Queryable.GroupBy<TSource, TKey, TElement> Method (IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, IEqualityComparer<TKey>)


Groups the elements of a sequence and projects the elements for each group by using a specified function. Key values are compared by using a specified comparer.

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)

public static IQueryable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, TKey>> keySelector,
	Expression<Func<TSource, TElement>> elementSelector,
	IEqualityComparer<TKey> comparer


Type: System.Linq.IQueryable<TSource>

An IQueryable<T> whose elements to group.

Type: System.Linq.Expressions.Expression<Func<TSource, TKey>>

A function to extract the key for each element.

Type: System.Linq.Expressions.Expression<Func<TSource, TElement>>

A function to map each source element to an element in an IGrouping<TKey, TElement>.

Type: System.Collections.Generic.IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys.

Return Value

Type: System.Linq.IQueryable<IGrouping<TKey, TElement>>

An IQueryable<IGrouping<TKey, TElement>> in C# or IQueryable(Of IGrouping(Of TKey, TElement)) in Visual Basic where each IGrouping<TKey, TElement> contains a sequence of objects of type TElement and a key.

Type Parameters


The type of the elements of source.


The type of the key returned by the function represented in keySelector.


The type of the elements in each IGrouping<TKey, TElement>.

Exception Condition

source or keySelector or elementSelector or comparer is null.

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, IEqualityComparer<TKey>) method generates a MethodCallExpression that represents calling GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, IEqualityComparer<TKey>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, IEqualityComparer<TKey>) depends on the implementation of the type of the source parameter. The expected behavior is that it groups the elements of source by a key value that is obtained by invoking keySelector on each element. Key values are compared by using comparer. The elementSelector parameter is invoked on each element to obtain a result element.

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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