73 lines
2.4 KiB
Markdown
73 lines
2.4 KiB
Markdown
|
# Static Delegates
|
||
|
|
|
||
|
|
* [x] Proposed
|
||
|
|
* [ ] Prototype: Not Started
|
||
|
|
* [ ] Implementation: Not Started
|
||
|
|
* [ ] Specification: Not Started
|
||
|
|
|
||
|
|
## Summary
|
||
|
|
[summary]: #summary
|
||
|
|
|
||
|
|
Provide a general-purpose, lightweight callback capability to the C# language.
|
||
|
|
|
||
|
|
## Motivation
|
||
|
|
[motivation]: #motivation
|
||
|
|
|
||
|
|
Today, users have the ability to create callbacks using the `System.Delegate` type. However, these are fairly heavyweight (such as requiring a heap allocation and always having handling for chaining callbacks together).
|
||
|
|
|
||
|
|
Additionally, `System.Delegate` does not provide the best interop with unmanaged function pointers, namely due being non-blittable and requiring marshalling anytime it crosses the managed/unmanaged boundary.
|
||
|
|
|
||
|
|
With a few minor tweaks, we could provide a new type of delegate that is lightweight, general-purpose, and interops well with native code.
|
||
|
|
|
||
|
|
## Detailed design
|
||
|
|
[design]: #detailed-design
|
||
|
|
|
||
|
|
One would declare a static delegate via the following:
|
||
|
|
|
||
|
|
```C#
|
||
|
|
static delegate int Func()
|
||
|
|
```
|
||
|
|
|
||
|
|
One could additionally attribute the declaration with something similar to `System.Runtime.InteropServices.UnmanagedFunctionPointer` so that the calling convention, string marshalling, and set last error behavior can be controlled. NOTE: Using `System.Runtime.InteropServices.UnmanagedFunctionPointer` itself will not work, as it is only usable on Delegates.
|
||
|
|
|
||
|
|
The declaration would get translated into an internal representation by the compiler that is similar to the following
|
||
|
|
|
||
|
|
```C#
|
||
|
|
struct <name>
|
||
|
|
{
|
||
|
|
IntPtr pFunction;
|
||
|
|
|
||
|
|
static int Func();
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
That is to say, it is internally represented by a struct that has a single member of type `IntPtr` (such a struct is blittable and does not incur any heap allocations). The member contains the address of the function that is to be the callback. Additionally, the type declares a method matching the method signature of the callback.
|
||
|
|
|
||
|
|
The value of the static delegate can only be bound to a static method that matches the signature of the callback.
|
||
|
|
|
||
|
|
Chaining callbacks together is not supported.
|
||
|
|
|
||
|
|
Invocation of the callback would be implemented by the `calli` instruction.
|
||
|
|
|
||
|
|
|
||
|
|
## Drawbacks
|
||
|
|
[drawbacks]: #drawbacks
|
||
|
|
|
||
|
|
TBD
|
||
|
|
|
||
|
|
## Alternatives
|
||
|
|
[alternatives]: #alternatives
|
||
|
|
|
||
|
|
TBD
|
||
|
|
|
||
|
|
## Unresolved questions
|
||
|
|
[unresolved]: #unresolved-questions
|
||
|
|
|
||
|
|
What parts of the design are still TBD?
|
||
|
|
|
||
|
|
## Design meetings
|
||
|
|
|
||
|
|
Link to design notes that affect this proposal, and describe in one sentence for each what changes they led to.
|
||
|
|
|
||
|
|
|