Queryable.GroupBy(Of TSource, TKey, TElement) Method (IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement)), IEqualityComparer(Of TKey))
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
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.
Assembly: System.Core (in System.Core.dll)
'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))
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(Of TKey, TElement).
Parameters
- source
- Type: System.Linq.IQueryable(Of TSource)
An IQueryable(Of T) whose elements to group.
- keySelector
- Type: System.Linq.Expressions.Expression(Of Func(Of TSource, TKey))
A function to extract the key for each element.
- elementSelector
- Type: System.Linq.Expressions.Expression(Of Func(Of TSource, TElement))
A function to map each source element to an element in an IGrouping(Of TKey, TElement).
- comparer
- Type: System.Collections.Generic.IEqualityComparer(Of TKey)
An IEqualityComparer(Of T) to compare keys.
Return Value
Type: System.Linq.IQueryable(Of IGrouping(Of TKey, TElement))An IQueryable<IGrouping<TKey, TElement>> in C# or IQueryable(Of IGrouping(Of TKey, TElement)) in Visual Basic where each IGrouping(Of 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(Of TSource). When you use instance method syntax to call this method, omit the first parameter.| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector or elementSelector or comparer is Nothing. |
This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).
The GroupBy(Of TSource, TKey, TElement)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement)), IEqualityComparer(Of TKey)) method generates a MethodCallExpression that represents calling GroupBy(Of TSource, TKey, TElement)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement)), IEqualityComparer(Of TKey)) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Of 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(Of TSource, TKey, TElement)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement)), IEqualityComparer(Of 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.