Question

I have a situation in which I am attempting to keep my model and implementation as loosely coupled as possible, however I am faced with a situation where the coupling could potentially get a lot closer than I want.

I have a selection of ‘Model’ classes, all implementing interfaces. Additionally I have ‘Data Access’ classes, which provide a number of functions, one of which is decoding integer lookup values into their full ‘object’ representation.

Within my model classes I want to provide access to these decoded values without requiring the model to know about the data access classes.

A simplified example is:

/// Core classes --

class Car : ICar
{
    public int MakeId { get {...} set { ... } }

    public IMakeInfo Make { get {...} }

    public string Registration { get { ... } set { ... } }

    public int CurrentOwnerId { get { ... } set { ... } }

    public IPerson CurrentOwner { get { ... } }
}

class MakeInfo : IMakeInfo
{
    public string Name { ... }
    public int Id { ... }
    public decimal Weight { ... }
    // etc etc
}

/// Data Access Classes --

class ResolveMake 
{
    public IMakeInfo GetMakeInfo(int id)
    { 
        // Implementation here...
    }

}

How do I enable the Car class to provide the IMakeInfo object to any consuming classes without directly making it aware of the ResolveMake class? In the actual instance I am working with the Car class is not in the same namespace as the ResolveMake class and it contains no references to any instances of it.

Some of my options:

• Implement a delegate in Car which can be supplied with an instance of the GetMakeInfo method.
• Some kind of dependency injection
• Closely couple Car to ResolveMake and be done with it.
• Any other options?

Any suggestions welcome!

Answer 1

Extension Methods?

namespace CarStuff
{
   class Car : ICar
   {
      public int MakeId { get {...} set { ... } }
      // no Make property...
      public string Registration { get { ... } set { ... } }
      public int CurrentOwnerId { get { ... } set { ... } }
      public IPerson CurrentOwner { get { ... } }
   }
}


namespace MakeExts
{
   class ResolveMake
   {
      public static IMakeInfo Make(this Car myCar)
      {
         //implementation here
      }
   }
}

elsewhere:

using MakeExts;

Car c = new Car();
Console.WriteLine(c.Make().ToString());

Edit: For using Extension Methods in .NET 2.0, you need something like:

• http://kohari.org/2008/04/04/extension-methods-in-net-20/
• http://www.danielmoth.com/Blog/2007/05/using-extension-methods-in-fx-20.html

Basically, a class containing:

namespace System.Runtime.CompilerServices 
{ 
   class ExtensionAttribute : Attribute
   {
   }
 }

and a “using System.Runtime.CompilerServices” scattered in relevant places.