{"id":105520,"date":"2021-08-05T07:00:00","date_gmt":"2021-08-05T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=105520"},"modified":"2021-08-05T07:35:17","modified_gmt":"2021-08-05T14:35:17","slug":"20210805-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20210805-00\/?p=105520","title":{"rendered":"An example of using Windows Runtime interop methods from C++\/WinRT: Request­Token­For­Window­Async<\/CODE>"},"content":{"rendered":"

A customer was trying to use the IWeb\u00adAuthentication\u00adCore\u00adManager\u00adInterop::Request\u00adToken\u00adFor\u00adWindow\u00adAsync<\/code> method<\/a> from C++\/WinRT. The IWeb\u00adAuthentication\u00adCore\u00adManager\u00adInterop<\/code> interface follows the interop pattern<\/a> and lets a Win32 program use the Windows Runtime Web\u00adAuthentication\u00adCore\u00adManager<\/code> by associating it with a HWND<\/code> instead of a Core\u00adWindow<\/code>.<\/p>\n

A customer was having trouble getting this to work, though:<\/p>\n

#include <WebAuthenticationCoreManagerInterop.h>\r\n#include <winrt\/Windows.Foundation.h>\r\n#include <winrt\/Windows.Security.Authentication.Web.Core.h>\r\n\r\nnamespace winrt\r\n{\r\n    using namespace Windows::Foundation;\r\n    using namespace Windows::Security::Authentication::Web::Core;\r\n}\r\n\r\n\/\/ Note: Code in italics is wrong; see discussion\r\nwinrt::IAsyncOperation<winrt::WebTokenRequestResult>\r\nRequestTokenForWindowAsync(HWND window, winrt::WebTokenRequest const& request)\r\n{\r\n    auto interop = winrt::get_activation_factory<winrt::WebAuthenticationCoreManager,\r\n                                                ::IWebAuthenticationCoreManagerInterop>();\r\n\r\n    winrt::com_ptr<winrt::IAsyncOperation<winrt::WebTokenRequestResult>> operation;\r\n    auto requestInspectable = static_cast<::IInspectable*>(winrt::get_abi(request));\r\n\r\n    winrt::check_hresult(\r\n        interop->RequestTokenForWindowAsync(\r\n        window,\r\n        requestInspectable,\r\n        __uuidof(operation),\r\n        operation.put_void()));<\/i>\r\n\r\n    co_return co_await operation;\r\n}\r\n<\/pre>\n
The __uuid(operation)<\/code> fails to compile, producing the error<\/p>\n
error C2787: 'winrt::com_ptr<winrt::Windows::Foundation::IAsyncOperation<winrt::Windows::Security::Authentication::Web::Core::Web\u00adToken\u00adRequest\u00adResult>>': no GUID has been associated with this object\r\n<\/pre>\n
What’s going on?<\/p>\n
The first order of business is understanding the error message.<\/p>\n
The __uuidof<\/code> nonstandard extension keyword can be applied to a type or a variable. If you apply it to a variable, then it uses the type of that variable. If the type is a pointer or reference, the pointed-to or referenced type is used. And then the compiler checks if the resulting type has a __declspec(uuid(...))<\/code> attribute.<\/p>\n
What happened here is that we passed a variable whose type is winrt::com_ptr<something><\/code>, and winrt::com_ptr<\/code> doesn’t have a __declspec(uuid(...))<\/code> attribute because the UUID associated with a winrt::com_ptr<\/code> depends on what the something<\/code> is.<\/p>\n
Okay, so let’s fix that by using the something<\/code>.<\/p>\n
    winrt::check_hresult(\r\n        interop->RequestTokenForWindowAsync(\r\n        window,\r\n        requestInspectable,\r\n        __uuidof(winrt::IAsyncOperation<winrt::WebTokenRequestResult>)<\/span>,\r\n        operation.put_void()));\r\n<\/pre>\n
That still doesn’t work. We just get the same error again:<\/p>\n
error C2787: 'winrt::Windows::Foundation::IAsyncOperation<winrt::Windows::Security::Authentication::Web::Core::Web\u00adToken\u00adRequest\u00adResult>': no GUID has been associated with this object\r\n<\/pre>\n
It’s the same problem again. winrt::IAsyncOperation<winrt::Web\u00adToken\u00adRequest\u00adResult><\/code> doesn’t have a __declspec(uuid(...))<\/code> because the UUID depends on the thing inside.<\/p>\n
But this time, we can’t unwrap it because we really do want the UUID of the IAsyncOperation<something><\/code>, not the UUID of the something<\/code>.<\/p>\n
It turns out that we ran astray much earlier:<\/p>\n
    winrt::com_ptr<winrt::IAsyncOperation<winrt::WebTokenRequestResult>> operation;\r\n<\/pre>\n
We created a winrt::com_ptr<\/code> to a winrt::IAsyncOperation<T><\/code>. But the catch is that winrt::IAsyncOperation<T><\/code> is itself already a smart pointer. You created a smart pointer to a smart pointer, and that’s the source of confusion.<\/p>\n
The type wrapped by a winrt::com_ptr<\/code> is expected to be a type that has methods Query\u00adInterface<\/code>, AddRef<\/code>, and Release<\/code>, according to the conventions of the ABI ::IUnknown<\/code>. The thing to pass here is not another smart pointer, but rather the ABI COM interface type.<\/p>\n
