Dynamic casting to COM implementation #3055

sivadeilra · 2024-05-23T21:57:29Z

This provides a new feature for COM developers using the windows-rs crate. It allows for safe dynamic casting from IUnknown to an implementation object. It is based on Rust's Any trait.

Any type that is marked with #[implement], except for those that contain non-static lifetimes, can be used with dynamic casting.

Example:

#[implement(IFoo)]
struct MyApp { ... }

fn main() {
    let my_app = ComObject::new(MyApp { ... });
    let iunknown: IUnknown = my_app.to_interface();
    do_stuff(&iunknown);
}

fn do_stuff(unknown: &IUnknown) -> Result<()> {
    let my_app: ComObject<MyApp> = unknown.cast_object()?;
    my_app.internal_method();
    Ok(())
}

kennykerr · 2024-05-24T12:36:46Z

As discussed offline, we can extend the identity vtable pointer such that IDynamicCast inherits from IInspectable so that the instance size remains the same and ideally optimize interlocked cost as appropriate.

windows-rs/crates/libs/implement/src/lib.rs

Line 132 in 40d35fa

identity: &'static ::windows_core::IInspectable_Vtbl,

crates/libs/core/src/dynamic_cast.rs

crates/libs/implement/src/lib.rs

This provides a new feature for COM developers using the windows-rs crate. It allows for safe dynamic casting from IUnknown to an implementation object. It is based on Rust's Any trait. Any type that is marked with #[implement], except for those that contain non-static lifetimes, can be used with dynamic casting. Example: ```rust struct MyApp { ... } fn main() { let my_app = ComObject::new(MyApp { ... }); let iunknown: IUnknown = my_app.to_interface(); do_stuff(&iunknown); } fn do_stuff(unknown: &IUnknown) -> Result<()> { let my_app: ComObject<MyApp> = unknown.cast_object()?; my_app.internal_method(); Ok(()) } ```

This provides a new feature for COM developers using the windows-rs crate. It allows for safe dynamic casting from IUnknown to an implementation object. It is based on Rust's Any trait. Any type that is marked with #[implement], except for those that contain non-static lifetimes, can be used with dynamic casting. Example: ```rust struct MyApp { ... } fn main() { let my_app = ComObject::new(MyApp { ... }); let iunknown: IUnknown = my_app.to_interface(); do_stuff(&iunknown); } fn do_stuff(unknown: &IUnknown) -> Result<()> { let my_app: ComObject<MyApp> = unknown.cast_object()?; my_app.internal_method(); Ok(()) } ``` Co-authored-by: Arlie Davis <[email protected]>

sivadeilra marked this pull request as ready for review May 23, 2024 23:16

kennykerr changed the title ~~Dynamic casting to implementation~~ Dynamic casting to COM implementation May 24, 2024

kennykerr reviewed May 24, 2024

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