Feature Request: Expression bodied member syntax for interface implemented by field

[mattbenic]

Expression-bodied members were a great addition to C#6.

Expanding from that feature and syntax, being able to simplify the common pattern of a class implementing an interface by containing an instance of another class that implements that field would be very helpful. Similarly to with automatic properties, at compile time expand this to common properties and methods directly calling the corresponding properties and methods on the internal field.

Example:

public interface IAnimationPlayer
{
	bool IsPlayingAnimation { get; }

	void PlayAnimation(AnimationData animationData);
}

public class AnimationPlayer :
	IAnimationPlayer
{
	public bool IsPlayingAnimation { get; private set; }

	public void PlayAnimation(AnimationData animationData)
	{
		// Do work to play animation...

		IsPlayingAnimation = true;
	}
}

public class Actor :
	IAnimationPlayer,
	IOtherActorInterface
{
	// IAnimationPlayer interface implementation is 
	// delegated to private field
	public IAnimationPlayer => animationPlayer;

	public Actor(IAnimationPlayer animationPlayer)
	{
		this.animationPlayer = animationPlayer;
	}

	// Implement additional functionality...

	private readonly IAnimationPlayer animationPlayer;
}

With Actor compiling to:

public class Actor :
	IAnimationPlayer,
	IOtherActorInterface
{
	public bool IsPlayingAnimation 
	{
		get
		{
			return animationPlayer.IsPlayingAnimation;
		}
	}

	public void PlayAnimation(AnimationData animationData)
	{
		animationPlayer.PlayAnimation(animationData);
	}

	public Actor(IAnimationPlayer animationPlayer)
	{
		this.animationPlayer = animationPlayer;
	}

	// Implement additional functionality...

	private readonly IAnimationPlayer animationPlayer;
}

[gafter]

Issue moved to dotnet/csharplang #477 via ZenHub