Introduction to Microsoft Interface Definition Language 3.0

Microsoft Interface Definition Language (MIDL) 3.0 is a simplified, modern syntax for defining Windows Runtime types inside of Interface Definition Language (IDL) files (.idl files). This new syntax will feel familiar to anyone experienced with C, C++, C#, and/or Java. MIDL 3.0 is a particularly convenient way to define C++/WinRT runtime classes, being dramatically more concise than previous versions of IDL (reducing designs by two thirds in length, and using reasonable defaults to reduce the need for decorating with attributes).

Here's how MIDL 3.0 looks; this example demonstrates most of the language syntax elements that you'll likely use.

// Photo.idl
namespace PhotoEditor
    delegate void RecognitionHandler(Boolean arg); // delegate type, for an event.

    runtimeclass Photo : Windows.UI.Xaml.Data.INotifyPropertyChanged // interface.
        Photo(); // constructors.
        Photo(Windows.Storage.StorageFile imageFile);

        String ImageName{ get; }; // read-only property.
        Single SepiaIntensity; // read-write property.

        Windows.Foundation.IAsyncAction StartRecognitionAsync(); // (asynchronous) method.

        event RecognitionHandler ImageRecognized; // event.

Note that the syntax of MIDL 3.0 is specifically and solely designed for defining types. You'll use a different programming language to implement those types. To use MIDL 3.0, you'll need Windows SDK version 10.0.17134.0 (Windows 10, version 1803) (midl.exe version 8.01.0622 or later, used with the /winrt switch).


Also see the Windows Runtime consolidated reference (The Windows Runtime type system, and Windows Metadata files).

MIDL 1.0, 2.0, and 3.0

Interface Definition Language (IDL) began with the Distributed Computing Environment/Remote Procedure Calls (DCE/RPC) system. The original MIDL 1.0 is DCE/RPC IDL with enhancements for defining COM interfaces and coclasses.

An updated MIDL 2.0 syntax (also known as MIDLRT) was then developed within Microsoft to declare Windows Runtime APIs for the Windows platform. If you look in the Windows SDK folder %WindowsSdkDir%Include<WindowsTargetPlatformVersion>\winrt then you'll see examples of .idl files that are written with the MIDL 2.0 syntax. These are built-in Windows Runtime APIs, declared in their application binary interface (ABI) form. These files exist primarily for tooling to use—you won't author nor consume these APIs in this form (unless you're writing very low-level code).

Also see Transition to MIDL 3.0 from classic MIDLRT.

MIDL 3.0 is a much simpler and more modern syntax, whose purpose is to declare Windows Runtime APIs. And you can use it in your projects, particularly to define C++/WinRT runtime classes. The headers, for use from C++/WinRT, for the built-in Windows Runtime APIs are part of the SDK, inside the folder %WindowsSdkDir%Include<WindowsTargetPlatformVersion>\cppwinrt\winrt.

Use cases for MIDL 3.0

In general, all Windows Runtime APIs are designed to be available to all Windows Runtime language projections. This is done, in part, by choosing to exclusively pass Windows Runtime types to and from Windows Runtime APIs. While it is a valid design decision to pass a raw COM interface to and from a Windows Runtime API, doing so limits the consumers of that particular Windows Runtime API to C++ applications. The technique can be seen in interoperation scenarios—for example, when interoperating between Direct3D and XAML. Since Direct3D is in the picture, the scenario is necessarily narrowed to C++ applications. So, an API that requires a COM interface doesn't impose any additional limitation over and above what's inherent. For example, a C++ application can obtain an IDXGISwapChain interface pointer, and then pass that to the ISwapChainPanelNative::SetSwapChain method. A C# application, for example, wouldn't be able to obtain an IDXGISwapChain to begin with, so it wouldn't be able to use that method for that reason. These interop-related exceptions live in interop headers, such as

If there are features or functionality of a COM component that you wish to expose to Windows Runtime language projections beyond C++, then you can create a C++ Windows Runtime component (WRC) that directly creates and uses the COM component (such as DirectX, for example), and exposes a replication of some subset of its features and functionality in the form of a Windows Runtime API surface that takes and returns Windows Runtime types only. You could then consume that WRC from an application written in any Windows Runtime language projection.

