IAsyncOperation array types in MIDL

[sichbo]

Jeez, who's bugging you guys more than me lately, eh? 🙄 So here's something that ostensibly "should" work in MIDL 3.0 but raises a compilation error. A solution/reason why it doesn't work would be great to have in the intro here.

Busted MIDL:

runtimeclass Thing
{
    Windows.Foundation.IAsyncOperation<Windows.Foundation.Collections.IVector<String>> ListThings();
}

Emits:

error MIDL2025: [msg]syntax error [context]: expecting > or , near ">>"
error MIDL2025: [msg]syntax error [context]: expecting > or , near "("

Another scenario is IAsyncOperation<Anything[]>. It seems like MIDL 3.0 simply doesn't support nested generic types or arrays on any level. If you use IAsyncOperation with a basic type it's fine.

I looked everywhere for an example MIDL returning an IAsyncOperation collection but came up empty. Any insights? or do we have to reinvent the wheel defining a custom collection wrapper every time we need to return an array asynchronously? Seems a bit much..

---

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 2dacbdf2-172f-dd41-829f-94295cd0c621
• Version Independent ID: b511accc-e0b1-9b30-d044-231d5204024d
• Content: Introduction to Microsoft Interface Definition Language 3.0 - Windows UWP applications
• Content Source: winrt-related-src/midl-3/intro.md
• Product: uwp
• GitHub Login: @stevewhims
• Microsoft Alias: stwhi

[stevewhims]

The first issue turns out to be a whitespace issue. I can repro the issue but then if I add whitespace like this, it's fine:

Windows.Foundation.IAsyncOperation< Windows.Foundation.Collections.IVector< String > > ListThingsAsync();

This used to be an issue with C++ templates, too (not sure whether it still is). I got into the habit of adding a single space of padding inside all my angle brackets.

And, I've confirmed that it's not valid to use an array in that context (arrays are represented as a single parameter in metadata but as two tied parameters at the application binary interface layer). However, see whether IReferenceArray\<T> can help you there.

I've added this info to the intro and to the troubleshooting topics.

Thanks! ~Steve

[sichbo]

Phew! Nice find! Being able to nest generics will reduce a lot of an API's footprint and boilerplate code. Good note about IReferenceArray. Thanks for all of that, Steve 🍺

[stevewhims]

Always welcome, Simon. Thanks to you for developing with MIDL 3.0 and C++/WinRT! :) ~Steve