Appending/concatenating two IEnumerable sequences

Assuming your objects are of the same type, you can use either Union or Concat. Note that, like the SQL UNION keyword, the Union operation will ensure that duplicates are eliminated, whereas Concat (like UNION ALL) will simply add the second list to the end of the first.

IEnumerable<T> first = ...;
IEnumerable<T> second = ...;

IEnumerable<T> combined = first.Concat(second);

or

IEnumerable<T> combined = first.Union(second);

If they are of different types, then you’ll have to Select them into something common. For example:

IEnumerable<TOne> first = ...;
IEnumerable<TTwo> second = ...;

IEnumerable<T> combined = first.Select(f => ConvertToT(f)).Concat(
                          second.Select(s => ConvertToT(s)));

Where ConvertToT(TOne f) and ConvertToT(TTwo s) represent an operation that somehow converts an instance of TOne (and TTwo, respectively) into an instance of T.