Appending/concatenating two IEnumerable sequences

c# ienumerable unions
MikeTWebb picture MikeTWebb · Feb 10, 2011 · Viewed 32.8k times · Source

I have two sets of datarows. They are each IEnumerable. I want to append/concatenate these two lists into one list. I'm sure this is doable. I don't want to do a for loop and noticed that there is a Union method and a Join method on the two Lists. Any ideas?

Answer

Adam Robinson picture Adam Robinson · Feb 10, 2011

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.

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