c# merge two lists as queryable

List<string> a = new List<string>();
List<string> b = new List<string>();

a.AddRange(b);

var list3 = list1.Concat(list2);

// or

var list4 = list1.Union(list2);
// Union is a set operation - it returns distinct values.

// Concat simply returns the items from the first sequence followed by the items from the second sequence; the resulting sequence can include duplicate items.

// You can think of Union as Concat followed by Distinct.