Windows Runtime

Windows Runtime (WinRT) is a platform-agnostic application architecture first introduced in Windows 8 and Windows Server 2012 in 2012. WinRT supports development in C++/WinRT (standard C++), C++/CX (Component Extensions, a language based on C++), JavaScript-TypeScript, and the managed code languages C# and Visual Basic .NET (VB.NET). WinRT applications natively support both the x86 and ARM processors, and may run inside a sandboxed environment to allow greater security and stability.[1][2] WinRT components are designed with interoperability between multiple languages and APIs in mind, including native, managed and scripting languages.

Windows Runtime
Other namesWinRT
Operating systemMicrosoft Windows
ReplacesWin32 API
TypeApplication programming interface

Windows Phone 8.1 uses a version of the Windows Runtime named the Windows Phone Runtime. It enables developing applications in C# and VB.NET, and Windows Runtime components in C++/CX.[3]


WinRT is implemented in the programming language C++[4] and is object-oriented by design.[4] Its underlying technology, the Windows API (Win32 API), is written mostly in the language C.[5] It is an unmanaged code application programming interface (API) based on Component Object Model (COM) that allows interfacing from multiple languages, as does COM. However, the API definitions are stored in .winmd files, which are encoded in ECMA 335 metadata format, which .NET Framework also uses with a few modifications.[6][7] This common metadata format allows significantly less overhead when invoking WinRT from .NET applications, relative to P/Invoke, and much simpler syntax.[8]

The new C++/CX (Component Extensions) language, which borrows some C++/CLI syntax, was introduced for writing and consuming WinRT components with less glue code visible to the programmer, relative to classic COM programming in C++, and imposes fewer restrictions relative to C++/CLI on mixing types. The Component Extensions of C++/CX are recommended for use at the API-boundary only, not for other purposes.[9] Regular C++ (with COM-specific discipline) can also be used to program with WinRT components,[10] with the help of the Windows Runtime C++ Template Library (WRL), which is similar in purpose to what Active Template Library provides for COM.[11] In 2019, Microsoft deprecated C++/CX in favor of the C++/WinRT header library.[12]

WinRT applications run within a sandbox and need explicit user approval to access critical OS features and underlying hardware. File access is restricted to several predetermined locations, such as the directories Documents or Pictures.

WinRT applications for Windows RT, Windows 8 and beyond are packaged in the .appx file format; based upon Open Packaging Conventions, it uses a ZIP format with added XML files.[13] WinRT applications are distributed mostly through an application store named Microsoft Store, where WinRT software (termed Windows Store apps) can be downloaded and purchased by users. WinRT apps can only be sideloaded from outside Windows Store on Windows 8 or RT systems that are part of a Windows domain, or equipped with a special activation key obtained from Microsoft.[14][15][16][17]

In a major departure from Win32 and similarly to .NET Framework 4.5, most APIs which are expected to take significant time to complete are implemented as asynchronous. The application dispatches the API call, which returns immediately, freeing the application to perform other tasks while waiting for results.[18] The asynchronous model requires new programming language constructs (keyword async and operator await in C# and Visual Basic, class task and method .then in C++, which are provided by the WinRT software development kit (SDK), keyword promise and function then in JavaScript-HTML5), similar to try/catch used in exception handling. Parts of the API needing asynchronous access include on-screen messages and dialogs, file access, Internet connectivity, sockets, streams, devices and services, and calendar, contacts and appointments.



The metadata describes the code written for the WinRT platform. It defines a programming model that makes it possible to write object-oriented code that can be shared across programming languages, and enables services like reflection.

Herb Sutter, C++ expert at Microsoft, explained during his session on C++ at the 2011 Build conference that the WinRT metadata is CLI metadata.[9] Native code (i.e., processor-specific machine code) cannot contain metadata and it is then stored in separate WINMD-files that can be reflected like ordinary CLI assemblies.[19]

Because it is CLI metadata, code written in native WinRT languages can be used from managed CLI languages.

Type system

WinRT has a rich object-oriented class-based type system that is built on the metadata. It supports constructs with corresponding constructs in the .NET framework: classes, methods, properties, delegates, and events.

