ActivatorUtilities.CreateInstance is creating non-empty generic collections

[miguelnunes94]

Description

Generic collections that have copy constructors (with IEnumerable<T> as parameter) are initialized with elements, when using ActivatorUtilities.CreateInstance/ActivatorUtilities.GetServiceOrCreateInstance.

Reproduction Steps

    public interface ITest { }

    public class Test : ITest { }

    public class Test2 : ITest { }

    public static void Main(string[] args)
    {

        var serviceCollection = new ServiceCollection();

        serviceCollection.AddTransient<ITest, Test>();
        serviceCollection.AddTransient<ITest, Test2>();

        var services = serviceCollection.BuildServiceProvider();

        var collection = ActivatorUtilities.CreateInstance<List<ITest>>(services);
    }

Expected behavior

The collection object is an empty instance of List, because ActivatorUtilities.CreateInstance uses the constructor public List()

Actual behavior

The collection object is an instance of List containing two elements: one instance of Test and one of Test2. Currently, ActivatorUtilities.CreateInstance is selecting the constructor public List(IEnumerable<T> collection).

Regression?

We found this issue while upgrading from .Net 6 to .Net 8

Known Workarounds

No response

Configuration

.Net SDK 8.0.403

Other information

In such cases, the parameterless constructor may be preferable - particularly for generic collections with copy constructors. Consider annotating the expected constructors with the ActivatorUtilitiesConstructorAttribute to ensure the correct constructor is chosen.

It should be related with:

• Type activator should be order agnostic #67493
• ActivatorUtilities not depending on ctor order for creating instances #75846

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[julealgon]

If you want an empty list... why go through ActivatorUtilities for that?

[miguelnunes94]

The ActivatorUtilities is being used in a serialization & deserialization context where many different types are involved, including system generic collections. We used to have a clean solution by using the it.