Mixin can be implemented in C# 3.0 using an extension method. 
Functionality can be added by extending 
the interface implemented by the class.
 
Consider the following example.
   1:  using System;
   2:   
   3:  namespace Mixin
   4:   
   5:  {
   6:   
   7:  class Person : IPerson
   8:   
   9:  {
  10:   
  11:  private int _age;
  12:   
  13:  private string _name;
  14:   
  15:  #region IPerson Members
  16:   
  17:  public int age
  18:   
  19:  {
  20:
a
  21:  get
  22:   
  23:  {
  24:   
  25:  return _age;
  26:   
  27:  }
  28:   
  29:  set
  30:   
  31:  {
  32:   
  33:  if (value > 0)
  34:   
  35:  {
  36:   
  37:  _age = value;
  38:   
  39:  }
  40:   
  41:   
  42:  }
  43:   
  44:  }
  45:   
  46:   
  47:  public string name
  48:   
  49:  {
  50:   
  51:  get
  52:   
  53:  {
  54:   
  55:  return _name;
  56:   
  57:  }
  58:   
  59:  set
  60:   
  61:  {
  62:   
  63:  if (!string.IsNullOrEmpty(value))
  64:   
  65:  {
  66:   
  67:  _name = value;
  68:   
  69:  }
  70:   
  71:  else
  72:   
  73:  {
  74:   
  75:  _name = "Unknown";
  76:   
  77:  }
  78:   
  79:  }
  80:   
  81:  }
  82:   
  83:  #endregion
  84:   
  85:  }
  86:   
  87:  interface IPerson
  88:   
  89:  {
  90:   
  91:  int age { get; set; }
  92:   
  93:  String name { get; set; }
  94:   
  95:  }
  96:   
  97:  static class ExtendIPerson
  98:   
  99:  {
 100:   
 101:  public static bool CanDrive(this IPerson testPerson, bool isDriver)
 102:   
 103:  {
 104:   
 105:  if (isDriver)
 106:   
 107:  return true;
 108:   
 109:  return false;
 110:   
 111:  }
 112:   
 113:  }
 114:   
 115:  class Program
 116:   
 117:  {
 118:   
 119:  static void Main(string[] args)
 120:   
 121:  {
 122:   
 123:  IPerson ip = new Person();
 124:   
 125:   
 126:  ip.name = "ABC";
 127:   
 128:  ip.age = 21;
 129:   
 130:  if(ip.CanDrive(true))
 131:   
 132:  {
 133:   
 134:  Console.WriteLine(ip.name +" can drive");
 135:   
 136:  }
 137:   
 138:  Console.ReadLine();
 139:   
 140:  }
 141:   
 142:  }
 143:   
 144:  }
 145:   
 
In this example, we implemented mixin by extending the IPerson interface
[line 87] using CanDrive extension method defined
in the ExtendIPerson static class[line 101]. 
Notice the code on line 130 ip.CanDrive - an example of C# mixin. 
Another explanation of the mixin can be found at
 
http://mikehadlow.blogspot.com/2008/05/interface-extension-methods-mixin.html