Definition structure, and calling midl.exe from the command line

The key organizational concepts in a MIDL 3.0 definition are namespaces, types, and members. A MIDL 3.0 source file (an .idl file) contains at least one namespace, inside which are types and/or subordinate namespaces. Each type contains zero or more members.

  • Classes, interfaces, structures, and enumerations are types.
  • Methods, properties, events, and fields are examples of members.

When you compile a MIDL 3.0 source file, the compiler (midl.exe) emits a Windows Runtime metadata file (typically a .winmd file).

// Bookstore.idl
namespace Bookstore
    runtimeclass BookSku : Windows.UI.Xaml.Data.INotifyPropertyChanged
        BookSku(Single price, String authorName, String coverImagePath, String title);

        Single Price;

        String AuthorName{ get; };
        Windows.UI.Xaml.Media.ImageSource CoverImage{ get; };
        String CoverImagePath{ get; };
        String Title{ get; };

        Boolean Equals(BookSku other);
        void ApplyDiscount(Single percentOff);

Since the namespace of a Windows Runtime type becomes part of the type name, the example above defines a runtime class named Bookstore.BookSku. There's no language-independent way of expressing BookSku without also expressing the namespace.

This class implements the Windows.UI.Xaml.Data.INotifyPropertyChanged interface. And the class contains several members: two constructors, a read-write property (Price), some read-only properties (AuthorName through Title), and two methods, named Equals and ApplyDiscount. Note the use of the type Single rather than float. And that String has an upper-case "S".


Visual Studio provides the best experience for compiling MIDL 3.0, by means of the C++/WinRT Visual Studio Extension (VSIX). See Visual Studio support for C++/WinRT, and the VSIX.

But you can also compile MIDL 3.0 from the command line. If the source code for this example is stored in a file named Bookstore.idl, then you can issue the command below. If necessary for your case, you can update the SDK version number used in the command (which is 10.0.17134.0).

midl /winrt /metadata_dir "%WindowsSdkDir%References\10.0.17134.0\\" /h "nul" /nomidl /reference "%WindowsSdkDir%References\10.0.17134.0\Windows.Foundation.FoundationContract\\Windows.Foundation.FoundationContract.winmd" /reference "%WindowsSdkDir%References\10.0.17134.0\Windows.Foundation.UniversalApiContract\\Windows.Foundation.UniversalApiContract.winmd" /reference "%WindowsSdkDir%\References\10.0.17134.0\Windows.Networking.Connectivity.WwanContract\\Windows.Networking.Connectivity.WwanContract.winmd" Bookstore.idl

The midl.exe tool compiles the example and produces a metadata file named Bookstore.winmd (by default, the name of the .idl file is used).


If you use more than one IDL file (for advice about that, see Factoring runtime classes into Midl files (.idl)), then merge all of the resulting .winmd files into a single file with the same name as the root namespace. That final .winmd file will be the one that the consumers of your APIs will reference.

In this case, BookSku is the only runtime class in the Bookstore namespace, so we saved a step and just named the .idl file for the namespace.

Incidentally, you can use the where command to find out where midl.exe is installed.

where midl

If you want to use the types defined in one .idl file from a different .idl file, then you use the import directive. For more details, and a code example, see XAML controls; bind to a C++/WinRT property. Of course, if you're consuming a built-in or third-party component, then you won't have access to the .idl file. For example, you might want to consume the Win2D Windows Runtime API for immediate-mode 2D graphics rendering. The command above used the /reference switch to reference a Windows Runtime metadata (.winmd) file. In this next example, we'll use that switch again, imagining the scenario where we have Bookstore.winmd, but not Bookstore.idl.

