TupleOverloadGenerator 1.0.0

TupleOverloadGenerator

Supported

• .NETCoreApp 3.1
• .NET 6.0 or greater
• .NETStandard 2.1 with System.Memory and System.Runtime.CompilerServices.Unsafe packages installed.

This is experimental and uses undefined behaviour! Only tested on Linux

Motivation

When producing a library we often which to allow a variable number of arguments to be passed to a given function, such as string Concatenation. Historically the params keyword followed by an array type string[] has been to conveniently indroduce a parameter with a variable number of arguments. However a array introduces a few problems, the most prevalent of which is that the array is allocated on the heap.

Modern libraries should therefore allow the user to pass a Span instead of an array, this approach is the most performant, yet calling the function is inconvenient and still requires a heap allocation for non managed, blittable types, where stackalloc is not usable.

Span<string> parts = stackalloc string[12];
parts[0] = "Hello"; [...]
AffixConcat concat = new("[", "]");
return concat.Concat(parts);

The solution is overloads with inline arrays. These can be represented by tuples with a single generic type parameter. This source generator generates these overloads for parameters decorated with the [TupleOverload] attribute.

AffixConcat concat = new("[", ", ", "]");
return concat.Concat(("Hello", "World"));

Example

using System;
using System.Text;

namespace BasicUsageExamples;

public partial readonly struct record AffixConcat(string Prefix, string Infix, string Suffix) {
    public string Concat(ReadOnlySpan<string> parts) {
        StringBuilder sb = new();
        sb.Append(Prefix);
        var en = parts.GetEnumerator();
        if (en.MoveNext()) {
            sb.Append(en.Current);
            while (en.MoveNext()) {
                sb.Append(Infix);
                sb.Append(en.Current);
            }
        }
        sb.Append(Suffix);
        return sb.ToString();
    }

    public string Concat([TupleOverload] params string[] parts) {
        ReadOnlySpan<string> partsSpan = parts.AsSpan();
        return ConcatInternal(partsSpan);
    }
}

The above example displays the required conditions required to use the attribute.

1. A namespace directly containing a type definition. Ommited namespace doesnt allow partial definitions, and nested types are not supported.
2. The partial type definition, e.g. sealed partial class, partial record, partial ref struct, ...
3. A method with a parameter array, e.g. params string[].
4. The parameter array decorated with the [TupleOverload] attribute.
5. The parameter exclusively called like with the AsSpan extension method. No other member can be used!

Please note that the example above is for demonstration purposes only! I advice using a ref struct and a ValueStringBuilder for real world applications.

Behind the scenes

Primarly the source generator replaces the parameter type with a given tuple type (e.g (string, string, string)) for two through seven parameters. Tuples are fast for 2-7 parameters.

Q: Tuples cannot be cast to a span can they? No, they cannot. At least not trivially. To obtain a span from a tuple, we have to cheat, and by cheat I mean unsafe hacks that may not work in the future.

The source generator adds the following extension methods to the value types types with 2-7 parameters:

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
[...]
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static ref T GetPinnableReference<T>(in this ValueTuple<T, T> tuple) {
    return ref Unsafe.AsRef(in tuple).Item1;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static ReadOnlySpan<T> AsSpan<T>(in this ValueTuple<T, T> tuple) {
    return MemoryMarshal.CreateReadOnlySpan(ref tuple.GetPinnableReference(), 2);
}

GetPinnableReference

GetPinnableReference returns a reference to the first element in the tuple, treating it as a linline array. This is unsafe, because RYU may reorder the the items, so that the following layout applies:

[Item2][Padding][Item1][padding][Item3][padding]

If the structure has padding, or is reordered this will not work! Therefore its best used with pointer sized values, such as nint, object, Func<>, etc..

AsSpan

As span creates a span from the reference to the first element in the tuple with length equal to the number of elements in the tuple.

The primary issue here is that MemoryMarshal.CreateReadOnlySpan is not speified to work with a tuple! At some point Microsoft may deicide that this should throw an exception instead of succeeding. We are working with undefined behaviour here!

Other then that the in keyword for the parameter too can be a problem. It specifies that the readonly-reference to the struct is passed instead of the struct itself. In and of itself this is not a problem, but the memory analyzer will complain when returing the span to a different context.

All in all I have tested this with 3.1.423, 6.0.401 and 7.0.0-rc.2.22472.3 on Linux. This is untested on MacOs and Windows.