IComparable<T> Vs. IComparer<T> System.Comparison<T>

Well they are not quite the same thing as IComparer<T> is implemented on a type that is capable of comparing two different objects while IComparable<T> is implemented on types that are able to compare themselves with other instances of the same type.

I tend to use IComparable<T> for times when I need to know how another instance relates to this instance. IComparer<T> is useful for sorting collections as the IComparer<T> stands outside of the comparison.

IComparer < T >是上实现一种能够比较两个不同的对象而IComparable < T >是上实现类型能够比较自己与其他相同类型的实例。

class Student : IComparable 
{
    public string Name { get; set; }
    public int MathScore { get; set; }
    public int EnglishScore { get; set; }

    public int TotalScore 
    {
        get
        {
            return this.MathScore + this.EnglishScore; 
        }
    }

    public int CompareTo(object obj)
    {
        return CompareTo(obj as Student);  
    }

    public int CompareTo(Student other)
    {
        if (other == null)
        {
            return 1;
        }
        return this.Name.CompareTo(other.Name);  
    }
}

But if a teacher 'A' wants to compare students based on MathScore, and teacher 'B' wants to compare students based on EnglishScore. It will be good idea to implement IComparer separately. (More like a strategy pattern)

class CompareByMathScore : IComparer<Student>
{
    public int Compare(Student x, Student y)
    {
        if (x.MathScore > y.MathScore)
          return 1;
        if (x.MathScore < y.MathScore)
          return -1;
        else
          return 0;
    }
}

public class NameComparer:IComparer<Student>
{
    public int Compare(Student x, Student y)
    {
        return x.Name.CompareTo(y.Name);
    }
}

 http://stackoverflow.com/questions/3498891/system-comparisont-understanding

System.Comparison<T> is defined as follows:

public delegate int Comparison<in T>(T x, T y);

That means that it's delegate, not a class. A method accepting a delegate as a parameter actually accepts a method, not an instance of a Comparison class.

This code can be rewritten as follows with a lambda expression:

collection.Sort((i1, i2) => i1.ToString().CompareTo(i2.ToString()));

The following snippet might explain better what happens:

public static class TestClass {

  public static void Main(string[] args){
      Comparison<Int32> comparisonDelegate = CompareWithCase;
      //We now can use comparisonDelegate as though it is a method;
      int result = comparisonDelegate(1,2);
  }

  public static int CompareWithCase(int i1, int i2)
  {
     return i1.ToString().CompareTo(i2.ToString());
  } 
}