How to use a COM VARIANT in Rust winapi programming?

Question

I'm trying to convert the C++ COM code for TaskScheduler to Rust and am stuck with the VARIANT argument of ITaskService::Connect:

extern crate winapi;
use winapi::{
    ctypes::c_void,
    shared::{
        guiddef::{GUID, REFCLSID, REFIID},
        ntdef::{HRESULT, NULL},
        rpcdce::{RPC_C_AUTHN_LEVEL_PKT_PRIVACY, RPC_C_IMP_LEVEL_IMPERSONATE},
        winerror::FAILED,
        wtypes::VARENUM,
        wtypesbase::CLSCTX_INPROC_SERVER,
    },
    um::{
        combaseapi::{CoCreateInstance, CoInitializeEx, CoInitializeSecurity, CoUninitialize},
        oaidl,
        objbase::COINIT_MULTITHREADED,
        taskschd::{ITaskService, TaskScheduler},
    },
    Class, Interface,
}; // 0.3.7

fn main() {
    //  Create an instance of the Task Service.
    let mut p_service: *mut ITaskService = unsafe { std::mem::zeroed() };

    let hr = unsafe {
        CoCreateInstance(
            &TaskScheduler::uuidof(),
            std::ptr::null_mut(),
            CLSCTX_INPROC_SERVER,
            &ITaskService::uuidof(),
            &mut p_service as *mut *mut ITaskService as *mut *mut c_void,
        )
    };

    // [...]
    // //  Connect to the task service.
    // let hr = pService->Connect(_variant_t(), _variant_t(),
    //     _variant_t(), _variant_t());

    let hr = unsafe {
        (&mut *p_service).Connect(
            oaidl::VARIANT { /* VT_EMPTY HERE */ },
            oaidl::VARIANT { /* VT_EMPTY HERE */ },
            oaidl::VARIANT { /* VT_EMPTY HERE */ },
            oaidl::VARIANT { /* VT_EMPTY HERE */ },
        )
    };
}

The docs say:

_variant_t( ) Constructs an empty _variant_t object, VT_EMPTY.

I found no mention on how to use the variant as stated in the README:

Use std::mem::zeroed() to create an instance of the union, and then assign the value you want using one of the variant methods.

When checking the docs for "variant", there's no _variant_t fn on the list, so not sure what it is called in Rust

Errors

field `0` of struct `winapi::um::oaidl::VARIANT_n1` is private

field `0` is private

for

  oaidl::VARIANT {
    n1: oaidl::VARIANT_n1 {
      0: [0, 0, 0]
    }
  }

_variant_t is a c++ class so I suppose you must bind something to one of its constructor. Bindgen should be able to do it for you. How do you bind all this API so far ? Your question lack of detail. — Stargateur
– Stargateur, Commented Jun 12, 2019 at 23:58
_variant_t is just a C++ helper class that wraps a COM VARIANT, it automates memory management and reference counting for VARIANT values. _variant_t doesn't exist in other languages, like Rust, so you will have to construct a VARIANT manually and populate it as needed. — Remy Lebeau
– Remy Lebeau, Commented Jun 13, 2019 at 0:07
A COM VARIANT is a tagged union, have a look at How Rust Implements Tagged Unions and A quick tour of Rust’s Type System Part 1: Sum Types (a.k.a. Tagged Unions) — Remy Lebeau
– Remy Lebeau, Commented Jun 13, 2019 at 0:08
to be completely clear: I'm stuck and have not made any progress, there's no reproducible example because of this, and winapi::um must have winuser enabled in Cargo features. VARIANT_n1 is private and can't be used when using VARIANT { n1: /*...*/ }. — pocesar
– pocesar, Commented Jun 13, 2019 at 21:55
@pocesar did you ever find a solution? Im trying to create a scheduled task in windows with rust as well but no luck so far. Can you help? — j-hc
– j-hc, Commented Apr 8, 2021 at 16:52

Francis Gagné · Accepted Answer · 2020-02-25 01:59:07Z

2

Initializing unions is explained in winapi's README:

How do I create an instance of a union?

Use std::mem::zeroed() to create an instance of the union, and then assign the value you want using one of the variant methods.

For instance, for VARIANT_n1, the methods are n2, n2_mut, decVal and decVal_mut. You will naturally need to use a _mut method to assign a value.

Note that, starting with winapi 0.3.7, you can also use Default::default() to obtain a zeroed union instead of using the unsafe std::mem::zeroed(). You need to enable the impl-default feature on the winapi crate for the Default trait to be implemented.

edited Feb 25, 2020 at 1:59

answered Jun 14, 2019 at 4:25

Francis Gagné

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4 Comments

pocesar Over a year ago

the trait 'std::default::Default' is not implemented for '*mut winapi::um::taskschd::ITaskService' and the trait 'std::default::Default' is not implemented for 'winapi::um::oaidl::VARIANT_n1'. while searching for those methods, it seems that they are defined in another external crate, called oaidl (that makes working with those variants less painful)

Stargateur Over a year ago

But _variant_t is not an union. It's class or I miss something

Stargateur Over a year ago

@pocesar Default is implemented for VARIANT_N1.

Francis Gagné Over a year ago

@pocesar Make sure you're using winapi 0.3.7 or later. The Default implementations were added in 0.3.7. Use cargo update to update your dependencies.

Jannis Froese · Accepted Answer · 2021-04-20 18:41:17Z

0

You can construct a variant with

use winapi::um::oaidl::VARIANT;

fn doStuff() {
  unsafe {
    let mut variant: VARIANT = std::mem::zeroed();
    // set values here
  }
}

variant.n1.n2().vt contains the type (e.g. winapi::shared::wtypes::VT_BOOL or winapi::shared::wtypes::VT_EMPTY, msdn describes the possible values here), the variant.n1.n2().n3 union contains the value.

answered Apr 20, 2021 at 18:41

Jannis Froese

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1 Comment

j-hc Over a year ago

Thanks for taking your time but I am still unable to instantiate a variant with a bstr value in order to pass as an userId to RegisterTaskDefinition()

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