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.
This method excludes duplicates from the return set. This is different behavior to the Concat<TSource> method, which returns all the elements in the input sequences including duplicates.
The default equality comparer, Default, is used to compare values of the types that implement the IEqualityComparer<T> generic interface. To compare a custom data type, you need to implement this interface and provide your own GetHashCode and Equals methods for the type.
When the object returned by this method is enumerated, Union enumerates first and second in that order and yields each element that has not already been yielded.