SaahilClaypool / DelegateTo

C# Source generator for automatic delegation to composed objects

Delegate To

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Inheritance is often used to share code, but this can lead to unintended complexity. One way around this is to favor composition over inheritance.

Some languages have a bit of syntactic sugar to make delegating behavior to a sub-entity easier:

• Ruby/Rails allows you to specify a "delegate" where methods of a given name are forwarded to a composite type

  class Greeter < ActiveRecord::Base
      def hello
          'hello'
      end
  end
  
  class Foo < ActiveRecord::Base
      belongs_to :greeter
      delegate :hello, to: :greeter
  end
  
  Foo.new.hello  # implicitly forward "hello" to the contained "Greeter"

• Go has a concept of promoted fields where methods are forwarded to an anonymous composed type

  struct Student {
      Year int
      Person // anonymous person delegate
  }
  
  struct Person {
      Age int
  }
  
  student := Student { Year: 2010, Person: Person { Age: 18 } }
  student.Age // -> 18

C# does not have this feature, so inheritance is the easiest way to share code or interfaces between classes.

until now

With source generators, we can forward, at compile time, all desired functionality from parent class to a composed class, and inherit all required interfaces.

Example:

partial class Student
{
    public int Year { get; set; }

    [GenerateDelegate] // signal source generator to create delegate functions
    Person Person { get; set; }
}

class Person
{
    public int Age { get; set; }
}

var student = new Student { Year = 2010, Person = new() { Age = 18 }};
student.Age // -> 18

This will be done by generating the following partial class for student with the needed functionality

partial class Student
{
    public Age { get => Person.Age; set => Person.Age = value; }
}

As a bonus, the Student or parent class should be able to automatically gain any and all interfaces implemented by the composed class as it will now fulfill all of those contracts.

See ./DelegateTo.Test for examples.