#windows #ffi #com


Utilities for implementing COM Client and Servers

5 unstable releases

✓ Uses Rust 2018 edition

0.2.0 Apr 9, 2020
0.1.0 Oct 1, 2019
0.0.3 Aug 26, 2017
0.0.2 Jun 10, 2015
0.0.1 Jun 8, 2015

#7 in Windows APIs

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Build Status Gitter Docs.rs

A one stop shop for all things related to COM programming in Rust.

This library exposes various macros, structs and functions to the user for both producing and consuming COM components in an idiomatic manner.

🚨 🚨 🚨 NOTE This crate is currently in heavy development as we decide on a stable API. 🚨 🚨 🚨

What is COM?

COM is a platform-independent, distributed, object-oriented system for creating binary software components that can interact.

COM has been superseded by WinRT which builds on COM to provide even more guarantees about the binary interface. As such, if you're not sure if you need to use COM, you probably shouldn't.


Defining a COM interface

To both consume or produce a COM component through an interface, you will first need to generate the Rust representation of said interface. The com_interface macro is the main tool for automatically generating this Rust representation.

pub trait IUnknown {
    unsafe fn query_interface(
        riid: *const IID,
        ppv: *mut *mut c_void
    ) -> HRESULT;
    fn add_ref(&self) -> u32;
    unsafe fn release(&self) -> u32;

pub trait IAnimal: IUnknown {
    unsafe fn eat(&self) -> HRESULT;

Short explanation: This generates the VTable layout for IUnknown and implements the trait on com::ComRc so that it dereferences the correct function pointer entry within the VTable.

Consuming a COM component

Interaction with COM components are always through an Interface Pointer (a pointer to a pointer to a VTable). We represent such an Interface Pointer with the com::ComRc struct, which helps manage the lifetime of the COM component through IUnknown methods.

use com::run_time::{create_instance, init_runtime};

// Initialises the COM library
init_runtime().expect("Failed to initialize COM Library");

// Get a COM instance's interface pointer, by specifying
// - The CLSID of the COM component
// - The interface of the COM component that you want
// create_instance returns a ComRc<dyn IAnimal> in this case.
let mut cat = create_instance::<dyn IAnimal>(&CLSID_CAT_CLASS).expect("Failed to get a cat");

// All IAnimal methods will be defined on ComRc<T: IAnimal>

Producing a COM component

Producing a COM component is relatively complicated compared to consumption, due to the many features available that we must support. Here, we will walk you through producing one of our examples, the BritishShortHairCat.

  1. Define the struct containing all the user fields you want.
  • Apply the #[co_class(...)] macro to the struct. This will expand the struct into a COM-compatible struct, by adding COM-specific fields.
  • You can then use the attribute argument implements(...) to indicate inheritance of any COM interfaces. The order of interfaces declared is important, as the generated vpointers are going to be in that order.
use com::co_class;

#[co_class(implements(ICat, IDomesticAnimal))]
pub struct BritishShortHairCat {
    num_owners: u32,
  1. Implement the necessary traits on the COM struct (in this case, BritishShortHairCat).
impl IDomesticAnimal for BritishShortHairCat {
    unsafe fn train(&self) -> HRESULT {

impl ICat for BritishShortHairCat {
    unsafe fn ignore_humans(&self) -> HRESULT {
        println!("Ignoring Humans...");

impl IAnimal for BritishShortHairCat {
    unsafe fn eat(&self) -> HRESULT {
  1. You will have to define a constructor with the below signature. This provides us with a standard constructor to instantiate your COM component.
fn new() -> Box<BritishShortHairCat>

Within this constructor, you need to

  • Call the provided BritishShortHairCat::allocate() function, passing in your user fields in the order they were declared. IMPORTANT
  • The allocate function in this case has the signature:
fn allocate(num_owners: u32) -> Box<BritishShortHairCat>
impl BritishShortHairCat {
    pub(crate) fn new() -> Box<BritishShortHairCat> {
        let num_owners = 20;


While COM specifies details about the ABI of method calls, it does little in terms of guranteeing the safety of those method calls. As such, it is left up to the programmer to verify the safety of COM APIs and to write safe wrappers for those APIs.

You can read more about what gurantees this library makes in the guide to safety.

Existing crates

There are many existing Rust crates that help with COM interactions. Depending on your use case, you may find these crates more suited to your needs. For example, we have

  • Intercom, which focuses on providing support for writing cross-platform COM components in Rust.
  • winapi-rs, which provides a straightforward macro that allows you to easily consume COM interfaces.


There are many advanced concepts in COM that our library aim to support. Relevant documentation on these advanced features can be found within the docs folder.


This library is Windows only, so the easiest way to contribute will be on a Windows machine. You can execute the examples like so:

cd examples\basic
cargo run --release

If you are on a Mac or Linux machine, you should still be able to make changes and check that they compile by running the following from the root of the project:

cargo check --target=x86_64-pc-windows-msvc


For further information on contributing, please take a look at the contributing doc

Code of Conduct

This project has adopted the Microsoft Open Source Code of Conduct. You can find out more in the code of conduct doc.


Is there IDL support?

As a foundation, we are attempting to create a library that doesn't necessarily rely on having an IDL file. However, it is in the pipeline for future improvements. We will have a command-line tool that will parse the IDL into the required macros.

Is there out-of-process COM support?

Currently, we only support production of in-process COM components. Also, production of a COM component can only be in the DLL format. There will be plans to enable out-of-process COM production as well as producing in the .EXE format.


~18K SLoC