public class Foo : IComparable<Foo>
{
    public Foo() { }

    public int Value { get; set; }

    public int CompareTo(Foo other)
    {
        if (other == null) { return 1; }

        return this.Value.CompareTo(other.Value);
    }
}

The trick is the line, which checks for the null comparison. Since null is always less than this, the method must return a value greater than zero (0).

Also see the remarks in the documentation:

Greater than zero: This object is greater than other.
...
By definition, any object compares greater than null (Nothing in Visual Basic), and two null references compare equal to each other.