`init` Properties on Interfaces With DIMs Causes Diagnostics

[JasonBock]

To Reproduce

Run this code in a test:

var code =
	"""
	using Rocks;
	
	[assembly: Rock(typeof(IBufferedChannel), BuildType.Create | BuildType.Make)]
	
	public interface IBufferedChannel
	{
		int MaxBufferSize { get; init; }
		void Consume(int count) { }
	}
	""";

Expected behavior
Rocks generates code with no issues.

Actual behavior
A compiler diagnostic is created:

error CS8852: Init-only property or indexer 'IBufferedChannel.MaxBufferSize' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.

Additional context
This is hard to describe why this is a problem. The issue is in the generate shim:

private sealed class ShimIBufferedChannel
	: global::IBufferedChannel
{
	private readonly Mock mock;
						
	public ShimIBufferedChannel(Mock @mock) =>
		this.mock = @mock;
						
	public int MaxBufferSize
	{
		get => ((global::IBufferedChannel)this.mock).MaxBufferSize!;
		init => ((global::IBufferedChannel)this.mock).MaxBufferSize = value!;
	}
}

For some reason, calling MaxBufferSize on the "parent" mock with the cast in the init isn't allowed.

One idea to fix this is to simply not do anything in the init:

private sealed class ShimIBufferedChannel
	: global::IBufferedChannel
{
	private readonly Mock mock;
						
	public ShimIBufferedChannel(Mock @mock) =>
		this.mock = @mock;
						
	public int MaxBufferSize
	{
		get => ((global::IBufferedChannel)this.mock).MaxBufferSize!;
		init;
	}
}

The init (and, for that matter, the get) will never be called from the mock instance itself. Therefore, it doesn't matter what they do. The property has to be implemented, but what it does is irrelevant.

If the property itself was a DIM, the same issue occurs within the mock when it tries to call the shim's property implementation from its' init implementation. Note that the get implementation is fine. This is really odd, because the assignment is within an init accessor, which is what the diagnostic is complaining about. But we want to call the DIM's implementation.

For reference: dotnet/roslyn#50053

This was found in DotNext.IO, DotNext.Buffers.IBufferedReader.

[JasonBock]

After doing some investigation, I think the way to address this is to simply not try to call the mock instance within the init of the shim. A ConstructorProperties type will be created that will be used to assign the init property within the mock's constructor, so there's no chance the shim's init will ever be called. The following is a scaled-down version of what happens when the code is generated with the init implementation removed in the shim:

#pragma warning disable CA1050 // Declare types in namespaces
#pragma warning disable CA1859 // Use concrete types when possible for improved performance

public interface IBufferedChannel
{
	void Consume(int count) { }
	int MaxBufferSize { get; init; }
}

public sealed class Mock
	: global::IBufferedChannel
{
	private readonly global::IBufferedChannel shimForIBufferedChannel;

	public Mock(ConstructorProperties? @constructorProperties)
	{
		this.shimForIBufferedChannel = new ShimIBufferedChannel(this);

		if (@constructorProperties is not null)
		{
			this.MaxBufferSize = @constructorProperties.MaxBufferSize;
		}
	}

	public int MaxBufferSize
	{
		get;
		init;
	}

	public void Consume(int @count) =>
		this.shimForIBufferedChannel.Consume(@count: @count!);

	private sealed class ShimIBufferedChannel
		: global::IBufferedChannel
	{
		private readonly Mock mock;

		public ShimIBufferedChannel(Mock @mock) =>
			this.mock = @mock;

		public int MaxBufferSize
		{
			get => ((global::IBufferedChannel)this.mock).MaxBufferSize!;
			init { }
		}
	}
}

public sealed class ConstructorProperties
{
	internal int MaxBufferSize { get; init; }
}

Ignore the #pragmas at the top, that's not important. The key is the init implementation in the shim. By basically making it a no-op, the error goes away.