GroupBy(TSource, TKey, TResult) Method (IQueryable(TSource), Expression(Func(TSource, TKey)), Expression(Func(TKey, IEnumerable(TSource), TResult)), IEqualityComparer(TKey))

Queryable.GroupBy(Of TSource, TKey, TResult) Method (IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TKey, IEnumerable(Of TSource), TResult)), IEqualityComparer(Of TKey))


Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Keys are compared by using a specified comparer.

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

Public Shared Function GroupBy(Of TSource, TKey, TResult) (
	source As IQueryable(Of TSource),
	keySelector As Expression(Of Func(Of TSource, TKey)),
	resultSelector As Expression(Of Func(Of TKey, IEnumerable(Of TSource), TResult)),
	comparer As IEqualityComparer(Of TKey)
) As IQueryable(Of TResult)


Type: System.Linq.IQueryable(Of TSource)

An IQueryable(Of T) whose elements to group.

Type: System.Linq.Expressions.Expression(Of Func(Of TSourceTKey))

A function to extract the key for each element.

Type: System.Linq.Expressions.Expression(Of Func(Of TKeyIEnumerable(Of TSource), TResult))

A function to create a result value from each group.

Type: System.Collections.Generic.IEqualityComparer(Of TKey)

An IEqualityComparer(Of T) to compare keys.

Return Value

Type: System.Linq.IQueryable(Of TResult)

An T:System.Linq.IQueryable`1 that has a type argument of TResult and where each element represents a projection over a group and its 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 result value returned by resultSelector.

Exception Condition

source or keySelector or resultSelector or comparer is null.

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

The GroupBy(Of TSource, TKey, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TKey, IEnumerable(Of TSource), TResult)), IEqualityComparer(Of TKey)) method generates a MethodCallExpression that represents calling GroupBy(Of TSource, TKey, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TKey, IEnumerable(Of TSource), TResult)), IEqualityComparer(Of TKey)) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Of 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(Of TSource, TKey, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TKey, IEnumerable(Of TSource), TResult)), IEqualityComparer(Of TKey)) depends on the implementation of the type of the source parameter. The expected behavior is that it groups the elements of source by key values that are obtained by invoking keySelector on each element. The comparer parameter is used to compare keys and the resultSelector parameter is used to obtain a result value from each group and its key.

Universal Windows Platform
Available since 4.5
.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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