AnhHao Trần AnhHao Trần - 1 year ago 112
C# Question

C# passing a compare class as a generic type

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 Source

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)) ||
          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.", 

      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>();
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