Add custom entry resolver when using DllImportAttribute

[temfordump]

It will be great to add method for setting custom entry resolver when loading native libraries using DllImportAttribute.

Example:

NativeLibrary.SetDllImportEntryResolver(Assembly.GetExecutingAssembly(), (libraryHandle, entryPoint) =>
{
    if (NativeLibrary.TryGetExport(libraryHandle, entryPoint, out IntPtr handle))
    {
        return handle;
    }

    if (NativeLibrary.TryGetExport(libraryHandle, "GetProcAddress", out IntPtr getProcAddressHandle))
    {
        var getProcAddress = Marshal.GetDelegateForFunctionPointer<GetProcAddressDelegate>(getProcAddressHandle);

        return getProcAddress.Invoke(entryPoint);
    }

    return IntPtr.Zero;
});

[jkotas]

Could you please explain why you need to dynamically change the names of the native entrypoints? Why can't the code have the right names of the entrypoints in the first place?

[temfordump]

I don't want to change the entry point dynamically.
For a example some of OpenGL static methods are extensions and are not exported and cannot be loaded using NativeLibrary.TryGetExport and is required to load wglGetProcAddress and call it to get the actual pointer to the method.

[temfordump]

It can also be used to mock some missing/unsupported entry point.

[DaZombieKiller]

I believe it would also be useful to pass in the DllImport's associated MethodInfo so you can use attributes to control how these resolvers would behave.

// use SDL_GL_GetProcAddress instead of wglGetProcAddress
[GLImportOptions(GLImportOptions.SDL), DllImport("OpenGL32")]
unsafe static extern void glGenFramebuffers(uint size, uint* frameBuffers);

[jkotas]

For a example some of OpenGL static methods are extensions and are not exported and cannot be loaded using NativeLibrary.TryGetExport and is required to load wglGetProcAddress and call it to get the actual pointer to the method.

You can do this today by wrapping the pointer using a delegate. C# 9 is adding support for function pointers that will allow it without the delegate overhead.

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