// MVVMApp.idl
namespace MVVMApp
    runtimeclass ViewModel
        Bookstore.BookSku BookSku{ get; };

If the source code for the example above is stored in a file named MVVMApp.idl, then you can issue the command below to reference Bookstore.winmd.

midl /winrt /metadata_dir "%WindowsSdkDir%References\10.0.17134.0\\" /h "nul" /nomidl /reference "%WindowsSdkDir%References\10.0.17134.0\Windows.Foundation.FoundationContract\\Windows.Foundation.FoundationContract.winmd" /reference "%WindowsSdkDir%References\10.0.17134.0\Windows.Foundation.UniversalApiContract\\Windows.Foundation.UniversalApiContract.winmd" /reference "%WindowsSdkDir%\References\10.0.17134.0\Windows.Networking.Connectivity.WwanContract\\Windows.Networking.Connectivity.WwanContract.winmd" /reference Bookstore.winmd MVVMApp.idl


A namespace is required. It prefixes the name of all types defined in the scope of the namespace block with the namespace name. A namespace can also contain subordinate namespace declarations. The name of types defined in a subordinate namespace scope have a prefix of all the containing namespace names.

The examples below are two ways of declaring the same Windows.Foundation.Uri class (as you can see, periods separate the levels of nested namespaces).

namespace Windows.Foundation
    runtimeclass Uri : IStringable
namespace Windows
    namespace Foundation
        runtimeclass Uri : IStringable

Here's another example showing that it's legal to declare namespaces and their types in a nested fashion.

namespace RootNs.SubNs1
    runtimeclass MySubNs1Class
        void DoWork();

    namespace SubNs2
        runtimeclass MySubNs2Class
            void DoWork();

But it's more common practice to close the previous namespace, and open a new one, like this.

namespace RootNs.SubNs1
    runtimeclass MySubNs1Class
        void DoWork();

namespace RootNs.SubNs1.SubNs2
    runtimeclass MySubNs2Class
        void DoWork();


There are two kinds of data types in MIDL 3.0: value types, and reference types. A variable of a value type directly contains its data. A variable of a reference type stores a reference to its data (such a variable is also known as an object).

It's possible for two reference type variables to reference the same object. Thus, an operation on one variable affects the object referenced by the other variable. With value types, the variables each have their own copy of the data, and it's not possible for an operation on one to affect the other.

MIDL 3.0's value types are further divided into simple types, enum types, struct types, and nullable types.

MIDL 3.0's reference types are further divided into class types, interface types, and delegate types.

Here's an overview of MIDL 3.0's type system. Unlike previous versions of MIDL, you can't use aliases for these types.

Category Description
Value types Simple types Signed integral: Int16, Int32, Int64
Unsigned integral: UInt8, UInt16, UInt32, UInt64
Unicode characters: Char (represents a UTF-16LE; a 16-bit Unicode code unit)
Unicode strings: String
IEEE floating point: Single, Double
Boolean: Boolean
128 bit UUID: Guid
Enum types User-defined types of the form enum E {...}
Struct types User-defined types of the form struct S {...}
Nullable types Extensions of all other value types with a null value
Reference types Class types Ultimate base class of all other types: Object
User-defined types of the form runtimeclass C {...}
Interface types User-defined types of the form interface I {...}
Delegate types User-defined types of the form delegate <returnType> D(...)

The seven integral types provide support for 8-bit unsigned data; and 16-bit, 32-bit, and 64-bit values in signed or unsigned form.

The two floating point types, Single and Double, represent data using the 32-bit single-precision and 64-bit Double-precision IEEE 754 formats, respectively.

MIDL 3.0's Boolean type represents boolean values; either true or false.

Characters and strings in MIDL 3.0 contain Unicode characters. The Char type represents a UTF-16LE code unit; and the String type represents a sequence of UTF-16LE code units.

The following table summarizes MIDL 3.0's numeric types.

