Which is faster: Union or Concat?

.net, c#, concatenation, linq, union

Solution

`Union` removes duplicates. `Concat` does not.

So, they produce different results if the sources either contain any items in common, or have any internal duplicates.

If you can guarantee there are no duplicates, or if there are few and you don't care about having them in your output, `Concat` will be faster since there's no need to test each value against what has already been yielded.

However, if there are many duplicates and you don't need them, the extra processing in `Union` to remove the dupes may be offset by the savings in your code that consumes the results.

Problem

Original source