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


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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)

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


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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