C# generic compare method [analogue of C++ template]

Question

Both C++ and C# support generics. However, I don't see any way to rewrite a simple C++ function template that compares any two args (arg1 > arg2 ?) into a singe C# generic method:

C++

template<typename T>
int compare(const T & st1, const T & st2) {
    if (std::less<T>() (st1, st2)) 
        return -1;
    return 1;
}

works both with int, std::string, std::vectoretc.

compare(33, 4);         // 1

std::vector<int> v1{ 1,0 }, v2{ 1,0,0 };
compare(v1, v2);        // -1

std::vector<std::string> vs1{ "hi", "bob" }, vs2{ "hi", "ben" };
compare(vs1, vs2);      // 1

C#

   class Demo
    {
        public static int Compare<T>(T v1, T v2) where T : IComparable<T>
        {
            if (v1.CompareTo(v2) < 0)
                return -1;
            return 1;
        }
    }

doesn't work with, say C# Lists<>:

List<int> v1 = new List<int> { 1, 2 };
List<int> v2 = new List<int> { 3, 4 };
Console.WriteLine($"Compare(v1, v2): {Compare(v1, v2)}");

error: no implicit reference conversion from 'System.Collections.Generic.List' to 'System.IComparable>'

Is the only way to make it work with both integral types and collections in C# to overload each time?

public static int Compare<T>(List<T> v1, List<T> v2) where T : IComparable<T>
{
    for (int i = 0; i < v1.Count; i++)
    {
        if (v1[i].CompareTo(v2[i]) < 0)
            return -1;
    }
    return 1;
}

Answer 1

The immediate cause of the error is that List<int> doesn't implement IComparer<List<int>> and this fact doesn't meet method's specification:

public static int Compare<T>(T v1, T v2) where T : IComparable<T>

Since T must implement IComparable<T>.

I suggest something like this (quick and in some case a dirty solution):

public static int Compare<T>(T v1, T v2, IComparer<T> comparer = null) {
  if (null == comparer)              // If we don't have tailored comparer
    comparer = Comparer<T>.Default;  // Try default one

  // If we don't know how to compare - throw exception
  if (null == comparer)
    throw new ArgumentNullException("comparer", 
      $"Type {typeof(T).Name} doesn't have default comparer; comparer must not be null.");

  // Taken from the question: 
  // if (v1.CompareTo(v2) < 0)
  //          return -1;
  //      return 1;
  // You, probably, may want just  
  // return comparer.Compare(v1, v2);
  return comparer.Compare(v1, v2) < 0 ? -1 : 1;
}

So you can put, in a simple case

int result = Compare(15, 25); // Comparer<int>.Default will be used

In a complex case with no default comparer you have to implement it:

public class MyComparer<T> : IComparer<IEnumerable<T>> {
  public int Compare(IEnumerable<T> x, IEnumerable<T> y) {
    if (Object.ReferenceEquals(x, y))
      return 0;
    else if (null == x)
      return -1;
    else if (null == y)
      return 1;

    Comparer<T> comparer = Comparer<T>.Default;

    using (var en_x = x.GetEnumerator()) {
      using (var en_y = y.GetEnumerator()) {
        if (!en_x.MoveNext()) 
          if (!en_y.MoveNext())
            return 0;
          else
            return 1;
        else if (en_y.MoveNext())
          return -1;

        if (comparer != null) {
          int result = comparer.Compare(en_x.Current, en_y.Current);

          if (result != 0)
            return result;
        }
      }
    }

    return 0;
  }
}

And provide the comparer

List<int> v1 = new List<int> { 1, 2 };
List<int> v2 = new List<int> { 3, 4 };

int another result = Compare(v1, v2, new MyComparer<int>());