Q: How to use UnmanagedCallersOnlyAttribute?

[hypeartist]

Is there any info on how to use that new attribute?
All I have found is:

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/UnmanagedCallersOnlyAttribute.cs

Line 8 in f185bd5

/// Any method marked with <see cref="System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute" /> can be directly called from

I have tried to apply it to some static POD method of a class library but what next? It doesn't show up in exports (used some third-party tools to check) and also I can't obtain its address having dll's handle via GetProcAddress.

Thanks in advance for any clarification.
PS: using .NET 5 Preview 7

[Dotnet-GitSync-Bot]

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[jkotas]

cc @AaronRobinsonMSFT

[AaronRobinsonMSFT]

@hypeartist The UnmanagedCallersOnlyAttribute is used to produce the most lite weight version of a function pointer to a managed function that can be call outside the runtime. The ability to create function pointers in C# was recently exposed in C# 9 - see proposal and tracking issue dotnet/roslyn#39865. Consuming function pointers isn't the only way this feature can be used, but is definitely the most obvious utilization of this attribute.

Another way to use this attribute is via the low level hosting APIs - load_assembly_and_get_function_pointer (PR #35763). Leveraging this low level API is how the DNNE package creates a native export.

This attribute is really a low level building block for tooling or providing symmetry for C# function pointers.

[tomrus88]

So this attribute itself doesn't create native export and you need to go extra mile if you need one?

Is it safe to use C# function pointers feature to get a pointer to C# method and manually pass that pointer to native code (using pinvoke or something)?

What's the difference compared to Marshal.GetFunctionPointerForDelegate?

[AaronRobinsonMSFT]

So this attribute itself doesn't create native export and you need to go extra mile if you need one?

Correct. It is an attribute that indicates to the JIT that this function will only be called from a non-managed context and therefore you should add that transition logic.

Is it safe to use C# function pointers feature to get a pointer to C# method and manually pass that pointer to native code (using pinvoke or something)?

Yes. That is exactly the intent - the function itself and subsequently the pointer can only be dispatched to in an unmanaged way. Note that the calli instruction does permit an unmanaged dispatch.

What's the difference compared to Marshal.GetFunctionPointerForDelegate?

Mostly performance and an allocation. Using Marshal.GetFunctionPointerForDelegate requires a delegate allocation but using the C# function pointers update to address-of (i.e. &) the address can be taken of a manged function with the above attribute and then immediately passed to native instead of allocating a delegate.