Generic new constraint instead of DynamicallyAccessedMembers

[Aziz-Dhiab]

I was working with Activator.CreateInstance<T>() and was looking for examples of how it is used.

I found a lot of public generic methods that uses it and decorate the generic parameter with [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicParameterlessConstructor)]

This got me curious. Is there a reason for not adding where T : new() so that calling such method cause a compilation error instead of a runtime error.

Example:

public sealed class MyObject
{
    public MyObject(int id) { }
}

MyObjecto = null;
_ = LazyInitializer.EnsureInitialized<MyObject>(ref o);

[MichalStrehovsky]

Is there a reason for not adding where T : new() so that calling such method cause a compilation error instead of a runtime error.

You mean a reason we didn't add where T : new() to Activator.CreateInstance<T>? It would be a breaking change at this point. It's very unfortunate the API wasn't decorated with T : new() from the beginning, but that ship has sailed.

We didn't want to break everyone just because of trimming (it would be breaking for everyone, whether they use trimming or not). For trimming analysis, new() and DynamicallyAccessedMemberTypes.PublicParameterlessConstructor are interchangeable (the T that is constrained with new() will meet DynamicallyAccessedMemberTypes.PublicParameterlessConstructor requirement so one can pass such typeof(T) to APIs that are marked with DynamicallyAccessedMembers without having to also put DynamicallyAccessedMemberTypes.PublicParameterlessConstructor on the T itself).

[Aziz-Dhiab]

Thank you for you explanation, so just to clarify the breaking change is the compile time error that would have been a runtime error anyways or is it a binary breaking because adding where T : new() will produce a different IL

[MichalStrehovsky]

Yes, compile time error for something that might not actually be reachable at runtime. E.g.

public object Activate<T>()
    => CanUseDefaultConstructor<T>() ? Activator.CreateInstance<T>() : CustomFactory<T>();