Category Bits Type Range/Precision
Signed integral 16 Int16 –32,768...32,767
32 Int32 –2,147,483,648...2,147,483,647
64 Int64 –9,223,372,036,854,775,808...9,223,372,036,854,775,807
Unsigned integral 8 UInt8 0...255
16 UInt16 0...65,535
32 UInt32 0...4,294,967,295
64 UInt64 0...18,446,744,073,709,551,615
Floating point 32 Single 1.5 × 10−45 to 3.4 × 1038, 7-digit precision
64 Double 5.0 × 10−324 to 1.7 × 10308, 15-digit precision

MIDL 3.0 source files use type definitions to create new types. A type definition specifies the name and the members of the new type. These MIDL 3.0 type categories are user-definable.

  • attribute types,
  • struct types,
  • interface types,
  • runtimeclass types,
  • delegate types, and
  • enum types.

An attribute type defines a Windows Runtime attribute that can be applied to other type definitions. An attribute provides metadata about the type to which the attribute is applied.

A struct type defines a Windows Runtime structure that contains data members (fields). Structs are value types, and they do not require heap allocation. A data member of a struct type must either be a value type or a nullable type. Struct types do not support inheritance.

An interface type defines a Windows Runtime interface, which is a named set of function members. An interface may specify that an implementation of the interface must also implement of one or more specified additional (required) interfaces. Every interface type directly derives from the Windows Runtime IInspectable interface.

A runtimeclass type defines a Windows Runtime class (runtime class). A runtime class contains members that can be properties, methods, and events.

A delegate type defines a Windows Runtime delegate, which represents a reference to a method with a particular parameter list and return type. Delegates make it possible to treat a method as an entity that can be passed as a parameter. A delegate is similar to the concept of a function pointer found in some other languages. Unlike function pointers, delegates are object-oriented, and type-safe.

An enum type is a distinct type with named constants. Every enum type has an implicit underlying type; either Int32 or UInt32. The set of values of an enum type is the same as the set of values of the underlying type.

MIDL 3.0 supports three additional type categories.

  • single-dimensional array types,
  • nullable value types, and
  • the Object type.

You don't need to declare a single-dimensional array before you can use it. Instead, array types are constructed by following a type name with square brackets. For example, Int32[] is a single-dimensional array of Int32.

Similarly, nullable value types also do not have to be defined before they can be used. For each non-nullable value type T (except String), there's a corresponding nullable type Windows.Foundation.IReference<T>, which can hold the additional value null. For instance, Windows.Foundation.IReference<Int32> is a type that can hold any 32-bit integer, or the value null. Also see IReference<T>.

Finally, MIDL 3.0 supports the Object type, which maps to the Windows Runtime IInspectable interface. The interface and runtimeclass reference types conceptually derive from the Object type; delegate does not.

Expressions in an enumerated value

With MIDL 3.0, you can only use an expression in the definition of the value of an enumerated type's named constants; in other words, in an enumeration initializer.

An expression is constructed from operands and operators. The operators in an expression indicate which operations to apply to the operands. Examples of operators include +, -, *, /, and new. Examples of operands include literals, fields, local variables, and expressions.

When an expression contains multiple operators, the precedence of the operators controls the order in which the individual operators are evaluated. For example, the expression x + y * z is evaluated as x + (y * z) because the * operator has higher precedence than the + operator. Logical operations are lower precedence than bitwise operations.

The following table summarizes MIDL 3.0's operators, listing the operator categories in order of precedence from highest to lowest. Operators in the same category have equal precedence.

Category Expression Description
Primary x++ Post-increment
x-- Post-decrement
Unary +x Identity
-x Negation
!x Logical negation
~x Bitwise negation
++x Pre-increment
--x Pre-decrement
Multiplicative x * y Multiplication
x / y Division
x % y Remainder
Additive x + y Addition, String concatenation, delegate combination
x – y Subtraction, delegate removal
Shift x << y Shift left
x >> y Shift right
Bitwise AND x & y Integer bitwise AND
Bitwise XOR x ^ y Integer bitwise XOR
Bitwise OR x | y Integer bitwise OR
Logical AND x && y Boolean logical AND
Logical OR x || y Boolean logical OR


Classes (or runtime classes) are the most fundamental of MIDL 3.0's types. A class is a definition of an aggregation of methods, properties, and events in a single unit. Classes support inheritance and polymorphism—mechanisms whereby derived classes can extend and specialize base classes.

