Troubleshooting Microsoft Interface Definition Language 3.0 issues

The table of troubleshooting symptoms and remedies below may be helpful to you whether you're cutting new code or porting an existing app.

Note

The purpose of Microsoft Interface Definition Language (MIDL) 3.0 is to define Windows Runtime types inside Interface Definition Language (IDL) files (.idl files). MIDL 3.0 is a particularly convenient way to define C++/WinRT runtime classes. For more info and background, see Introduction to Microsoft Interface Definition Language 3.0.

IDL files have traditionally been used to define COM types; so, MIDL 3.0 represents a new and different way of using IDL files, with an expanded syntax for them. For more info about using IDL files to define non-Windows Runtime types (COM types), see Microsoft Interface Definition Language.

Symptoms and remedies

Symptom	Remedy
Compiling in Visual Studio results in "error MIDL2003: [msg]redefinition [context]: IUnknown", and many other similar errors.	Your toolchain is set up to automatically reference any types that are in system namespaces. In your IDL files, remove any import directives for Windows namespaces; you only need to import any types that you've defined in your project.
error MIDL2009 : [msg]undefined symbol [context]: IInspectable.	Your toolchain is not set up to automatically import types in system namespaces. If you're using midl.exe from the command line, then see Definition structure, and calling midl.exe from the command line for the correct command-line syntax; specifically the use of the /reference switch. Alternatively, compile your IDL files using Visual Studio with the C++/WinRT Visual Studio Extension (VSIX) (see Visual Studio support for C++/WinRT, and the VSIX). If you do any of these things, then it won't be necessary to add an import directive for Windows.Foundation.idl to your IDL file. But you will need the import directive to import additional IDL if you're referencing types that you've defined in your project.
error MIDL2011 : [msg]unresolved type declaration [context]:, followed by a type name.	In your IDL file, add an import directive for the IDL file(s) that contain the definitions of any type(s) that you reference that you've defined in your project.
error MIDL2025: [msg]syntax error [context]: expecting > or, near ">>"	Insert a space between the two > characters so the pair of template-closing characters is not misinterpreted as a right-shift operator.
error MIDL2025: [msg]syntax error [context]: expecting > or, near "["	This error can occur if you use an array as a parameter type argument to a parameterized interface. Doing so is not valid. But for more details and a potential solution see Parameterized types.
The Windows App Certification Kit tests produce an error that one of your runtime classes "does not derive from a Windows base class. All composable classes must ultimately derive from a type in the Windows namespace".	Any runtime class (that you define in your application) that derives from a base class is known as a composable class. The ultimate base class of a composable class must be a type originating in a Windows.* namespace; for example, Windows.UI.Xaml.DependencyObject. See XAML controls; bind to a C++/WinRT property for more details.
Compiling in Visual Studio results in "error MIDL5148: [msg]Classic WinRT IDL constructs cannot be used in Modern WinRT IDL types".	You're using MIDL 1.0 or 2.0 syntax in your MIDL 3.0 file. For background info, see MIDL 1.0, 2.0, and 3.0.
MIDL4035 [msg]The [in] parameter has a pointer type that is not valid.	Some input parameter is passed by pointer, but the type of the pointer is invalid. This is generally a sign that ref const was used with a non-struct type, or ref was used with a value type. Note that the [in] attribute mentioned in the error message actually means that the error concerns an input parameter, which is any parameter without the out keyword. The [in] attribute itself is not a valid attribute in MIDL 3.0 and therefore won't appear in code.