List.Sort in C#: comparer being called with null object
- by cbp
I am getting strange behaviour using the built-in C# List.Sort function with a custom comparer.
For some reason it sometimes calls the comparer class's Compare method with a null object as one of the parameters. But if I check the list with the debugger there are no null objects in the collection.
My comparer class looks like this:
public class DelegateToComparer<T> : IComparer<T>
{
private readonly Func<T,T,int> _comparer;
public int Compare(T x, T y)
{
return _comparer(x, y);
}
public DelegateToComparer(Func<T, T, int> comparer)
{
_comparer = comparer;
}
}
This allows a delegate to be passed to the List.Sort method, like this:
mylist.Sort(new DelegateToComparer<MyClass>(
(x, y) => {
return x.SomeProp.CompareTo(y.SomeProp);
});
So the above delegate will throw a null reference exception for the x parameter, even though no elements of mylist are null.
UPDATE: Yes I am absolutely sure that it is parameter x throwing the null reference exception!
UPDATE: Instead of using the framework's List.Sort method, I tried a custom sort method (i.e. new BubbleSort().Sort(mylist)) and the problem went away. As I suspected, the List.Sort method passes null to the comparer for some reason.
