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.
If comparer is null, the default equality comparer, Default, is used to hash and compare keys.
A join refers to the operation of correlating the elements of two sources of information based on a common key. Join<TOuter, TInner, TKey, TResult> brings the two information sources and the keys by which they are matched together in one method call. This differs from the use of SelectMany, which requires more than one method call to perform the same operation.
Join<TOuter, TInner, TKey, TResult> preserves the order of the elements of outer, and for each of these elements, the order of the matching elements of inner.
In relational database terms, the Join<TOuter, TInner, TKey, TResult> method implements an inner equijoin. 'Inner' means that only elements that have a match in the other sequence are included in the results. An 'equijoin' is a join in which the keys are compared for equality. A left outer join operation has no dedicated standard query operator, but can be performed by using the GroupJoin<TOuter, TInner, TKey, TResult> method. See Join Operations.