One of the major additions to WinRT relative to COM is the cross-application binary interface (ABI), .NET-style generics. In C++/CX these are declared using the keyword generic with a syntax very similar to that of keyword template. WinRT classes (ref classes) can also be genericized using C++ templates, but only template instantiations can be exported to .winmd metadata (with some name mangling), unlike WinRT generics which preserve their genericity in the metadata. WinRT also provides a library of generic containers that parallel those in the C++ Standard Library, and some reciprocal (back-and-forth) conversion functions. The consumption of WinRT collections in .NET languages (e.g., C# and VB) and in JavaScript is more transparent than in C++, with automated mappings into their natural equivalents occurring behind the scenes. When authoring a WinRT component in a managed language, some extra, COM-style rules must be followed, e.g. .NET framework collection types cannot be declared as return types, but only the WinRT interfaces that they implement can be used at the component boundary.

WinRT components

Classes that are compiled to target the WinRT are called WinRT components. They are classes that can be written in any supported language and for any supported platform. The key is the metadata. This metadata makes it possible to interface with the component from any other WinRT language. The runtime requires WinRT components that are built with .NET Framework to use the defined interface types or .NET type interfaces, which automatically map to the first named. Inheritance is as yet not supported in managed WinRT components, except for XAML classes.[20]

Programming interfaces

Programs and libraries targeted for the WinRT runtime can be created and consumed from several platforms and programming languages. Notably C/C++ (either with language extensions offering first-class support for WinRT concepts, or with a lower-level template library allowing to write code in standard C++), .NET (C# and Visual Basic .NET (VB.NET)) and JavaScript. This is made possible by the metadata.

In WinRT terminology, a language binding is termed a language projection.

C++ (C++/WinRT, Component Extensions, WRL)

Native C++ is a first-class citizen of the WinRT platform. As of version 10.0.17134.0 (Windows 10, version 1803), the Windows SDK contains C++/WinRT. C++/WinRT is an entirely standard modern C++17 language projection for Windows Runtime (WinRT) APIs, implemented as a header-file-based library, and designed to provide first-class access to the modern Windows API. With C++/WinRT, Windows Runtime APIs can be authored and consumed using any standards-compliant C++17 compiler. WinRT is a native platform and supports any native (and standard) C++ code, so that a C++ developer can reuse existing native C/C++ libraries. With C++/WinRT, there are no language extensions.

Prior to C++/WinRT being officially released in the Windows SDK, from October 2016,[21] Microsoft offered on GitHub C++/WinRT.[22] It does not rely on C++/CX code, with the result of producing smaller binaries and faster code.[23]

There are two other legacy options for using WinRT from C++: WRL, an ATL-style template library, and C++/CX (C++ with Component Extensions) which resembles C++/CLI.[24] Because of the internal consumption requirements at Microsoft, WRL is exception-free, meaning its return-value discipline is HRESULT-based just like that of COM.[25] C++/CX on the other hand wraps-up calls to WinRT with code that does error checking and throws exceptions as appropriate.[26]

C++/CX has several extensions that enable integration with the platform and its type system. The syntax resembles the one of C++/CLI although it produces native (although not standard) code and metadata that integrates with the runtime. For example, WinRT objects may be allocated with ref new, which is the counterpart of gcnew from C++/CLI. The hat operator ^ retains its meaning, however in the case where both the caller and callee are written in C++ and living in the same process, a hat reference is simply a pointer to a vptr to a virtual method table (vtable, VMT).[26]

Along with C++/CX, relative to traditional C++ COM programming, are partial classes, again inspired by .NET. These allow instance XAML code to be translated into C++ code by tools, and then combined with human-written code to produce the complete class while allowing clean separation of the machine-generated and human-edited parts of a class implementation into different files.


The .NET Framework and the Common Language Runtime (CLR) are integrated into the WinRT as a subplatform. It has influenced and set the standards for the ecosystem through the metadata format and libraries. The CLR provides services like JIT-compilation code and garbage collection. WinRT applications using .NET languages use the new Windows Runtime XAML Framework, and are primarily written in C#, VB.NET, and for the first time for XAML, with native code using C++/CX. Although not yet officially supported, programs can also be written in other .NET languages.


Classes defined in WinRT components that are built in managed .NET languages must be declared as sealed, so they cannot be derived from. However, non-sealed WinRT classes defined elsewhere can be inherited from in .NET, their virtual methods overridden, and so on; but the inherited managed class must still be sealed.

