Testing parameter types: IEnumerable<string> versus List<string>

Benchmark.cs

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Engines;
using BenchmarkDotNet.Running;

_ = BenchmarkRunner.Run<TestParamType>();

[MemoryDiagnoser]
public class TestParamType
{
    private List<string> list = new();
    private Consumer consumer = new();

    [GlobalSetup]
    public void Setup()
    {
        list.AddRange(Enumerable.Range(1, 1_000_000).Select(x => Guid.NewGuid().ToString()));
    }

    [Benchmark(Baseline = true)]
    public void Passing_IEnumerable()
    {
        Test(list);

        void Test(IEnumerable<string> list)
        {
            foreach (var item in list)
            {
                consumer.Consume(item);
            }
        }
    }

    [Benchmark]
    public void Passing_List()
    {
        Test(list);

        void Test(List<string> list)
        {
            foreach (var item in list)
            {
                consumer.Consume(item);
            }
        }
    }
}

Hello @JerryNixon thanks for you time to take a look into this.

I just take a look into .net 8's implementation for the List<> IEnumerable<T>.GetEnumerator() and the allocation
is supposed to happen as it calls GetEnumerator() which will return a Enumerator and that Enumerator will then be casted
to IEnumerable which is supposed to cause the boxing (for what I learn boxing happen whenever a value is type implicitly converted to
a Interface)

Ps: Maybe your Benchmark setup is missing something?

I think this can also help investigating the issue

https://stackoverflow.com/questions/3032750/structs-interfaces-and-boxing/3033357#3033357

https://stackoverflow.com/a/3032765/2766753 (Jon Skeet)