GroupBy with elementSelector和resultSelector

Question

该Enumerable.GroupBy和Queryable.GroupBy扩展有8个重载.其中两个(for Enumerable.GroupBy)是:

// (a)
IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    Func<TKey, IEnumerable<TSource>, TResult> resultSelector);

// (b)
IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    Func<TSource, TElement> elementSelector,
    Func<TKey, IEnumerable<TElement>, TResult> resultSelector);

(Queryable.GroupBy同样,只是Expression<Func<...代替Func<...)

(b)有一个额外的elementSelector参数.

MSDN上是对于过载的(a)的例子和对过载(b)中的例子.它们都使用相同的示例源集合:

List<Pet> petsList = new List<Pet>
{
    new Pet { Name="Barley", Age=8.3 },
    new Pet { Name="Boots", Age=4.9 },
    new Pet { Name="Whiskers", Age=1.5 },
    new Pet { Name="Daisy", Age=4.3 }
};

示例(a)使用此查询:

var query = petsList.GroupBy(
    pet => Math.Floor(pet.Age), // keySelector
    (age, pets) => new          // resultSelector
    {
        Key = age,
        Count = pets.Count(),
        Min = pets.Min(pet => pet.Age),
        Max = pets.Max(pet => pet.Age)
    });

示例(b)使用此查询:

var query = petsList.GroupBy(
    pet => Math.Floor(pet.Age), // keySelector
    pet => pet.Age,             // elementSelector
    (baseAge, ages) => new      // resultSelector
    {
        Key = baseAge,
        Count = ages.Count(),
        Min = ages.Min(),
        Max = ages.Max()
    });

两个查询的结果完全相同.

问题1:是否有任何类型的查询无法通过resultSelector单独使用以及我真正需要的地方来表达elementSelector？或者这两个重载的能力总是相同的,使用一种或另一种方式只是一种品味问题？

问题2:使用LINQ查询语法时,是否存在两个不同重载的对应项？

(作为一个附带问题:当Queryable.GroupBy与Entity Framework一起使用时,是否会将两个重载都转换为完全相同的SQL？)

Answer 1

对于IEnumerable:

petsList.GroupBy(
    pet => Math.Floor(pet.Age), // keySelector
    (age, pets) => new          // resultSelector
    {
        Key = age,
        Count = pets.Count(),
        Min = pets.Min(pet => pet.Age),
        Max = pets.Max(pet => pet.Age)
    });

是符合以下条件的:

var query = petsList.GroupBy(
    pet => Math.Floor(pet.Age), // keySelector
    pet => pet,             // elementSelector
    (baseAge, ages) => new      // resultSelector
    {
        Key = baseAge,
        Count = ages.Count(),
        Min = ages.Min(pet => pet.Age),
        Max = ages.Max(pet => pet.Age)
    });

使用elementSelector可以简化resultSelector中的表达式(比较next和previous):

var query = petsList.GroupBy(
    pet => Math.Floor(pet.Age), // keySelector
    pet => pet.Age,             // elementSelector
    (baseAge, ages) => new      // resultSelector
    {
        Key = baseAge,
        Count = ages.Count(),
        Min = ages.Min(), //there is no lambda due to element selector
        Max = ages.Max() ////there is no lambda due to element selector
    });

在IQueryable中,它并非如此简单.您可以查看此方法的来源:

public static IQueryable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector, Expression<Func<TSource, TElement>> elementSelector, Expression<Func<TKey, IEnumerable<TElement>, TResult>> resultSelector)
        {
            if (source == null)
                throw Error.ArgumentNull("source"); 
            if (keySelector == null)
                throw Error.ArgumentNull("keySelector"); 
            if (elementSelector == null) 
                throw Error.ArgumentNull("elementSelector");
            if (resultSelector == null) 
                throw Error.ArgumentNull("resultSelector");
            return source.Provider.CreateQuery<TResult>(
                Expression.Call(
                    null, 
                    ((MethodInfo)MethodBase.GetCurrentMethod()).MakeGenericMethod(typeof(TSource), typeof(TKey), typeof(TElement), typeof(TResult)),
                    new Expression[] { source.Expression, Expression.Quote(keySelector), Expression.Quote(elementSelector), Expression.Quote(resultSelector) } 
                    )); 
        }

public static IQueryable<TResult> GroupBy<TSource, TKey, TResult>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector,Expression<Func<TKey, IEnumerable<TSource>, TResult>> resultSelector)
        {
            if (source == null)
                throw Error.ArgumentNull("source"); 
            if (keySelector == null)
                throw Error.ArgumentNull("keySelector"); 
            if (resultSelector == null) 
                throw Error.ArgumentNull("resultSelector");
            return source.Provider.CreateQuery<TResult>( 
                Expression.Call(
                    null,
                    ((MethodInfo)MethodBase.GetCurrentMethod()).MakeGenericMethod(typeof(TSource), typeof(TKey), typeof(TResult)),
                    new Expression[] { source.Expression, Expression.Quote(keySelector), Expression.Quote(resultSelector) } 
                    ));
        } 

正如您所看到的,它们返回不同的表达式,因此我不确定结果SQL查询在所有情况下是否相同,但我认为与没有elementSelector的重载相比,使用elementSelector + resultSelector进行重载的SQL查询将不会更慢.

答案1:不,对于IEnumerable,没有任何查询无法通过resultSelector单独使用来表达.

答案2.不,使用LINQ查询语法时,两个不同的重载没有对应物.与LINQ查询语法相比,扩展方法具有更多可能性.

答案3(对于附带问题):不保证sql查询对于此重载是相同的.