MSDN Library

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

Groups the elements of a sequence according to a specified key selector function and compares the keys by using a specified comparer.

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

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

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.
Type: System.Collections.Generic.IEqualityComparer<TKey>
An IEqualityComparer<T> to compare keys.

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> 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.


source or keySelector 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>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, IEqualityComparer<TKey>) method generates a MethodCallExpression that represents calling GroupBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, 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>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, 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. The key value is obtained by invoking keySelector on each element, and key values are compared by using comparer.


Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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