C# passing a compare class as a generic type

Question

I'm new in C#. I tried to create a heap structure and I came up with this question: How do I pass a "compare-class" to my heap structure? I mean, I want to create a heap like: Heap<int, cmp<int>> heap = new Heap<int, cmp<int>>(); where "cmp" is a compare-class which makes the heap in a priority order (I took the idea of priority_queue in C++). I have succeed (I think) in making a Heap that takes a max-min comparer:

public class Heap<T, Priority>
    where Priority : IPriority<T>, new()
    where T : IComparable
{
    private List<T> storage = new List<T>();        
    private Priority HeapPriority = new Priority();
    private void UpHeap(int position)
    {            
        for(var i = position; i > 0; i = (i - 1) >> 1)
        {
            // Check whether storage[i] is more Priority than storage[(i - 1) >> 1]
            if (HeapPriority.MorePriority(storage[i], storage[(i - 1) >> 1])
                .CompareTo(storage[i]) == 0)
            {
                storage.Swap(i, (i - 1) >> 1);
            }
            else break;
        }
    }   
}

and here is the IPriority interface:

public interface IPriority<T>
    where T : IComparable
{
    T MorePriority(T a, T b);
}

and I used the Heap like this:

public class Min<T> : IPriority<T>
        where T : IComparable
    {
        public Min() { }
        public T MorePriority(T a, T b)
        {
            return a.CompareTo(b) <= 0 ? a : b;
        }
    }
static public void TestHeap()
    {
        var heap = new Heap<Pair<long, int>, Min<Pair<long, int>>>();            
        heap.Add(Pair<long, int>(10, 20));
        heap.Add(Pair<long, int>(21, 100));
        // ...
    }

but I want a heap that sorts the items by any way that I want, not only max-min order. Moreover, is there a way to use "Ipriority.MorePriority" as a static method?, because it's working just like a static method. Can anyone give me some advices? Sorry for my bad English.

Answer 1

I suggest treating IComparer<T> as a dependence and pass it to constructor; something like this:

  // You can create a heap of any type, right? 
  // But in some cases (e.g. Heap<Button>) you should provide a comparer:
  // how to compare Button instances   
  public class Heap<T> {
    //TODO: implement here Heap as well as Unheap method having IComparer<T> m_Comparer
    ...
    private IComparer<T> m_Comparer;

    // comparer = null - if comparer is not provided, try to use default one
    // if it's possible (e.g. in case of Heap<double>)
    public Heap(IComparer<T> comparer = null): base() {
      // Do we have a default comparer (e.g. for int, double, string)?
      if (null == comparer) 
        if (typeof(IComparable).IsAssignableFrom(typeof(T)) ||
            typeof(IComparable<T>).IsAssignableFrom(typeof(T)))
          comparer = Comparer<T>.Default;

      if (null == comparer)
        throw new ArgumentNullException("comparer", string.Format(
          "There's no default comparer for {0} class, you should provide it explicitly.", 
          typeof(T).Name));

      m_Comparer = comparer;
    }
    ...
  }

So you can create heaps

  // heap of integers, default comparer
  Heap<int> heap1 = new Heap<int>();

  // heap of integers, custom comparer (via lambda)
  Heap<int> heap2 = new Heap<int>(Comparer<int>.Create((x, y) => -x.CompareTo(y)));

  // heap of Buttons, custome comparer 
  Heap<Button> heap3 = new Heap<Button>(Comparer<Button>.Create((x, y) => ...));

And this will throw exception: no default comparer for Button class

  Heap<Button> heapErr = new Heap<Button>();

Answer 2

You should just use IComparer<T>, which is what all the collections in .NET use. For example:

public class Heap<T>
{
    private IComparer<T> comparer;
    private List<T> storage = new List<T>();     

    public Heap() : this(Comparer<T>.Default)
    {
    }

    public Heap(IComparer<T> comparer)
    {
        this.comparer = comparer;
    }

    private void UpHeap(int position)
    {            
        for(var i = position; i > 0; i = (i - 1) >> 1)
        {
            // Check whether storage[i] is more Priority than storage[(i - 1) >> 1]
            if (comparer.Compare(storage[i], storage[(i - 1) >> 1]) > 0)
            {
                storage.Swap(i, (i - 1) >> 1);
            }
            else break;
        }
    }   
}

This has several advantages:

• You don't need the extra type parameter for the comparer, which is a lot more convenient when referencing the heap in other methods.
• There's no need to constrain T to be IComparable.
• However, if it does implement ICompatable (like Int32 does), Comparer<T>.Default will use that implementation.