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)
Syntax
'Declaration
<ExtensionAttribute> _
Public Shared Function GroupBy(Of TSource, TKey, TElement) ( _
source As IQueryable(Of TSource), _
keySelector As Expression(Of Func(Of TSource, TKey)), _
elementSelector As Expression(Of Func(Of TSource, TElement)), _
comparer As IEqualityComparer(Of TKey) _
) As IQueryable(Of IGrouping(Of TKey, TElement))
public static IQueryable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(
this IQueryable<TSource> source,
Expression<Func<TSource, TKey>> keySelector,
Expression<Func<TSource, TElement>> elementSelector,
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.
- TElement
The type of the elements in each IGrouping<TKey, TElement>.
Parameters
- source
Type: System.Linq.IQueryable<TSource>
An IQueryable<T> whose elements to group.
- keySelector
Type: System.Linq.Expressions.Expression<Func<TSource, TKey>>
A function to extract the key for each element.
- elementSelector
Type: System.Linq.Expressions.Expression<Func<TSource, TElement>>
A function to map each source element to an element in an IGrouping<TKey, TElement>.
- comparer
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.
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 elementSelector 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, 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.
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.