(this BindingList<T> list, Func<T, bool> predicate)
    {
        // first check predicates -- uses System.Linq
        // could collapse into the foreach, but still must use 
        // ToList() or ToArray() to avoid deferred execution                       
        var toRemove = list.Where(predicate).ToList();

        // then loop and remove after
        foreach (var item in toRemove)
        {
            list.Remove(item);
        }
    }
}

对于那些对细节感兴趣的人，似乎 ToList() 和 ToArray() 的性能非常接近（事实上，根据具体情况，每个都可以更快），可以忽略不计： “https://stackoverflow.com/questions/1826658/what-is-faster-or-preferred-ienumerable-toarray-or-tolist">我需要迭代和计数。哪一个最快或首选：ToArray() 还是 ToList()？

I'd say:

public static class BindingListExtensions
{
    public static void RemoveAll<T>(this BindingList<T> list, Func<T, bool> predicate)
    {
        // first check predicates -- uses System.Linq
        // could collapse into the foreach, but still must use 
        // ToList() or ToArray() to avoid deferred execution                       
        var toRemove = list.Where(predicate).ToList();

        // then loop and remove after
        foreach (var item in toRemove)
        {
            list.Remove(item);
        }
    }
}

And for those interested in the minutia, seems ToList() and ToArray() are so close to the same performance (and in fact each can be faster based on the circumstance) as to be negligible: I need to iterate and count. What is fastest or preferred: ToArray() or ToList()?

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