C# – Order Comparison

Order Comparison

Protocols are:

1. The IComparable Interfaces

2. The < and > Operators

IComparable Interfaces

Used for general-purpose sorting algorithms.

System.String implements IComparable interfaces.

string[] colors = { "Green", "Red", "Blue" };
Array.Sort (colors);
foreach (string c in colors) Console.Write (c + " ");   // Blue Green Red

// The IComparable interfaces are defined as follows:
//   public interface IComparable       { int CompareTo (object other); }
//   public interface IComparable { int CompareTo (T other);      }

Console.WriteLine ("Beck".CompareTo ("Anne"));       // 1
Console.WriteLine ("Beck".CompareTo ("Beck"));       // 0
Console.WriteLine ("Beck".CompareTo ("Chris"));      // -1

If a comes after b, a.CompareTo(b) returns a positive number.

If a is the same as b, a.CompareTo(b) returns 0.

If a comes before b, a.CompareTo(b) returns a negative number.

< and > Operators

The < and > operators are more specialized and mostly used for numeric types. They are efficient for bytecode translation and used for intensive algorithms.

// Some types define < and > operators:
bool after2010 = DateTime.Now > new DateTime (2010, 1, 1);

// The string type doesn't overload these operators (for good reason):
bool error = "Beck" > "Anne";       // Compile-time error

public struct Note : IComparable, IEquatable, IComparable
{
	int _semitonesFromA;
	public int SemitonesFromA => _semitonesFromA;
	
	public Note (int semitonesFromA)
	{
		_semitonesFromA = semitonesFromA;
	}
	
	public int CompareTo (Note other)            // Generic IComparable
	{
		if (Equals (other)) return 0;    // Fail-safe check
		return _semitonesFromA.CompareTo (other._semitonesFromA);
	}
	
	int IComparable.CompareTo (object other)     // Nongeneric IComparable
	{
		if (!(other is Note))
			throw new InvalidOperationException ("CompareTo: Not a note");
		return CompareTo ((Note) other);
	}
	
	public static bool operator < (Note n1, Note n2)
		=> n1.CompareTo (n2) < 0;
	
	public static bool operator > (Note n1, Note n2)
		=> n1.CompareTo (n2) > 0;
	
	public bool Equals (Note other)    // for IEquatable
		=> _semitonesFromA == other._semitonesFromA;
	
	public override bool Equals (object other)
	{
		if (!(other is Note)) return false;
		return Equals ((Note) other);
	}
	
	public override int GetHashCode()
		=> _semitonesFromA.GetHashCode();
	
	public static bool operator == (Note n1, Note n2)
		=> n1.Equals (n2);
	
	public static bool operator != (Note n1, Note n2)
		=> !(n1 == n2);
}

static void Main()
{
	Note n1 = new Note (1);	
	Note n2 = new Note (2);
	(n2 > n1).Dump();
}