You define a new class type using a class definition. A class definition starts with a header that specifies the runtimeclass keyword, the name of the class, the base class (if given), and the interfaces implemented by the class. The header is followed by the class body, which consists of a list of member declarations written between the delimiters { and }.

Here's a definition of a simple class named Area.

runtimeclass Area
    Area(Int32 width, Int32 height);

    Int32 Height;
    Int32 Width;

    static Int32 NumberOfAreas { get; };

This defines a new Windows Runtime class named Area, which has a constructor that take two Int32 parameters, two Int32 read-write properties named Height and Width, and a static read-only property named NumberOfAreas.

By default, a runtimeclass is sealed, and derivation from it is disallowed. See Base classes.

In order to bind XAML to a view model, the view model runtime class needs to be defined in MIDL. See XAML controls; bind to a C++/WinRT property for more details.

You can declare that a class supports no instances (and consequently must contain only static members) by prefixing the runtime class definition with the static keyword. Adding a non-static member to the class then causes a compilation error.

static runtimeclass Area
    static Int32 NumberOfAreas { get; };

A static class is different from an empty class. Also see Empty classes.

You can indicate that a class definition is incomplete by prefixing the runtime class definition with the partial keyword. All of the partial class definitions encountered by the compiler are combined into a single runtime class. This feature is primarily for XAML authoring scenarios, where some of the partial classes are machine-generated.

Modifier Meaning
static Class has no instances. Consequently, only static members are permitted.
partial Class definition is incomplete.

See Composition and activation for advanced modifiers.

Member access modifiers

As MIDL 3.0 is a definition language for describing the public surface of Windows Runtime types, there's no need for explicit syntax to declare the public accessibility of a member. All members are implicitly public. That's why MIDL 3.0 doesn't require nor allow the (effectively redundant) public keyword.

Base classes

A class definition may specify a base class by following the class name and type parameters with a colon and the name of the base class. Omitting a base class specification is the same as deriving from type Object (in other words, from IInspectable).


Your view model classes—in fact, any runtime class that you define in your application—need not derive from a base class.

Any runtime class that you define in the application that does derive from a base class is known as a composable class. And there are constraints around composable classes. For an application to pass the Windows App Certification Kit tests used by Visual Studio and by the Microsoft Store to validate submissions (and therefore for the application to be successfully ingested into the Microsoft Store), a composable class must ultimately derive from a Windows base class. Meaning that the class at the very root of the inheritance hierarchy must be a type originating in a Windows.* namespace.

See XAML controls; bind to a C++/WinRT property for more details.

In the next example, the base class of Volume is Area, and the base class of Area is Windows.UI.Xaml.DependencyObject.

unsealed runtimeclass Area : Windows.UI.Xaml.DependencyObject
    Area(Int32 width, Int32 height);
    Int32 Height;
    Int32 Width;

runtimeclass Volume : Area
    Volume(Int32 width, Int32 height, Int32 depth);
    Int32 Depth;


Here, Area and Volume are defined in the same source file. For a discussion of the pros and cons, see Factoring runtime classes into Midl files (.idl).

A class inherits the members of its base class. Inheritance means that a class implicitly contains all members of its base class, except for the constructor(s) of the base class. A derived class can add new members to those it inherits, but it cannot remove the definition of an inherited member.

In the previous example, Volume inherits the Height and Width properties from Area. So, every Volume instance contains three properties: Height, Width, and Depth.

In general, type resolution rules require that a type name is fully qualified when referenced. An exception is when the type has been defined in the same namespace as the current type. The example above works as written if Area and Volume are both in the same namespace.

Implemented interfaces

A class definition may also specify a list of interfaces that the class implements. You specify the interfaces as a comma-separated list of interfaces following the (optional) base class.