Members that interface with another language must have a signature with WinRT types or a managed type that is convertible to these.[20]


WinRT applications can also be coded using HTML with JavaScript in code-behind, which are run using the Trident rendering engine and Chakra JavaScript engine, both of which are also used by Internet Explorer. When coding a WinRT app in JavaScript, its features are adapted to follow JavaScript naming conventions, and namespaces are also mapped to JavaScript objects.



WinRT comes with an application programming interface (API) in the form of a class library that exposes the features of Windows 8 for the developer, like its immersive interface API. It is accessible and consumable from any supported language.

Runtime classes

The Windows Runtime classes is a set SDKs that provide access to all functionality from the XAML parser to the camera function. The SDKs are implemented as native C/C++ libraries (unmanaged).

Naming conventions

The naming conventions for the components (classes and other members) in the API are heavily influenced by the .NET naming conventions which uses camel case (specifically PascalCase). Microsoft recommends users to follow these rules in case where no others are given.

These conventions are projected differently in some languages, like JavaScript, which converts it to its conventions and the other way around. This is to give a native and consistent experience regardless of the programming language.

Restrictions and rules

Since Windows Runtime is projected to various languages, some restrictions on fundamental data types exist so as to host all such languages. Programmers must be careful with the behavior of those types when used with public access (for method parameters, method return values, properties, etc.).[27]

  • Basic types
    • In .NET languages and C++, a rich set of data types exists, representing various numerals.
    • In JavaScript, a Number can only represent up to 53 bits of precision.
    • In WinRT, the only lacking numeral data type is 8-bit signed integer relative to .NET and C++. JavaScript developers must be careful when dealing with big numbers while coding for WinRT.
    • Strings are immutable in .NET and JavaScript, but mutable in C++.
    • A null pointer passed as a string to WinRT by C++ is converted to an empty string
    • In .Net, null being passed as a string to WinRT is converted to an empty string
    • In JavaScript, null being passed as a string to WinRT is converted to a string with the word null. This is due to JavaScript's keyword null being represented as a null object
      • Similar results occur when passing undefined to WinRT from JavaScript
    • In .NET and C++, structs are value types, and such a struct can contain any type in it.
    • JavaScript does not directly support structs.
    • In WinRT, use of structs is allowed only for containing types that have value semantics, including numerals, strings, and other structs. Pointers or interface references are disallowed.
    • In .NET, objects are passed by reference, whereas numerals and structs are passed by value.
    • In C++, all types can be passed by reference or value.
    • In WinRT, interfaces are passed by reference; all other types are passed by value.
    • In .NET, C++, and JavaScript arrays are reference types.
    • In WinRT, arrays are value types.
    • In .NET and C++, clients subscribe to events using += operator.
    • In JavaScript, addEventListener function or setting on<EventName> property is used to subscribe to events.
    • In WinRT, all languages can use their own way to subscribe to events.
    • Some .NET collections map directly to WinRT collections.
    • WinRT Vector type resembles arrays and the array syntax is used to consume them.
    • WinRT Map type is a key/value pair collection, and is projected as Dictionary in .NET languages.
    Method overloading
    • All WinRT languages (.NET, C++, JavaScript) support overloading on parameters
    • .NET and C++ also support overloading on type.
    • In WinRT, only parameter number is used for overloading.
    • All WinRT methods are designed such that any method taking longer than 50 milliseconds is an async method.
    • The established naming pattern to distinguish asynchronous methods is <Verb>[<Noun>]Async. For the full runtime library, all methods that have a chance to last longer than 50 ms are implemented as asynchronous methods only.

Version history

Windows version
Windows 8 Windows Runtime
Windows 8.1
Windows 10 Universal Windows Platform (UWP)

Windows Phone Runtime

Starting from Windows Phone 8 it is possible to develop apps using a version of the Windows Runtime called the Windows Phone Runtime (WPRT). Although WP8 brought limited support, the platform did eventually converge with Windows 8.1 in Windows Phone 8.1.

Windows Phone 8

Windows Phone 8 has limited support for developing and consuming Windows Runtime components through Windows Phone Runtime. Many of the Windows Runtime APIs in Windows 8 that handle core operating system functions have been ported to Windows Phone 8.[28] Support for developing native games using C++/CX and DirectX has been added, by request from the game development industry.

However, the Windows Phone XAML Framework is still based on the same Microsoft Silverlight framework, as in Windows Phone 7, for backward compatibility. Thus, as of 2016, XAML development is impossible in C++/CX. Development using either HTML5 or WinJS is unsupported on Windows Phone 8.

