Simplest way to form a union of two lists

Question

What is the easiest way to compare the elements of two lists say A and B with one another  and add the elements which are present in B to A only if they are not present in A   To illustrate  Take list A    1 2 3  list B    3 4 5   So after the operation AUB I want list A    1 2 3 4 5

User · Answer

Using LINQ s Union  Enumerable Union ListA ListB     or  ListA Union ListB

User · Answer

If it is a list  you can also use AddRange method   var listB   new List lt int gt  3  4  5     var listA   new List lt int gt  1  2  3  4  5    listA AddRange listB      listA now has elements of listB also    If you need new list  and exclude the duplicate   you can use Union    var listB   new List lt int gt  3  4  5       var listA   new List lt int gt  1  2  3  4  5     var listFinal   listA Union listB     If you need new list  and include the duplicate   you can use Concat    var listB   new List lt int gt  3  4  5       var listA   new List lt int gt  1  2  3  4  5     var listFinal   listA Concat listB     If you need common items  you can use Intersect   var listB   new List lt int gt  3  4  5     var listA   new List lt int gt  1  2  3  4     var listFinal   listA Intersect listB     3 4

User · Answer

The easiest way is to use LINQ s Union method   var aUb   A Union B  ToList

User · Answer

I think this is all you really need to do   var listB   new List lt int gt  3  4  5   var listA   new List lt int gt  1  2  3  4  5    var listMerged   listA Union listB

User · Answer

If it is two IEnumerable lists you can t use AddRange  but you can use Concat   IEnumerable lt int gt  first   new List lt int gt  1 1 2 3 5   IEnumerable lt int gt  second   new List lt int gt  8 13 21 34 55    var allItems   first Concat second      1 1 2 3 5 8 13 21 34 55

[c#] Simplest way to form a union of two lists

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