In the example below, the Area class implements the IStringable interface; and the Volume class implements both IStringable and the hypothetical IEquatable interface.

unsealed runtimeclass Area : Windows.Foundation.IStringable
    Area(Int32 width, Int32 height);
    Int32 Height;
    Int32 Width;

runtimeclass Volume : Area, Windows.Foundation.IStringable, IEquatable
    Volume(Int32 width, Int32 height, Int32 depth);
    Int32 Depth;

In the MIDL, you don't declare the interface's members on the class. You do, of course, have to declare and define them on the actual implementation.


The members of a class are either static members or instance members. A static member belongs to a class. An instance member belongs to an object (that is, an instance of a class).

This table shows the kinds of member that a class can contain.

Member kind Description
Constructors Actions required to initialize an instance of the class, or to initialize the class itself
Properties Actions associated with reading and writing named properties of an instance of the class, or of the class itself
Methods Computations and actions that can be performed by an instance of the class, or by the class itself
Events Notifications that can be raised by an instance of the class


MIDL 3.0 supports the declaration of instance constructors. An instance constructor is a method that implements the actions required to initialize an instance of a class. Constructors may not be static.

A constructor is declared like an instance method (but with no return type), and with the same name as the containing class.

Instance constructors can be overloaded. For example, the Test class below declares three instance constructors; one with no parameters (the default constructor), one that takes an Int32 parameter, and one that takes two Double parameters (parameterized constructors).

runtimeclass Test
    Test(Int32 x);
    Test(Double x, Double y);

For details on the syntax for parameter lists, see Methods below.

Instance properties, methods, and events are inherited. Instance constructors are not inherited (with one exception), and a class has no instance constructors other than those actually declared in the class. If no instance constructor is supplied for a class, then you cannot directly instantiate the class. For such a class, you'd typically have a factory method elsewhere that returns an instance of the class.

The exception is unsealed classes. An unsealed class can have one or more protected constructors.


Properties are conceptually similar to fields (for example, C# fields; or the fields of a MIDL 3.0 struct). Both properties and fields are members with a name and an associated type. However, unlike fields, properties don't denote storage locations. Instead, properties have accessors that specify the function to be executed when you read or write a property.

A property is declared like a struct's field, except that the declaration ends with a get keyword and/or a set keyword written between the delimiters { and }, and ending in a semicolon.

A property that has both a get keyword and a set keyword is a read-write property. A property that has only a get keyword is a read-only property. The Windows Runtime doesn't support write-only properties.

For example, the class Area, seen previously, contains two read-write properties named Height and Width.

unsealed runtimeclass Area
    Int32 Height { get; set; };
    Int32 Width; // get and set are implied if both are omitted.

The declaration of Width omits the braces and the get and set keywords. The omission implies that the property is read-write, and is semantically identical to providing the get and set keywords in that order—get, followed by set.

Additionally, you can specify only the get keyword to indicate that the property is read-only.

// Read-only instance property returning mutable collection.
Windows.Foundation.Collections.IVector<Windows.UI.Color> Colors { get; };

The Windows Runtime doesn't support write-only properties. But you can specify only the set keyword to revise an existing read-only property into a read-write property. Take this version of Area as an example.

unsealed runtimeclass Area
    Color SurfaceColor { get; };

If you want subsequently to make the SurfaceColor property read-write, and you don't need to maintain binary compatibility with prior definitions of Area (for example, the Area class is a type in an application that you recompile each time), then you can simply add the set keyword to the existing SurfaceColor declaration like this.

unsealed runtimeclass Area
    Color SurfaceColor { get; set; };

If, on the other hand, you do need binary stability (for example, the Area class is a component in a library that you ship to customers), then you cannot add the set keyword to the existing property declaration. Doing so changes the binary interface to your class.

In that case, add the property set keyword to an additional definition of the property at the end of the class like this.

unsealed runtimeclass Area
    Color SurfaceColor { get; };
    Color SurfaceColor { set; };

The compiler produces an error for a write-only property. But that's not what's being done here. Because of the preceding declaration of the property as read-only, the addition of the set keyword doesn't declare a write-only property, but instead a read-write property.

The Windows Runtime implementation of a property is one or two accessor methods on an interface. The order of the get and set keywords in the property declaration determine the order of the get and set accessor methods in the backing interface.

