Invalid codegen for Enumerable.Empty returned from ParamsSource

[BoundedChenn31]

Description:
Attempt to parameterize property of type IEnumerable<T> with Enumerable.Empty<T>() or Enumerable.Repeat<T>(element, count) using ParamsSource attribute causes compilation error in codegen compilation error CS0266: Cannot implicitly convert type 'object' to 'System.Collections.Generic.IEnumerable'. An explicit conversion exists (are you missing a cast?) because of lines like

instance.A = (System.Object)BenchmarkDotNet.Parameters.ParameterExtractor.GetParameter(instance.ValuesForA, 1);;

Benchmark code:

public class ParamsSourceIntro
{
    public IEnumerable<IEnumerable<int>> ValuesForA()
    {
        yield return Enumerable.Empty<int>();
        yield return Enumerable.Repeat(1, 1);
    }

    [ParamsSource(nameof(ValuesForA))]
    public IEnumerable<int> A { get; set; }

    [Benchmark]
    public int[] Benchmark() => A.ToArray();
}

class Program
{
    static void Main(string[] args)
    {
        var _ = BenchmarkRunner.Run<ParamsSourceBug>();
    }
}

Logs:
Log with compilation error: BenchmarkDotNetParamsSourceBug.ParamsSourceBug-20210923-021529.log
Autogenerated project: 397b1fac-e83d-4694-8454-8c0fcba111a5.zip

Workarounds:

1. Use ArgumentsSource attribute;
2. Change type of parameterized property to object and cast it back later;
3. Wrap IEnumerable in it's own type record Wrapper<T>(IEnumerable<T> Collection).

[adamsitnik]

Hi @BoundedChenn31

Does it work if you apply the following change:

- public IEnumerable<IEnumerable<int>> ValuesForA()
+ public IEnumerable<object> ValuesForA()

[BoundedChenn31]

Nope, the same error.

Not sure if it helps, but I was able to reproduce the issue following the same pattern Enumerable.Empty does.

Declare in another assembly internal type and expose it through interface:

// another assembly
internal class MyInternalEnumerable<T> : IEnumerable<T> { ... }
public class MyPublicEnumerable<T> : IEnumerable<T> { ... }
internal class MyDerivedEnumerable<T> : MyPublicEnumerable<T> {  }

public static class MyEnumerable
{
    public static IEnumerable<T> InternalEmpty<T>() => new MyInternalEnumerable<T>();
    public static IEnumerable<T> PublicEmpty<T>() => new MyPublicEnumerable<T>();
    public static IEnumerable<T> DerivedEmpty<T>() => new MyDerivedEnumerable<T>();
}

// Benchmark assembly
public class ParamsSourceIntro
{
    public IEnumerable<object> ValuesForA() // or IEnumerable<IEnumerable<int>>, doesn't matter
    {
        yield return MyEnumerable.InternalEmpty<int>();
        // yield return MyEnumerable.PublicEmpty<int>();
        // yield return MyEnumerable.DerivedEmpty<int>();
    }

    [ParamsSource(nameof(ValuesForA))]
    public IEnumerable<int> A { get; set; }

    [Benchmark]
    public int[] Benchmark() => A.ToArray();
}

class Program
{
    static void Main(string[] args)
    {
        var _ = BenchmarkRunner.Run<ParamsSourceIntro>();
    }
}

For public and internal derived from public types codegen contains line following line and runs ok:

instance.A = (AnotherAssembly.MyPublicEnumerable<System.Int32>)BenchmarkDotNet.Parameters.ParameterExtractor.GetParameter(instance.ValuesForA(), 0);;

But for MyInternalEnumerable<T> the Object type is the only public base type and that results in

instance.A = (System.Object)BenchmarkDotNet.Parameters.ParameterExtractor.GetParameter(instance.ValuesForA(), 0);;