I think this was included for compatibility with IronPython. How would you handle out parameters that still have automatic conversions likeint andstr?

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Merging#1308 (ff89811) intomaster (57f4a80) willdecrease coverage by13.84%.
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@filmor this is the confusing part. This change does not affectGetInt(out int val) functions. It affectsGetInt([Out] int val) functions, where[Out] parameter does nothing. The only real use case I see is that inRead([Out] byte[] buf)[Out] indicates, that the data will be put into thebuf. The data can not be put intostring orint withoutref int/out int, and these are handled separately.

This annotation is normally meant for P/Invoke marshaling scenarios, where you have to add[Out] tobyte[] arr into PInvoke signature for data to be properly marshaled back intoarr upon exiting C function. I am not even sure whyMemoryStream.Read has this annotation except for informational purposes. I believe passing Python lists in place ofbuf is why it was added, but considering lack of[Out] annotations in most places, I don't think it is a good thing to rely on.

For an instance of API difference between implementations in regards to[Out] attribute:

In .NET CoreMemoryStream.Read has none:https://github.com/dotnet/runtime/blob/5432a66018c2499b88a54276f468ea903ed2b8de/src/libraries/System.Private.CoreLib/src/System/IO/MemoryStream.cs#L333

In Mono it is present for COM-compatibility:https://github.com/mono/mono/blob/c5b88ec4f323f2bdb7c7d0a595ece28dae66579c/mcs/class/referencesource/mscorlib/system/io/memorystream.cs#L339

Without this change the same Python code would not work with both Mono and .NET Core.

Thanks for the explanation. I ran into this problem while testing .NET Core and couldn't really make sense of it, wasn't aware of[Out].