IComparer use

Question

I'v created IComparer for SortedList collection but compiler do not like how I'm using it:

class IntReverse : IComparer
{   public int Compare(object x, object y)
    {  int xcopy = (int)x, ycopy = (int)y;
            if (xcopy > ycopy) return -1; else return 1;
      }
 }
  class Program
 {  static void Main()
     {  IComparer comp = new IntReverse();
         SortedList<int,string> a = new SortedList<int, string>(comp);
      }
 }

 Error message:
Error    CS1503    Argument 1: cannot convert from 'System.Collections.IComparer' to 'int'   

Answer 1

Try another comparer:

class IntReverse : IComparer<int>

{

   public int Compare( int x, int y )

   {

      . . .

   }

}
. . .

IComparer<int> comp = new IntReverse( );

SortedList<int, string> a = new SortedList<int, string>( comp );

Note that Compare must return 0 too. For example:

class IntReverse : IComparer<int>

{

   public int Compare( int x, int y )

   {

      return y - x;

   }

}

The documentation for IComparer<T> recommends deriving from the Comparer<T> abstract class instead of implementing the IComparer<T> interface.

Answer 2

The error occurs because you are mixing up the non-Generic version of the comparer with the Generic version of the SortedList. You need to be consistent: Either both have to be Generic, or both non-Generic.

For example, you can do

... new SortedList(comp)

note that there is no <int, string>.

Or you can create a class that is an IComparer<int> (instead of an IComparer) and then use this class to initialize a new SortedList<int, string>(comparer).

But the way you have written it, the compiler thinks that you are calling this overload of the constructor for the Generic SortedList:

SortedList<TKey,TValue>(Int32)

and therefore it complains that it cannot convert from your IComparer to an int.

Note that there is a different constructor like this:

SortedList<TKey,TValue>(IComparer<TKey>)

which presumably is the one that you were trying to invoke. But notice that this takes as an argument an IComparer<TKey>, which is not the same as an IComparer.