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)
Type Parameters
- TSource
The type of the elements of source.
- TKey
The type of the key returned by the function represented in keySelector.
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.
- comparer
- 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.| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector or comparer is nullptr. |
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.
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