Linq中默认值的平均扩展方法 [英] Average extension method in Linq for default value

问题描述

任何人都知道如何为平均值设置默认值吗?我有这样一条线...

Anyone know how I can set a default value for an average? I have a line like this...

dbPlugins = (from p in dbPlugins
                select new { Plugin = p, AvgScore = p.DbVersions.Average(x => x.DbRatings.Average(y => y.Score)) })
            .OrderByDescending(x => x.AvgScore)
            .Select(x => x.Plugin).ToList();

如果我还没有评级，这会引发错误.如果没有，我希望平均值默认为0.我想这应该是一种扩展方法，在其中可以指定默认值.

which throws an error becase I have no ratings yet. If I have none I want the average to default to 0. I was thinking this should be an extension method where I could specify what the default value should be.

推荐答案

有: DefaultIfEmpty .

我不确定您的DbVersions和DbRatings是什么，以及哪个集合正好有零个项目，但这是这样的想法:

I 'm not sure about what your DbVersions and DbRatings are and which collection exactly has zero items, but this is the idea:

var emptyCollection = new List<int>();
var average = emptyCollection.DefaultIfEmpty(0).Average();

更新 :(重复以下评论中的内容以提高知名度)

Update: (repeating what's said in the comments below to increase visibility)

如果发现自己需要在类类型的集合上使用DefaultIfEmpty，请记住，您可以将LINQ查询更改为项目之前进行汇总.例如:

If you find yourself needing to use DefaultIfEmpty on a collection of class type, remember that you can change the LINQ query to project before aggregating. For example:

class Item
{
    public int Value { get; set; }
}

var list = new List<Item>();
var avg = list.Average(item => item.Value);

如果您不想/无法构造默认的Item，且Value等于0，则可以先投影到int s的集合，然后然后提供一个默认值:

If you don't want to/can not construct a default Item with Value equal to 0, you can project to a collection of ints first and then supply a default:

var avg = list.Select(item => item.Value).DefaultIfEmpty(0).Average();

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