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Queryable.GroupBy(TSource, TKey) Method (IQueryable(TSource), Expression(Func(TSource, TKey)))

Groups the elements of a sequence according to a specified key selector function.

Namespace:  System.Linq
Assemblies:   System.Linq.Queryable (in System.Linq.Queryable.dll)
  System.Core (in System.Core.dll)

static member GroupBy : 
        source:IQueryable<'TSource> * 
        keySelector:Expression<Func<'TSource, 'TKey>> -> IQueryable<IGrouping<'TKey, 'TSource>> 

Type Parameters


The type of the elements of source.


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


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.

Return Value

Type: System.Linq.IQueryable(IGrouping(TKey, TSource))
An IQueryable<IGrouping<TKey, TSource>> in C# or IQueryable(Of IGrouping(Of TKey, TSource)) in Visual Basic where each IGrouping(TKey, TElement) object contains a sequence of objects and a key.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable(TSource). When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).


source or keySelector is a null reference (Nothing in Visual Basic).

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)(IQueryable(TSource), Expression(Func(TSource, TKey))) method generates a MethodCallExpression that represents calling GroupBy(TSource, TKey)(IQueryable(TSource), Expression(Func(TSource, 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)(IQueryable(TSource), Expression(Func(TSource, 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.

The following code example demonstrates how to use GroupBy(TSource, TKey)(IQueryable(TSource), Expression(Func(TSource, TKey))) to group the elements of a sequence.

No code example is currently available or this language may not be supported.

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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