    \/\/ Oh my goodness, please don't make me type this.\r\n    winrt::com_ptr<ABI::Windows::Foundation::IAsyncOperation<\r\n        ABI::Windows::Security::Authentication::Web::Core::WebTokenRequestResult*>>\r\n        operation;\r\n<\/pre>\n
That is a horrible mouthful, and requires you to include the Windows Runtime ABI header files. Mixing the Windows Runtime ABI header files with C++\/WinRT does work, but it usually creates mass confusion for the developer, so let’s try not to do that.<\/p>\n
Even if you wade into this world, you have to take the COM ABI version of IAsyncOperation<WebTokenResult><\/code> and convert it back to its C++\/WinRT equivalent before you can co_await<\/code> it, which is another level of annoying typing.<\/p>\n
Fortunately, there’s a way out of this mess. One is to realize that winrt::IAsyncOperation<winrt::WebTokenRequestResult><\/code> is the thing we want: A smart pointer around a raw ABI pointer. So use that.<\/p>\n
winrt::IAsyncOperation<winrt::WebTokenRequestResult>\r\nRequestTokenForWindowAsync(HWND window, winrt::WebTokenRequest const& request)\r\n{\r\n    auto interop = winrt::get_activation_factory<winrt::WebAuthenticationCoreManager,\r\n                                                ::IWebAuthenticationCoreManagerInterop>();\r\n\r\n    winrt::IAsyncOperation<winrt::WebTokenRequestResult><\/span> operation;\r\n    auto requestInspectable = static_cast<::IInspectable*>(winrt::get_abi(request));\r\n\r\n    winrt::check_hresult(\r\n        interop->RequestTokenForWindowAsync(\r\n        window,\r\n        requestInspectable,\r\n        winrt::guid_of<decltype(operation)>()<\/span>,\r\n        operation.put_void()));\r\n\r\n    co_return co_await operation;\r\n}\r\n<\/pre>\n
Once we have it in this form, we can call upon our old friend  winrt::capture<\/code><\/a>, whose job is to help obtain COM ABI objects and convert them to C++\/WinRT objects.<\/p>\n
winrt::IAsyncOperation<winrt::WebTokenRequestResult>\r\nRequestTokenForWindowAsync(HWND window, winrt::WebTokenRequest const& request)\r\n{\r\n    auto interop = winrt::get_activation_factory<winrt::WebAuthenticationCoreManager,\r\n                                                ::IWebAuthenticationCoreManagerInterop>();\r\n\r\n    auto requestInspectable = static_cast<::IInspectable*>(winrt::get_abi(request));\r\n\r\n    co_return co_await\r\n        winrt::capture<winrt::IAsyncOperation<winrt::WebTokenRequestResult>>(\r\n            interop,\r\n            &::IWebAuthenticationCoreManagerInterop::RequestTokenForWindowAsync,\r\n            window,\r\n            requestInspectable);\r\n}\r\n<\/pre>\n
For style points, you could collapse the entire function into a one-liner.<\/p>\n
winrt::IAsyncOperation<winrt::WebTokenRequestResult>\r\nRequestTokenForWindowAsync(HWND window, winrt::WebTokenRequest const& request)\r\n{\r\n    return\r\n        winrt::capture<winrt::IAsyncOperation<winrt::WebTokenRequestResult>>(\r\n            winrt::get_activation_factory<winrt::WebAuthenticationCoreManager,\r\n                                          ::IWebAuthenticationCoreManagerInterop>(),\r\n            &::IWebAuthenticationCoreManagerInterop::RequestTokenForWindowAsync,\r\n            window,\r\n            static_cast<::IInspectable*>(winrt::get_abi(request)));\r\n}\r\n<\/pre>\n
Replacing co_return co_await<\/code> with a simple return<\/code> works here because we are just returning the operation, so we can pass it through instead of wrapping it inside a coroutine. (This sort of trick is  not available in general<\/a>, but we can use it here.)<\/p>\n
One final tweak is using the WIL helper method com_raw_ptr<\/code> to extract the ABI IInspectable*<\/code> from a C++\/WinRT object.<\/p>\n
winrt::IAsyncOperation<winrt::WebTokenRequestResult>\r\nRequestTokenForWindowAsync(HWND window, winrt::WebTokenRequest const& request)\r\n{\r\n    return\r\n        winrt::capture<winrt::IAsyncOperation<winrt::WebTokenRequestResult>>(\r\n            winrt::get_activation_factory<winrt::WebAuthenticationCoreManager,\r\n                                          ::IWebAuthenticationCoreManagerInterop>(),\r\n            &::IWebAuthenticationCoreManagerInterop::RequestTokenForWindowAsync,\r\n            window,\r\n            wil::com_raw_ptr(request));\r\n}\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
Understanding how the C++\/WinRT interop pieces fit together.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-105520","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Understanding how the C++\/WinRT interop pieces fit together.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/105520","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=105520"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/105520\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=105520"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=105520"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=105520"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}