The get accessor corresponds to a parameterless method with a return value of the property type—the property getter.

A set accessor corresponds to a method with a single parameter named value, and no return type—the property setter.

Consequently, these two declarations produce different binary interfaces.

Color SurfaceColor { get; set; };
Color SurfaceColor { set; get; };
Static and instance properties

Similar to methods, MIDL 3.0 supports both instance properties and static properties. Static properties are declared with the static modifier prefixed, and instance properties are declared without it.


A method is a member that implements a computation or action that can be performed by an instance of the class, or by the class itself. A static method is accessed through the class. An instance method is accessed through an instance of the class.

A method has a (possibly empty) list of parameters, which represent values or variable references passed to the method. A method also has a return type, which specifies the type of the value computed and returned by the method. A method's return type is void if it doesn't return a value.

// Instance method with no return value.
void AddData(String data);

// Instance method *with* a return value.
Int32 GetDataSize();

// Instance method accepting/returning a runtime class.
// Notice that you don't say "&" nor "*" for reference types.
BasicClass MergeWith(BasicClass other);

// Asynchronous instance methods.
Windows.Foundation.IAsyncAction UpdateAsync();
Windows.Foundation.IAsyncOperation<Boolean> TrySaveAsync();

// Instance method that returns a value through a parameter.
Boolean TryParseInt16(String input, out Int16 value);

// Instance method that receives a reference to a value type.
Double CalculateArea(ref const Windows.Foundation.Rect value);

// Instance method accepting or returning a conformant array.
void SetBytes(UInt8[] bytes);
UInt8[] GetBytes();

// instance method that writes to a caller-provided conformant array
void ReadBytes(ref UInt8[] bytes);

The signature of a method must be unique in the class in which the method is declared. The signature of a method consists of the name of the method, the types of its parameters, and/or the number of its parameters. The signature of a method doesn't include the return type.

Method visibility modifiers

A method may have one of two optional visibility modifiers when the method is present in a derived class.

The overridable modifier states that this method may be overridden by a method (with the same name and signature) belonging to a subclass.

The protected modifier states that this method is only accessible by members in a subsequently derived class.

Method overloading

Method overloading permits multiple methods in the same class to have the same name as long as their parameters differ in number (in other words the methods have different arity).

runtimeclass Test
    static void F();
    static void F(Double x);
    static void F(Double x, Double y);


All methods with the same name should have differing arity. That's because weakly-typed programming languages don't support overloading by type.


Parameters are used to pass values or variable references to a method. A parameter describes a slot with a type and a name, and optionally some modifier keyword. An argument is an actual value passed in that slot from the caller of the method to the callee.

Parameters of a method get their value from the specific argument that is specified when the method is invoked. The way arguments are passed between caller and callee depends on the type of the parameter. By default, all parameters are input parameters, that is they are marshaled from the caller to the callee only. The modifier keywords ref, ref const, and out can be added to modify the default direction of marshaling between caller and callee, and create output parameters. Not all keywords are valid with all parameter types, though; valid combinations are detailed below.


The Common Language Runtime (CLR) has concepts and modifier keywords that might appear to be similar to the ones described in this section. However in practice those are unrelated, and the effect of these modifiers is specific to the design and functioning of the Windows Runtime.

Value types are implicitly input parameters, and by default a copy of the argument is passed from the caller to the callee. Value parameters can be transformed into output parameters with the out keyword; in that case the argument is marshaled instead from the callee back to the caller only.

runtimeclass Test
    static void Divide(Int32 x, Int32 y, out Int32 result, out Int32 remainder);

As a special performance optimization, struct types (and no other type), which are normally passed by value as a full copy, can be made to be passed by pointer to the immutable struct. This is achieved with the ref const (not const ref) keyword, which marks the struct parameter as an input parameter, but instructs the marshaler to pass a pointer to the struct's storage, instead of passing a full copy of the struct. Note however that the struct is immutable; the pointer is conceptually a const pointer. There is no boxing involved. This is a practical choice when accepting a value as large as a Matrix4x4, for example.

