out ref parameters

[bill-poole]

C# currently has the ability for a method to return a "ref" or "ref readonly" value, but no such ability exists with "out" parameters. That is, I'd like the ability to specify a "ref" or "ref readonly" out parameter of a method. I have an example below where I would use this feature if available. I've only included the pertinent parts from the code as I'd write it if the feature were available.

public class ApportionedValue
{
  private Portion[] _portions;

  public int UnspecifiedPortionIndex { get; } = -1;

  public bool HasUnspecifiedPortion => UnspecifiedPortionIndex >= 0;

  public bool IsPartlySpecifiedNumeric(out ref readonly Portion specifiedPortion)
  {
    specifiedPortion = ref Portion.Empty;
    if (_portions.Length != 2)
    {
      return false;
    }

    if (!HasUnspecifiedPortion)
    {
      return false;
    }

    ref readonly var portion = ref _portions[1 - UnspecifiedPortionIndex];
    if (portion.Value.IsNumeric)
    {
      specifiedPortion = ref portion;
      return true;
    }

    return false;
  }

  // ...
}

public struct Value
{
  public bool IsNumeric { get; }

  // ...
}

public struct Portion
{
  public static readonly Portion Empty;

  public Value Value { get; }

  // ...
}

I'd be keen to get people's thoughts.

[HaloFour]

I'm not sure I understand the purpose of this proposal. Are you trying to achieve a ref to a ref?

[bill-poole]

No, I want the IsPartlySpecifiedNumeric method to return 'true' if the portions in the _portions array field meet the criteria of the ApportionedValue being a "partly-specified numeric" (which is an ApportionedValue with exactly two portions, where one portion is the specified portion and the other portion is the unspecified portion) and have the out parameter be a ref to the specified portion, which is one of the elements in the _portions array.

If I were to do this as a regular out parameter (i.e. "out Portion portion" instead of "out ref readonly Portion portion", then the specified portion element in the _portions array would be copied into the out parameter, rather than just a reference to that portion.

[HaloFour]

out already means ref, just with additional rules that the compiler assign a value.

[bill-poole]

But isn't it a ref to a value (i.e. a Portion instance) that has been allocated on the stack prior to the method being invoked? I don't want an empty Portion instance to be allocated on the stack and then copied into by the method from the relevant element in the _portions array. I want the out value to be a 64-bit pointer (on a 64-bit system) to the relevant element in the _portions array.

[YairHalberstadt]

Out is always passed by ref.

[bill-poole]

Well, you could have a method that does something rather expensive and the output of that method is two or more "ref return" outputs. If you did that as two calls, then you'd need to do that rather expensive thing twice.

But let me say, the ugly syntax I mentioned isn't my primary motivator here. Ultimately when I'm designing/writing high performance code and I establish a way that it should theoretically be possible to work, but the C# language syntax or class libraries get in the way, it introduces unnecessary overheads - typically in the form of unnecessary copying of memory.

That as I understand it was the motivation for the wonderful additions to the language and class libraries in recent years - making specific reference to the features added in C# 7, 7.1, 7.2 and 7.3, and also Span, etc. It's been a great journey with a lot of progress. I just hope we one day reach a point with the language and class libraries where we have the expressiveness we need to direct the machine to do what we want/need without constraint (without threatening memory safety that is).

[bill-poole]

Right. That would likely be the preferred way for this to go for us. this use case woudl then just fall out cleanly.

Yes, this would (hopefully) allow the following solution (or something along these lines - apologies, I'm not really fully up to speed on the current state of the proposal for ref fields in ref structs).

public ref struct PartlySpecifiedNumeric
{
  public readonly ref Portion SpecifiedPortion;

  public PartlySpecifiedNumeric(ref readonly Portion specifiedPortion)
  {
    SpecifiedPortion = ref specifiedPortion;
  }
}

public bool IsPartlySpecifiedNumeric(out PartlySpecifiedNumeric partlySpecifiedNumeric)
{
  partlySpecifiedNumeric = new PartlySpecifiedNumeric(ref Portion.Empty);
  if (_portions.Length != 2)
  {
    return false;
  }

  if (!HasUnspecifiedPortion)
  {
    return false;
  }

  ref readonly var portion = ref _portions[1 - UnspecifiedPortionIndex];
  if (portion.Value.IsNumeric)
  {
    partlySpecifiedNumeric = new PartlySpecifiedNumeric(ref portion);
    return true;
  }

  return false;
}

This would then be invoked as follows:

if (apportionedValue.IsPartlySpecifiedNumeric(out var partlySpecifiedNumeric)
{
  // Do something with partlySpecifiedNumeric.SpecifiedPortion
}

It's not really clear to me what the ref fields of a ref struct that can contain ref fields would be initialised to for a default instance of the ref struct. Maybe the compiler will prevent that from happening?

[bill-poole]

Thanks also to @CyrusNajmabadi for your help with this.

[bill-poole]

Would it be possible to use the (currently internal) ByReference<T> type to support this proposal? For example, consider the following code.

public void MyMethod(out ref MyStruct x)
  => ref _values[0];

Could that be implemented using ByReference<T> as follows?

public void MyMethod(out ByReference<MyStruct> x)
  => new ByReference<MyStruct>(ref _values[0]);

[bill-poole]

By way of example, the following is valid in C# today.

public readonly ref struct ByReference<T>
{
    private static T Null;

    private readonly Span<T> _span;

    public readonly ref T Value
        => ref (_span.IsEmpty ? ref Null : ref _span[0]);

    public ByReference(ref T value)
    {
        _span = MemoryMarshal.CreateSpan(ref value, 1);
    }
}

public readonly struct MyStruct
{
}

public class Whatever
{
    private MyStruct[] _values = new MyStruct[1];

    public void WriteRefToOutParameter(out ByReference<MyStruct> value)
        => value = new ByReference<MyStruct>(ref _values[0]);

    public void UseRefInParameter(in MyStruct value)
    {
        // Do something with value
    }

    public void GetRefOutParameterAndPassAsInParameter()
    {
        ReturnRefInOutParameter(out var valueRef);
        UseRefInParameter(valueRef.Value);
    }
}