Windows-rs: Support loading dll at runtime

[roblabla]

Sorry if this is out of scope, but it's a cool idea I had: It'd be nice if windows-rs provided a way to generate stubs that load the dlls at runtime and call the function indirectly (using LoadLibraryEx/GetProcAddress), returning an error if the function does not exist.

This would provide an easy way to target older versions of Windows. I imagine the API would look similar to what windows-dll provides.

[thisKai]

I plan to add an option to use this crate in windows-dll under the hood at some point, I'm not sure how much if any extra work it would be to generate windows-rs compatible function signatures though.

[kennykerr]

Thanks for the suggestion @roblabla! We could certainly add an option to generate delay load wrappers, perhaps as an option in the build macro for all or select functions.

[l0calh05t]

Would love to see this too. It even seems windows-rs already does this internally for some functions using the demand_load! macro.

For performance reasons, I think it would make sense to have it limited to a selected set of functions. Does https://github.com/microsoft/win32metadata also include data on the Windows version in which the corresponding API was introduced? If so, this could potentially be done automatically using a minimum version parameter.

[l0calh05t]

Does https://github.com/microsoft/win32metadata also include data on the Windows version in which the corresponding API was introduced? If so, this could potentially be done automatically using a minimum version parameter.

It seems it does in form of SupportedOSPlatform:

[kennykerr]

Yep, I'm planning to find some way to use the SupportedOSPlatform attribute to conditionally delay load. It's very tailored to C# but I'm hopeful I can make it work.

[l0calh05t]

Yep, I'm planning to find some way to use the SupportedOSPlatform attribute to conditionally delay load. It's very tailored to C# but I'm hopeful I can make it work.

In the Win32 metadata file we're looking at here, are there any of these version strings that aren't just a version (series of numbers separated by dots) prefixed by "windows"? From a cursory look in ILspy, I couldn't find any, but that wasn't an exhaustive check. I did find several functions that lacked the appropriate metadata, though 😕

[kennykerr]

Yes, this is a .NET invention that I'll have to parse. Looks like a reasonably closed set of possibilities.