Proposal: Composite Implementation Routing

[Sologix]

Composition is a powerful tool in object oriented design, but I'm somewhat bothered by having to reroute the interface members to implementation members, which are usually one-on-one calls.

I'm thinking about a C# language proposal to alleviate this burden and I'm posting here to discuss as to whether this might be a good idea.

The code is as follows:

NAME: COMPOSITE IMPLEMENTATION ROUTING

public interface IComposite1
{
  void Method1();
}

public interface IComposite2
{
  void Method2();
}

public Composite2Impl : IComposite2
{
  
}

CURRENT:

public class MyCompositeClass : IComposite1, IComposite2
{
  private IComposite1 _composite1;
  private Composite2Impl _composite2;

  public MyCompositeClass(IComposite1 composite1)
  {
    _composite1 = composite1;
    _composite2 = new Composite2Impl();
  }
  
  private void Method1()
  {
    _composite1.Method1();
  }
  
  private void Method2()
  {
    _composite2.Method2();
  }
}

PROPOSED: 

public class MyCompositeClass : IComposite1(_composite1), IComposite2(_composite2)
{
  protected IComposite1 _composite1;
  private Composite2Impl _composite2;
  
  public MyCompositeClass(IComposite1 composite1)
  {
    _composite1 = composite1;
    _composite2 = new Composite2Impl();
  }
}

public class MyInheritedCompositeClass : MyCompositeClass
{
  public override Method1()
  {
    
  }
}

[Joe4evr]

Looks like mostly a duplicate of #900, just with a slightly different syntax.

[yaakov-h]

Looks more like a duplicate of #234 (with different syntax) to me.

[YairHalberstadt]

C# 9.0 Source generators should make it possible to write something like this:

public partial class MyCompositeClass
{
  [ImplementInterface]
  protected IComposite1 _composite1;
    
  [ImplementInterface(typeof(IComposite2))]
  private Composite2Impl _composite2;
  
  public MyCompositeClass(IComposite1 composite1)
  {
    _composite1 = composite1;
    _composite2 = new Composite2Impl();
  }
}

and have the source generator generate the rest.

[bigpilot]

Nice!

[canton7]

Note that Visual Studio has a refactoring to insert all of the boilerplate for you, as well.

If you write:

public class MyCompositeClass : IComposite1
{
  private IComposite1 _composite1;

  public MyCompositeClass(IComposite1 composite1)
  {
    _composite1 = composite1;
  }
}

Then put your cursor over IComposite1 on the first line and hit Ctrl+., you should get the option to implement IComposite1 through _composite1.