trjhtr

This question is NOT about passing a Comparison to a Sort.

The question is how to pass a Comparison to an object for object to object (1:1) Comparison. Some users must be able to pass in a Comparison. They want 2 as want be able to tell when they give different comparison.

The way this solution approaches it is to pass a Comparison delegate to the constructor.

static int CompareMotorPrice(Motorcycle motorcycle1, Motorcycle motorcycle2)

 return motorcycle1.Price.CompareTo(motorcycle2.Price);

static int CompareMotorSpeed(Motorcycle motorcycle1, Motorcycle motorcycle2)

 return motorcycle1.Speed.CompareTo(motorcycle2.Speed);

public class Motorcycle : IComparable

 public string Name get; 
 public int Speed get; set; 
 public int Price get; set; 
 public Comparison<Motorcycle> CompareDelegate get; set; 
 public Comparison<Motorcycle> CompareDefault get; set; 
 public int CompareViaDelegate(Motorcycle motorcycle2)
 motorcycle2 == null)
 
 throw new ArgumentNullException();
 
 return (CompareDelegate(this, motorcycle2));
 
 public int CompareTo(object obj)
 
 if (obj == null) return 1;

 Motorcycle otherMotorcycle = obj as Motorcycle;
 if (otherMotorcycle != null)
 
 if (CompareDefault == null)
 
 return (this.Name.CompareTo(otherMotorcycle.Name));
 
 else
 
 return CompareDefault(this, otherMotorcycle);
 
 
 else
 
 throw new ArgumentException("Object is not a otherMotorcycle");
 
 
 public Motorcycle(string name, int price, int speed, Comparison<Motorcycle> compareDefault = null, Comparison<Motorcycle> compareDelegate = null)
 
 Name = name;
 Price = price;
 Speed = speed;
 CompareDelegate = compareDelegate;
 CompareDefault = compareDefault;
 

test:

Motorcycle motorcycle1 = new Motorcycle("harley", 1000, 120, CompareMotorSpeed, CompareMotorPrice);
Motorcycle motorcycle2 = new Motorcycle("yamaha", 1900, 130, CompareMotorSpeed, CompareMotorPrice);
Motorcycle motorcycle3 = new Motorcycle("susuki", 1800, 150, CompareMotorSpeed, CompareMotorPrice);
Debug.WriteLine(motorcycle1.CompareViaDelegate(motorcycle2));
List<Motorcycle> Motorcycles = new List<Motorcycle>() motorcycle1, motorcycle2, motorcycle3 ;
foreach (Motorcycle m in Motorcycles)

 Debug.WriteLine($"name m.Name price m.Price speed m.Speed");

Motorcycles.Sort(CompareMotorPrice);
foreach (Motorcycle m in Motorcycles)

 Debug.WriteLine($"name m.Name price m.Price speed m.Speed");

Motorcycles.Sort();
foreach (Motorcycle m in Motorcycles)

 Debug.WriteLine($"name m.Name price m.Price speed m.Speed");

1)

 if (CompareDelegate == null || motorcycle2 == null)
 
 throw new ArgumentNullException();
 

This exception is misleading. First, it does not specify which parameter is null. Second, strictly speaking CompareDelegate is not a parameter.

2) I would set the default value for CompareDefault:

public Comparison<Motorcycle> CompareDefault get; set; = 
 other => Name.CompareTo(other.Name);

this way you can probably remove some of the null checks and simplify CompareTo implementation.

3) Overall, I don't like this approach.

• it scales poorly.


• it hides comparison logic: there is no way to learn how two objects are going to be compared just by reading the code, the comparison logic is hidden inside some delegate.


• most importantly, you break asymmetric property of comparison. If a.CompareTo(b) > 0 then I expect that b.CompareTo(a) < 0. But it is not the case, since nothing in your code prevents two objects from using different delegates for comparison. This alone looks like a huge source of problems to me.

IMHO, just write a bunch of regular IComparer<T> implementations and be done with it. It has none of the problems I mentioned above:

//custom comparers are implemented as stand alone classes, 
//they are not part of Motorcycle class.
public class Motorcycle

 public string Name get; 
 public int Speed get; set; 
 public int Price get; set; 

 public Motorcycle(string name, int price, int speed)
 
 Name = name;
 Price = price;
 Speed = speed;
 


//It also guarantees asymmetry:
//if SpeedComparer.Compare(a,b) < 0 then SpeedComparer.Compare(b,a) > 0 is also true
class SpeedComparer : IComparer<Motorcycle>

 public int Compare(Motorcycle first, Motorcycle second)
 
 //add null checks

 return first.Price.CompareTo(second.Price);
 


class NameComparer : IComparer<Motorcycle>

 public int Compare(Motorcycle first, Motorcycle second)
 
 //add null checks

 return first.Name.CompareTo(other.Name);
 

Usage:

public void Test()

 var motorcycle1 = new Motorcycle("harley", 1000, 120);
 var motorcycle2 = new Motorcycle("yamaha", 1900, 130);
 var motorcycle3 = new Motorcycle("susuki", 1800, 150);
 var motorcycles = new List<Motorcycle>() motorcycle1, motorcycle2, motorcycle3 ;

 IComparer<Motorcycle> selectedComparer = new NameComparer();
 //compares by name
 TestComparison();

 selectedComparer = new SpeedComparer();
 //selectedComparer is changed, so now speeds are compared
 TestComparison();

 void TestComparison()
 
 Debug.WriteLine($"Using selectedComparer.GetType().Name...");
 Debug.WriteLine(selectedComparer.Compare(motorcycle1, motorcycle2));
 motorcycles.Sort(selectedComparer);
 motorcycles.ForEach(m => Debug.WriteLine($"name m.Name price m.Price speed m.Speed"));
 
 

As pointed out by Nikita the problem is a.CompareTo(b) might not by symmetric with b.CompareTo(a). Some times users ask for stuff that just does not make sense. A reality is often it is just easier to do it with limitations.

I think I have something that will work:

public delegate int DelegateDeclaration(Car car1, Car car2);
// implementation
public static int DelegateImplementation1(Car car1, Car car2)

 return car1.HP.CompareTo(car2.HP);

public class Car : IComparer<Car>

 public IComparer<Car> Comparer get; set; 
 public int Compare(Car car1, Car car2)
 
 if (car1 == null)
 
 throw new ArgumentNullException("car1");
 
 if (car1 == null)
 
 throw new ArgumentNullException("car1");
 
 if (Comparer == null)
 
 return car1.Name.CompareTo(car2.Name);
 
 return Comparer.Compare(car1, car2); //dangerous as they might not have the same Comparer
 
 public int ComareTo(Car other)
 
 return Compare(this, other);
 
 public bool CompareSame(Car other)
 
 return Compare(this, other) == -Compare(other, this);
 
 public int ComareTo(Car other, DelegateDeclaration dgate)
 
 return dgate(this, other);
 
 public bool ComareSame(Car other, DelegateDeclaration dgate1, DelegateDeclaration dgate2)
 
 return dgate1(this, other) == dgate2(this, other);
 
 public String Name get; 
 public int HP get; set; 
 public Car(string name, int hp, IComparer<Car> comparer = null)
 
 Name = name;
 HP = hp;