IComparable<T>.CompareTo compares with null in Sort

Question

I want to sort a list with the help of IComparable<T>.CompareTo for a type T called Path. I wrote

var shortest = new List<Path>();
//Fill shortest with elements != null
if (shortest.Contains(null))
      throw new System.Exception("Path is null");
shortest.Sort();
if (shortest.Contains(null))
      throw new System.Exception("Path is null");

To my surprise, the method

    int IComparable<Path>.CompareTo(Path other)
    {
        if (other == null)
            return -1;

        if (!other.valid)
            return 1;

        if (pfad.Count() > other.pfad.Count())
        {
            return -1;
        }
        else if (pfad.Count() < other.pfad.Count())
        {
            return 1;
        }
        else
        {
            if (length > other.length)
                return -1;
            else
                return 1;

        }

    }

from the class

 public class Path : IComparable<Path>

is called from Sort() with other==null. I am even more surprised that in the first code-block, the second exception is thrown, meaning that shortest contains a null value after the sort and not before.

Answer 1

Your CompareTo function is broken. It doesn't return 0 when an object is compared to itself and it always returns 1 when two objects with valid == false are compared. So there might be two objectsa and b with a.CompareTo(b) == 1 and b.CompareTo(a) == 1 and this might lead to Sort() behave strangely.

Also, as already noted in the other answer, it should return 1 if other == null. (shouldn't really matter when the list doesen't contain null)