Remove duplicates while merging lists using Union in LINQ

Question

I am trying to merge two lists using list.Union in LinqPad but I can't get it to work and wanted to check my understanding is correct.

Given this simple class:

public class Test 
{
   public int Id { get; set;}
   public int field1 { get; set; }

   public bool Equals(Test other)
   {        
      return this.Id.Equals(other.Id);
   }
}

And two lists populated like this:

List<Test> list = new List<Test>();
list.Add( new Test { Id = 1, field1 = 1});
list.Add( new Test { Id = 1, field1 = 2});
list.Add( new Test { Id = 2, field1 = 3});
list.Add( new Test { Id = 2, field1 = 4});

List<Test> list2 = new List<Test>();
list2.Add( new Test { Id = 1, field1 = 1});
list2.Add( new Test { Id = 1, field1 = 2});
list2.Add( new Test { Id = 2, field1 = 3});
list2.Add( new Test { Id = 2, field1 = 4});

I then try: var mergedList = list.Union(list2).ToList(); and output the data using a simple foreach loop and get this output:

ID: 1 -------- 1
ID: 1 -------- 2
ID: 2 -------- 3
ID: 2 -------- 4
ID: 1 -------- 1
ID: 1 -------- 2
ID: 2 -------- 3
ID: 2 -------- 4

I was under the impression that Union should remove the duplicates to return:

ID: 1 -------- 1
ID: 1 -------- 2
ID: 2 -------- 3
ID: 2 -------- 4

Am I doing something wrong or have I misunderstood?

Also, should it work without explicitly overriding the Equals method in the Test class?

Thanks

Answer 1

You can try something like that if not happy with default comparer (which is, in turn, utilizes GetHashCode method as @IlyaIvanov had mentioned):

// get all items that "other than in first list", so Where() and Any() are our filtering expressions
var delta = list2.Where(x2 => !list.Any(x1 => (x1.Id == x2.Id) && (x1.field1 == x2.field1)));

// now let merge two enumerables that have nothing "equal" between them
var merged = list.Union(delta).ToList();