C#LINQ SelectMany默认 [英] C# LINQ SelectMany with Default

问题描述

我正在寻找一种优雅的解决方案，将一个集合中的一个子集合聚合为一个大集合.我的问题是某些子集合可能为空.

I am looking for an elegant solution to aggregate a child collection in a collection into one large collection. My issue is when certain child collections could be null.

EG:

var aggregatedChildCollection = parentCollection.SelectMany(x=> x.ChildCollection);

如果任何子集合对象为null，则将引发异常.一些替代方法是:

This throws an exception should any of the child collection objects be null. Some alternatives are:

// option 1
var aggregatedChildCollection = parentCollection
    .Where(x=>x.ChildCollection != null)
    .SelectMany(x => x.ChildCollection);

// option 2
var aggregatedChildCollection = parentCollection
    .SelectMany(x => x.ChildCollection ?? new TypeOfChildCollection[0]);

这两种方法都可以，但是我正在对父级上的许多子级集合执行某种操作，并且这变得有些不合时宜了.

Both would work but I am doing a certain operation on quite a few child collections on the parent, and it is becoming a bit unweilding.

我想创建一个扩展方法，该方法检查集合是否为null，选项2是否为null-添加一个空数组.但是，我对Func的了解还不足以使我知道如何编写此扩展方法.我确实知道我想要的语法是这样的:

What I would like is to create an extension method that checks if the collection is null and if so does what option 2 does - adds an empty array. But my understanding of Func is not to a point where I know how to code this extension method. I do know that the syntax I would like is like this:

var aggregatedChildCollection = parentCollection.SelectManyIgnoringNull(x => x.ChildCollection);

有没有一种简单的扩展方法可以实现这一目标?

Is there a simple extension method that would accomplish this?

推荐答案

您可以使用自定义扩展方法:

You can use a custom extension method:

public static IEnumerable<TResult> SelectManyIgnoringNull<TSource, TResult>(
    this IEnumerable<TSource> source, 
    Func<TSource, IEnumerable<TResult>> selector)
{
    return source.Select(selector)
        .Where(e => e != null)
        .SelectMany(e => e);
}

并像这样使用:

var aggregatedChildCollection = parentCollection
    .SelectManyIgnoringNull(x => x.ChildCollection);

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