Sunday, October 18, 2009

Generic Comparer

A requirement in most of our applications is a default sorting for our codetable data. For example if you want to populate a combobox with all the countries in the world, it must be sorted by default in alphabetical order.

I heard my colleagues discussing about it during one of our whiteboard design meetings. Possibilities passed by from using a custom attribute to a metadata class. While they were discussing I build the following solution; a generic comparer.

Specify the properties to sort on using expressions and pass it on to the Sort method. No direct coupling between a class and it's sorting, type safety and extensible. What do you need more?

The implementation of the Comparer is very simple, it just iterate over the expressions array and compares each property in both objects: 

   1:  public class GenericComparer<T> : IComparer<T>
   2:  {
   3:          private readonly Func<T, IComparable>[] _expressions;
   4:   
   5:          public GenericComparer(params Func<T, IComparable>[] expressions)
   6:          {
   7:              _expressions = expressions;
   8:          }
   9:   
  10:          public int Compare(T x, T y)
  11:          {
  12:              foreach (var expression in _expressions)
  13:              {
  14:                  int result = Compare(expression.Invoke(x), expression.Invoke(y));
  15:                  if (result != 0)
  16:                      return result;
  17:              }
  18:              return 0;
  19:          }
  20:   
  21:          private int Compare(IComparable x, IComparable y)
  22:          {
  23:              if (x == null && y == null)
  24:                  return 0;
  25:              if (x == null)
  26:                  return -1;
  27:              if (y == null)
  28:                  return 1;
  29:              return x.CompareTo(y);
  30:          }
  31:   }

And an example:

   1:  countries.Sort(new GenericComparer<Country>(c => c.CountryCode, c => p.Name));

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