Union Vs Concat in Linq

Question

I have a question on Union and Concat. I guess both are behaving same in case of List .

var a1 = (new[] { 1,          


        

Answer 1

Concat literally returns the items from the first sequence followed by the items from the second sequence. If you use Concat on two 2-item sequences, you will always get a 4-item sequence.

Union is essentially Concat followed by Distinct.

In your first two cases, you end up with 2-item sequences because, between them, each pair of input squences has exactly two distinct items.

In your third case, you end up with a 4-item sequence because all four items in your two input sequences are distinct.