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

Microsoft Silverlight will reach end of support after October 2021. Learn more.

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)

Syntax

'Declaration
<ExtensionAttribute> _
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)

public static IQueryable<TResult> GroupBy<TSource, TKey, TResult>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, TKey>> keySelector,
    Expression<Func<TKey, IEnumerable<TSource>, TResult>> resultSelector,
    IEqualityComparer<TKey> comparer
)

Type Parameters

  • TSource
    The type of the elements of source.
  • TKey
    The type of the key returned by the function represented in keySelector.
  • TResult
    The type of the result value returned by resultSelector.

Parameters

Return Value

Type: System.Linq.IQueryable<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.

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.

Exceptions

Exception Condition
ArgumentNullException

source or keySelector or resultSelector or comparer is nulla null reference (Nothing in Visual Basic).

Remarks

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, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, TResult>>, IEqualityComparer<TKey>) method generates a MethodCallExpression that represents calling GroupBy<TSource, TKey, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, TResult>>, 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, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, TResult>>, 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 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.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

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