c# - Why does List<T>.Sort method reorder equal IComparable<T> elements?

Question

I have a problem with how the List Sort method deals with sorting. Given the following element:

class Element : IComparable<Element>
{
    public int Priority { get; set; }
    public string Description { get; set; }

    public int CompareTo(Element other)
    {
        return Priority.CompareTo(other.Priority);
    }
}

If I try to sort it this way:

List<Element> elements = new List<Element>()
                             {
                                 new Element()
                                     {
                                         Priority = 1,
                                         Description = "First"
                                     },
                                 new Element()
                                     {
                                         Priority = 1,
                                         Description = "Second"
                                     },
                                 new Element()
                                     {
                                         Priority = 2,
                                         Description = "Third"
                                     }
                             };
elements.Sort();

Then the first element is the previously second element "Second". Or, in other words, this assertion fails:

Assert.AreEqual("First", elements[0].Description);

Why is .NET reordering my list when the elements are essentially the same? I'd like for it to only reorder the list if the comparison returns a non-zero value.

Answer 1

From the documentation of the List.Sort() method from MSDN:

This method uses Array.Sort, which uses the QuickSort algorithm. This implementation performs an unstable sort; that is, if two elements are equal, their order might not be preserved. In contrast, a stable sort preserves the order of elements that are equal.

Here's the link: http://msdn.microsoft.com/en-us/library/b0zbh7b6.aspx

Essentially, the sort is performing as designed and documented.