Windows Phone 8.1

Windows Runtime support on Windows Phone 8.1 converges with Windows 8.1. The release brings a full Windows Runtime API to the platform, including support for Windows Runtime XAML Framework, and language bindings for C++/CX, and HTML5-JavaScript. There is also a project type called Universal apps to enable apps to share code across 8.1 versions of Windows Phone and Windows.

The Windows Phone 8 Silverlight Framework has been updated. It can exploit some of the new features in the Windows Runtime.

Windows Phone Runtime uses the AppX package format from Windows 8, after formerly using Silverlight XAP.

See also


  1. Avram, Abel (21 September 2011). "Design Details of the Windows Runtime". InfoQ.
  2. Klug, Brian; Smith, Ryan (13 September 2011). "Microsoft Build: Windows 8, A Pre-Beta Preview". AnandTech.
  3. "Windows Phone API reference". Windows Phone API reference. Microsoft. July 21, 2014.
  4. Michael, Mayberry (2012). WinRT Revealed. New York City: Apress. p. 3. ISBN 978-1-4302-4585-8.
  5. "Creating Win32 Applications (C++)". MSDN. Microsoft. Retrieved 12 January 2014.
  6. "Windows Metadata (WinMD) files". API reference for UWP apps. Microsoft Docs. Retrieved 2019-07-20.
  7. De Icaza, Miguel (15 September 2011). "WinRT demystified". Personal blog of Miguel de Icaza. Self-published. Retrieved 15 January 2014.
  8. "What is the COM marshaling overhead in calling the WinRT API from C#?". MSDN forum. Self-published. 20 September 2011. Retrieved 15 January 2014.
  9. "Using the Windows Runtime from C++ | Build2011 | Channel 9". 2011-09-14. Retrieved 2012-04-24.
  10. Sivakumar, Nish (2011-09-29). "Visual C++ and WinRT/Metro - Some fundamentals - CodeProject®". Retrieved 2012-04-24.
  11. "Using the Windows Runtime from C++ | Build2011 | Channel 9". 2011-09-14. Retrieved 2012-04-24.
  12. "Introduction to C++/WinRT - Windows UWP applications". Microsoft.
  13. "Designing a simple and secure app package – APPX". Windows 8 app developer blog. Retrieved 30 December 2013.
  14. "How to Add and Remove Apps". TechNet. Microsoft. 31 May 2012. Retrieved 4 October 2012. To enable sideloading on a Windows 8 Enterprise computer that is not domain-joined or on any Windows® 8 Pro computer, you must use a sideloading product activation key. To enable sideloading on a Windows® RT device, you must use a sideloading product activation key. For more information about sideloading product activation keys, see Microsoft Volume Licensing.
  15. "Windows 8: The Metro Mess". PC Magazine. Retrieved 8 September 2012.
  16. "Microsoft now using 'Modern UI Style' to refer to Windows 8 'Metro Style' apps". Retrieved 10 August 2012.
  17. "What's a Microsoft Store app?". Windows Dev Center. Retrieved 1 October 2012.
  18. "Asynchronous programming (Windows Store apps)". MSDN. Microsoft. Retrieved 12 January 2014.
  19. ".NET Gets a New Lease of Life". Archived from the original on September 24, 2011. Retrieved September 16, 2011.
  20. "Using the Windows Runtime from C# and Visual Basic | Build2011 | Channel 9". 2011-09-14. Retrieved 2012-04-24.
  21. "Initial preview release of C++/WinRT". 2016-10-05. Retrieved 2016-10-05.
  22. "C++/WinRT is a standard C++ language projection for the Windows Runtime". 2016-09-14. Retrieved 2016-09-14.
  23. "Standard C++ and the Windows Runtime (C++/WinRT)". 2016-11-28. Retrieved 2016-11-28.
  24. "Inside the C++/CX Design - Visual C++ Team Blog - Site Home - MSDN Blogs". 2011-10-20. Retrieved 2012-04-24.
  25. Charles (2011-10-26). "GoingNative 3: The C++/CX Episode with Marian Luparu | C9::GoingNative | Channel 9". Retrieved 2012-04-24.
  26. Under the covers with C++ for Metro style apps with Deon Brewis at //Build
  27. "Ten Tips When Writing a Hybrid Language Metro style Application - Build2011 - Channel 9". Channel 9. Microsoft.
  28. "Windows Phone Runtime API". Microsoft.
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