Linq所有空集合

问题描述

我需要检查所有定义是否都包含某些特定数据.除GroupBy返回空集合的情况外，此方法都可以正常工作.

I need to check if all definitions contains some specific data. It works fine except the case when GroupBy returns empty collection.

var exist = dbContext.Definitions
                     .Where(x => propertyTypeIds.Contains(x.PropertyTypeId) && x.CountryId == countryId)
                     .GroupBy(x => x.PropertyTypeId)
                     .All(...some condition...);

如何重写此代码，以便在空集合中所有人都将返回false?

How to rewrite this so All would return false on empty collection?

更新:这是SQL的LINQ，我想在一次调用中执行.

UPDATE:It is a LINQ to SQL and I wanted to execute this in single call.

UPDATE2:我认为这可行:

UPDATE2:I think this works:

var exist = dbContext.Definitions
                     .Where(x => propertyTypeIds.Contains(x.PropertyTypeId) && x.CountryId == countryId)
                     .GroupBy(x => x.PropertyTypeId)
                     .Count(x => x
                        .All(...some condition...)) == propertyTypeIds.Count;

推荐答案

如果您使用的是LINQ to Objects，我会编写自己的扩展方法.我的 Edulinq项目具有 All ，并进行调整非常简单:

If you're using LINQ to Objects, I'd just write my own extension method. My Edulinq project has sample code for All, and adapting that is pretty simple:

public static bool AnyAndAll<TSource>(
    this IEnumerable<TSource> source,
    Func<TSource, bool> predicate)
{
    if (source == null)
    {
        throw new ArgumentNullException(nameof(source));
    }
    if (predicate == null)
    {
        throw new ArgumentNullException(nameof(predicate));
    }

    bool any = false;
    foreach (TSource item in source)
    {
        any = true;
        if (!predicate(item))
        {
            return false;
        }
    }
    return any;
}

这避免了多次评估输入.

This avoids evaluating the input more than once.

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