HashSetContainsEquality.cs

class Foo : IEquatable<Foo>
{
    public int Id { get; private set; }

    public Foo(int id)
    {
        this.Id = id;
    }

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

var l = new List<Foo> { new Foo(1) };
Console.WriteLine(l.Contains(new Foo(1))); // true
var h = new HashSet<Foo> { new Foo(1) };
Console.WriteLine(h.Contains(new Foo(1))); // false