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Clarify the rules on UnmanagedCallersOnly #3823

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Merged
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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21 changes: 14 additions & 7 deletions proposals/csharp-9.0/function-pointers.md
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Original file line number Diff line number Diff line change
Expand Up @@ -39,8 +39,8 @@ The ECMA-335 definition of method pointers includes the calling convention as pa
The default calling convention will be `managed`. Unmanaged calling conventions can by specified by putting an `unmanaged`
keyword afer the `delegate*` syntax, which will use the runtime platform default. Specific unmanaged conventions can then
be specified in brackets to the `unmanaged` keyword by specifying any type starting with `CallConv` in the
`System.Runtime.CompilerServices` namespace. These types must come from the program's core library, and the set of valid
combinations is platform-dependent.
`System.Runtime.CompilerServices` namespace, leaving off the `CallConv` prefix. These types must come from the program's
core library, and the set of valid combinations is platform-dependent.

``` csharp
//This method has a managed calling convention. This is the same as leaving the managed keyword off.
Expand All @@ -51,10 +51,13 @@ delegate* managed<int, int>;
delegate* unmanaged<int, int>;

// This method will be invoked using the cdecl calling convention
delegate* unmanaged[CallConvCdecl] <int, int>;
// Cdecl maps to System.Runtime.CompilerServices.CallConvCdecl
delegate* unmanaged[Cdecl] <int, int>;

// This method will be invoked using the stdcall calling convention, and suppresses GC transition
delegate* unmanaged[CallConvStdCall, CallConvSuppressGCTransition] <int, int>;
// Stdcall maps to System.Runtime.CompilerServices.CallConvStdcall
// SuppressGCTransition maps to System.Runtime.CompilerServices.CallConvSuppressGCTransition
delegate* unmanaged[Stdcall, SuppressGCTransition] <int, int>;
```

Conversions between `delegate*` types is done based on their signature including the calling convention.
Expand Down Expand Up @@ -358,9 +361,13 @@ 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.
Expand Down