Why can I not use IComparable<T> on ancestor class and compare child classes?

Question

I'm trying to sort a list of objects using List.Sort(), but at runtime it tells me that it cannot compare elements in the array.

Failed to compare two elements in the array

Class structure:

public abstract class Parent : IComparable<Parent> {
    public string Title;
    public Parent(string title){this.Title = title;}

    public int CompareTo(Parent other){
        return this.Title.CompareTo(other.Title);
    }
}

public class Child : Parent {
    public Child(string title):base(title){}
}

List<Child> children = GetChildren();
children.Sort(); //Fails with "Failed to compare two elements in the array."

Why can I not compare subclasses of a base that implements IComparable<T>? I'm probably missing something, but I cannot see why this should not be allowed.

Edit: Should clarify that I'm targeting .NET 3.5 (SharePoint 2010)

Edit2: .NET 3.5 is the problem (see answer below).

Answer 1

I assume this is a .NET version before .NET 4.0; after .NET 4.0 it is IComparable<in T>, and should work OK in many cases - but this requires the variance changes in 4.0

The list is List<Child> - so sorting it will try to use either IComparable<Child> or IComparable - but neither of those is implemented. You could implement IComparable at the Parent level, perhaps:

public abstract class Parent : IComparable<Parent>, IComparable {
    public string Title;
    public Parent(string title){this.Title = title;}

    int IComparable.CompareTo(object other) {
        return CompareTo((Parent)other);
    }
    public int CompareTo(Parent other){
        return this.Title.CompareTo(other.Title);
    }
}

which will apply the same logic via object.