Queryable.GroupBy<TSource, TKey, TResult> Method (IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, TResult>>, IEqualityComparer<TKey>)
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.
Assemblies: System.Linq.Queryable (in System.Linq.Queryable.dll)
System.Core (in System.Core.dll)
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 )
The type of the elements of source.
The type of the key returned by the function represented in keySelector.
The type of the result value returned by resultSelector.
- Type: System.Linq.Expressions.Expression<Func<, >>
A function to extract the key for each element.
- Type: System.Linq.Expressions.Expression<Func<, IEnumerable<>, >>
A function to create a result value from each group.
Return ValueType: System.Linq.IQueryable<>
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 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 resultSelector or comparer is null.
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 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.