{"id":103586,"date":"2020-03-24T07:00:00","date_gmt":"2020-03-24T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103586"},"modified":"2020-03-24T05:52:48","modified_gmt":"2020-03-24T12:52:48","slug":"20200324-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200324-00\/?p=103586","title":{"rendered":"Accessing a member of a Windows Runtime class raises an Invalid­Cast­Exception<\/CODE> \/ throws a hresult_<\/CODE>no_<\/CODE>interface<\/CODE>, what does this mean?"},"content":{"rendered":"

You’re minding your own business and you decide to call a method on some object. Everything compiles fine, but it crashes at runtime with Invalid\u00adCast\u00adException<\/code>:<\/p>\n

\/\/ C#\r\nvar options = new LauncherOptions();\r\noptions.IgnoreAppUriHandlers = true; \/\/ System.InvalidCastException\r\n\r\n\/\/ C++\/WinRT\r\nLauncherOptions options;\r\noptions.IgnoreAppUriHandlers(true); \/\/ winrt::hresult_no_interface \r\n\r\n\/\/ C++\/CX\r\nauto options = ref new LauncherOptions();\r\noptions->IgnoreAppUriHandlers = true; \/\/ Platform::InvalidCastException\r\n<\/pre>\n
Why am I getting an “invalid cast exception” when there is no casting going on at all?<\/p>\n
The clue is in the C++\/WinRT example, which throws winrt::<\/code>hresult_<\/code>no_<\/code>interface<\/code>.<\/p>\n
Under the covers, Windows Runtime objects are COM objects, and their members are methods on COM interfaces. When you use a member, what happens behind the scenes is that the language projection queries the object for the COM interface that implements the desired member, and then it calls the corresponding interface method.<\/p>\n
One of the rules of COM is that interfaces are immutable. Therefore, in order to add new members to the object, those new members need to be put on a new interface.<\/p>\n
For example, the members of the Launcher\u00adOptions<\/code> runtime class were introduced as follows:<\/p>\n\n\n\n\n\n
Windows 8
\nILauncher\u00adOptions<\/tt><\/td>\n		Treat\u00adAs\u00adUntrusted
\nDisplay\u00adApplication\u00adPicker
\nUI
\nPreferred\u00adApplication\u00adPackage\u00adFamily\u00adName
\nPreferred\u00adApplication\u00adDisplay\u00adName
\nFallback\u00adUri
\nContent\u00adType<\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/tt><\/td>\n<\/tr>\n
Windows 10 version 1507
\nILauncher\u00adOptions2<\/tt><\/tt><\/td>\n		Target\u00adApplication\u00adPackage\u00adFamily\u00adName
\nNeighboring\u00adFiles\u00adQuery<\/tt><\/tt><\/tt><\/tt><\/td>\n<\/tr>\n
Windows 10 version 1607
\nILauncher\u00adOptions3<\/tt><\/tt><\/td>\n		Ignore\u00adApp\u00adUri\u00adHandlers<\/tt><\/tt><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Internally, a Windows Runtime object is represented by its “default interface”. Deciding upon a default interface is usually a no-brainer, because a freshly-introduced object typically implements only one interface anyway.\u00b9 For example, in Windows 8, the only interface supported by Launcher\u00adOptions<\/code> is ILauncher\u00adOptions<\/code>, which makes ILauncher\u00adOptions<\/code> the default interface, seeing as you have no choice.<\/p>\n
Using one of the Windows 8 properties goes like this:<\/p>\n
\/\/ C#:        options.TreatAsUntrusted = true;\r\n\/\/ C++\/CX:    options->TreatAsUntrusted = true;\r\n\/\/ C++\/WinRT: options.TreatAsUntrusted(true);\r\n\r\n\/\/ options is already a ILauncherOptions.\r\noptions->put_TreatAsUntrusted(true);\r\n<\/pre>\n
But using one of the properties added later takes a little more work:<\/p>\n
\/\/ C#:        options.IgnoreAppUriHandlers = true;\r\n\/\/ C++\/CX:    options->IgnoreAppUriHandlers = true;\r\n\/\/ C++\/WinRT: options.IgnoreAppUriHandlers(true);\r\n\r\nILauncherOptions3* options3;\r\noptions->QueryInterface(IID_PPV_ARGS(&options3));\r\noptions3->put_IgnoreAppUriHandlers(true);\r\noptions3->Release();\r\n<\/pre>\n
If you take a program that uses Ignore\u00adApp\u00adUri\u00adHandlers<\/code> and run it on on a version of Windows that doesn’t support the property, the Query\u00adInterface<\/code> call fails with E_<\/code>NO\u00adINTERFACE<\/code>. The language projection then converts this into a language-specific exception.<\/p>\n