Question

I have three seperate ToSelectList items and i wanted to combine each list into one dropdown list box and was wondering if i could use a Union for it. Or is it that a union is only for 2 toselectlist items only.

Thanks!

Answer 1

When item exists in more than one list, do you want item to appear once, or as many times as they exists in all lists?

As I understand you want just concat:

var combinedList = list1.Concat(list2).Concat(list3).ToList();

If you want to avoid duplicates:

var unionList = list1.Union(list2).Union(list3).ToList();

Union is more expensive, as it has to go through the list and take care of duplicates.
If your item is a reference type and there is no IComparable or IEquable interfaces and you don’t provide IEqualityComparer, you likely don’t need Union.