GroupBy with elementSelector and resultSelector
For 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)
});
is equevalent to:
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)
});
using of elementSelector could simplifies expressions in resultSelector (compare next and 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
});
In IQueryable, it is not so simple. You can look at sources of this methods:
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) }
));
}
As you can see, they returns different expressions, so I'm not sure that result SQL query will be same in all cases, but i suppose that SQL query for overload with elementSelector + resultSelector will be not slower compare to overload without elementSelector.
Answer 1: No, for IEnumerable there is no query that you cannot express by using the resultSelector
alone.
Answer 2. No, there are no counterpart for the two different overloads when using LINQ query syntax. Extension methods have more possibilities compare to LINQ query syntax.
Answer 3 (For side question): it is not guaranteed that sql queries will be same for this overloads.
Related videos on Youtube
Slauma
Updated on September 14, 2022Comments
-
Slauma over 1 year
The
Enumerable.GroupBy
andQueryable.GroupBy
extensions have 8 overloads. Two of them (forEnumerable.GroupBy
) are:// (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);
(for
Queryable.GroupBy
the same, just withExpression<Func<...
instead ofFunc<...
)(b)
has an additionalelementSelector
as parameter.On MSDN is an example for overload (a) and an example for overload (b). They both work with the same example source collection:
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 } };
Example (a) uses this query:
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) });
And example (b) uses this query:
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() });
The result of both queries is exactly the same.
Question 1: Is there any kind of query that I cannot express by using the
resultSelector
alone and where I really would need theelementSelector
? Or are the capabilities of the two overloads always equivalent and it is just a matter of taste to use one or the other way?Question 2: Is there a counterpart for the two different overloads when using LINQ query syntax?
(As a side question: When using
Queryable.GroupBy
with Entity Framework, will both overloads be translated into the exact same SQL?)