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.
Assembly: System.Core (in System.Core.dll)
static member GroupBy : source:IQueryable<'TSource> * keySelector:Expression<Func<'TSource, 'TKey>> * comparer:IEqualityComparer<'TKey> -> IQueryable<IGrouping<'TKey, 'TSource>>
The type of the elements of source.
The type of the key returned by the function represented in keySelector.
- Type: System.Linq.Expressions.Expression(Func(, ))
A function to extract the key for each element.
Return ValueType: System.Linq.IQueryable(IGrouping(, ))
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 NoteIn Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable(). When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).
source or keySelector or comparer is a null reference (Nothing in Visual Basic).
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 method generates a MethodCallExpression that represents calling 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 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.