Question

I basically have 2 list of the same object type.
The first one (A,B,C,D) is a property of one of my objects.

I need to join the second list(B, E, F) in my object, but exclude the duplicates.

This means i can’t just do:

    ListA.AddRange(ListB)

I’ll have to change it to

    ForEach item in ListB
       If Not ListA.Contains(item)
          ListA.Add(item)
       EndIf
    Next

Or add:

    ListA = ListA.Distinct()

Is there a faster and smoother way to code this?

Answer 1

If you have already overridden the Equals and GetHashcode you can use Enumerable.Union directly:

var ListC = ListA.Union(ListB).ToList();

Otherwise you could implement IEqulityCompararer<Foo> and use ListA.Union(ListB, comparer).

Assuming Foo is the type of your class and Name is a property you want to use to detect duplicates.

public class FooComparer : IEqualityComparer<Foo>
{
    public bool Equals(Foo x, Foo y)
    {
        if (x == null || y == null) return false;
        // 'A' = 'a' just to demonstrate case-insensitive duplicates
        return x.Name.Equals(y.Name, StringComparison.OrdinalIgnoreCase);
    }

    public int GetHashCode(Foo obj)
    {
        if (obj == null) return int.MinValue;
        return obj.Name.GetHashCode();
    }
}

Now use this comparer for Union:

var ListC = ListA.Union(ListB, new FooComparer()).ToList();