Constructor ambiguity fallback

[cwinland]

Description:
Improve constructor selection logic for auto-injection. Features:

• Try alternate constructors if default fails
• Fallback to parameterless or best-fit constructor
• Log fallback attempts for debugging
• Ensure compatibility with provider abstraction

[cwinland]

This is half-implemented. In certain circumstances, errors will be thrown. However, it will go through the constructors to pick from ones that work.

[cwinland]

Context

When a class under test has multiple constructors that could be satisfied by FastMoq’s injection logic, the current behavior may:

• Pick the wrong constructor.
• Fail with an ambiguous match exception.
• Require manual intervention in tests.

We need a deterministic fallback strategy to handle constructor ambiguity gracefully.

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High‑Level Goals

• Define a constructor selection algorithm that:
  1. Prefers the constructor with the most resolvable parameters.
  2. Breaks ties using explicit developer hints or attributes.
  3. Falls back to parameterless constructors if no match is found.
• Allow per‑test overrides for constructor selection.

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Proposed Structure

1. Constructor Selector

internal static class ConstructorSelector
{
    public static ConstructorInfo FindBestConstructor(Type targetType, Mocker mocker)
    {
        var ctors = targetType.GetConstructors(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);

        // Score constructors by resolvable parameters
        var scored = ctors
            .Select(c => new
            {
                Constructor = c,
                Score = c.GetParameters().Count(p => mocker.CanResolve(p.ParameterType))
            })
            .OrderByDescending(x => x.Score)
            .ToList();

        // Return highest score, break ties by attribute or parameter count
        var best = scored.FirstOrDefault();
        return best?.Constructor ?? targetType.GetConstructor(Type.EmptyTypes);
    }
}

2. Optional Attribute for Explicit Selection

[AttributeUsage(AttributeTargets.Constructor)]
public sealed class PreferredConstructorAttribute : Attribute { }

• If present, this constructor is always chosen.

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Example Usage

public class MyService
{
    public MyService(IDepA depA, IDepB depB) { }
    public MyService(IDepA depA) { }

    [PreferredConstructor]
    internal MyService() { }
}

• Without the attribute, the two‑parameter constructor is chosen if both deps are resolvable.
• With the attribute, the parameterless constructor is chosen.

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Benefits

• Deterministic: Always picks the same constructor given the same conditions.
• Configurable: Developers can override selection via attributes.
• Resilient: Falls back gracefully when no perfect match exists.

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Next Steps:

• Implement ConstructorSelector with scoring and attribute support.
• Integrate into MockerConstructionHelper so both generic and non‑generic bases benefit.
• Add tests for:
  • Multiple constructors with equal scores.
  • Attribute‑driven selection.
  • Fallback to parameterless constructor.
• Document constructor selection rules in Milestone 5.