This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.
The default equality comparer, Default, is used to hash and compare keys.
GroupJoin<TOuter, TInner, TKey, TResult> produces hierarchical results, which means that elements from outer are paired with collections of matching elements from inner. GroupJoin enables you to base your results on a whole set of matches for each element of outer.
If there are no correlated elements in inner for a given element of outer, the sequence of matches for that element will be empty but will still appear in the results.
The resultSelector function is called only one time for each outer element together with a collection of all the inner elements that match the outer element. This differs from the Join<TOuter, TInner, TKey, TResult> method, in which the result selector function is invoked on pairs that contain one element from outer and one element from inner.
GroupJoin preserves the order of the elements of outer, and for each element of outer, the order of the matching elements from inner.
GroupJoin<TOuter, TInner, TKey, TResult> has no direct equivalent in traditional relational database terms. However, this method does implement a superset of inner joins and left outer joins. Both of these operations can be written in terms of a grouped join. See Join Operations.
In query expression syntax, a join … into (Visual C#) or Group Join (Visual Basic) clause translates to an invocation of GroupJoin<TOuter, TInner, TKey, TResult>.