Union Vs Concat in Linq

Question

I have a question on Union and Concat. I guess both are behaving same in case of List .

var a1 = (new[] { 1,          


        

Answer 1

Union and Concat behave the same since Union can not detect duplicates without a custom IEqualityComparer. It's just looking if both are the same reference.

public class XComparer: IEqualityComparer
{
    public bool Equals(X x1, X x2)
    {
        if (object.ReferenceEquals(x1, x2))
            return true;
        if (x1 == null || x2 == null)
            return false;
        return x1.ID.Equals(x2.ID);
    }

    public int GetHashCode(X x)
    {
        return x.ID.GetHashCode();
    }
}

Now you can use it in the overload of Union:

var comparer = new XComparer();
a5 = lstX1.Cast().Union(lstX2.Cast(), new XComparer());