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Clarify the rules on UnmanagedCallersOnly #3823

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merged 4 commits into from
Aug 26, 2020
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Clarify the rules on UnmanagedCallersOnly #3823

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7ff6cb3
Clarify the rules on UnmanagedCallersOnly
333fred Aug 25, 2020
55df4c7
Update with feedback
333fred Aug 25, 2020
3e179c0
Address PR feedback:
333fred Aug 26, 2020
3213aa0
Update incorrect examples.
333fred Aug 26, 2020
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10 changes: 7 additions & 3 deletions proposals/csharp-9.0/function-pointers.md
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Expand Up @@ -358,9 +358,13 @@ the attribute:

* It is an error to directly call a method annotated with this attribute from C#. Users must obtain a function pointer to
the method and then invoke that pointer.
* It is an error to apply the attribute to anything other than a static method. The C# compiler will mark any non-static
methods imported from metadata with this attribute as unsupported by the language.
* It is an error to have managed types as parameters or the return type of a method marked with the attribute.
* It is an error to apply the attribute to anything other than an ordinary static method or ordinary static local function.
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Just wondering, but why are operators/properties restricted here? Is it just a language limitation, similar to binding against a signature that takes explicit this for instance methods?

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Correct, this is a language limitation.

The C# compiler will mark any non-static or static non-ordinary methods imported from metadata with this attribute as
unsupported by the language.
* It is an error for a method marked with the attribute to have a parameter or return type that is not an `unmanaged_type`.
* It is an error for a method marked with the attribute to have type parameters, even if those type parameters are
constrained to `unmanaged`.
* It is an error for a method in a generic type to be marked with the attribute.
* It is an error to convert a method marked with the attribute to a delegate type.
* It is an error to specify any types for `UnmanagedCallersOnly.CallConvs` that do not meet the requirements for calling
convention `modopt`s in metadata.
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