runtimeclass Test
    static Boolean IsIdentity(ref const Windows.Foundation.Numerics.Matrix4x4 m);

Reference types are also implicitly input parameters, meaning that the caller is responsible for allocating the object and passing a reference to it as argument; however since the argument is a reference to the object, modifications to that object by the callee are observed by the caller after the call. Alternatively, a reference type can be made an output parameter with the out keyword. In that case the roles are reversed; the callee is the one allocating the object and returning it back to the caller. Again, the ref keywords cannot be used in general with reference types (see exception below).

runtimeclass Test
    static void CreateObjectWithConfig(Config config, out MyClass newObject);

The following table summarizes the behavior of the marshaling keywords for value parameters and reference parameters:

Behavior Allocated by Keyword Types Remarks
Input parameter Caller (none) All types Default behavior
ref const Struct only Performance optimization
Output parameter Callee out All types

Windows Runtime supports array types, whose behavior as parameter is somewhat different. An array is a data structure that contains a number of variables stored sequentially and accessed via an index. The variables contained in an array—also called the elements of the array—are all of the same type, and this type is called the element type of the array.

MIDL 3.0 supports declarations of a single-dimensional array.

An array parameter is a reference type, and like all reference types is by default an input parameter. In that case, the caller allocates the array to the callee, which can read its elements but cannot modify them (read-only). This is called the pass array pattern. Alternatively, the fill array pattern can be used by adding the ref keyword to the parameter; in that setup, the array is still allocated by the caller, but is conceptually an output parameter in the sense that the callee will fill the values of the array elements. Finally, the last pattern is the receive array where (like all output reference parameters) the callee is both allocating and initializing the argument before it is returned to the caller.

runtimeclass Test
    // Pass array pattern: read-only array from caller to callee
    void PassArray(Int32[] values);

    // Fill array pattern: caller allocates array for callee to fill
    void FillArray(ref Int32[] values);

    // Receive array pattern: callee allocates and fill an array returned to caller
    void ReceiveArray(out Int32[] values);

The following table summarizes the behavior for arrays and their elements:

Array pattern Keyword Allocated by Elements access by callee
"Pass array" (none) Caller Read-only
"Fill array" ref Caller Write-only
"Receive array" out Callee Read-write

For more info about using C-style array parameters—also known as conformant arrays—with C++/WinRT, see Array parameters.

Static and instance methods

A method declared with a static modifier prefixed is a static method. A static method has no access to a specific instance, and therefore can only directly access other static members of the class.

A method declared without a static modifier is an instance method. An instance method has access to a specific instance, and can access both static and instance members of the class.

The following Entity class has both static and instance members.

runtimeclass Entity
    Int32 SerialNo { get; };
    static Int32 GetNextSerialNo();
    static void SetNextSerialNo(Int32 value);

Each Entity instance contains its own serial number (and presumably some other information that is not shown here). Internally, the Entity constructor (which is like an instance method) initializes the new instance with the next available serial number.

The SerialNo property provides access to the serial number for the instance on which you invoke the property get method.

The GetNextSerialNo and SetNextSerialNo static methods can access the internal next available serial number static member of the Entity class.

Overridable and protected methods

All methods in a Windows Runtime type are effectively virtual. When a virtual method is invoked, the run-time type of the instance for which that invocation takes place determines the actual method implementation to invoke.

A method can be overridden in a derived class. When an instance method declaration includes an overridable modifier, the method can be overridden by derived classes. Whether a derived class does actually override an overridable base class method is determined by the implementation; it's not present in the metadata. If a derived class redeclares a method in the base class, then it declares a new method that sits alongside the derived class method, rather than overriding it.

When an instance method declaration includes a protected modifier, the method is visible only to derived classes.


An event declaration is a member that specifies that a class is an event source. Such an event source provides notifications to any recipient that implements a delegate (a method with a specific signature).

You declare an event using the event keyword, followed by the delegate type name (which describes the required method signature), followed by the name of the event. Here's an example event that uses an existing delegate type from the platform.

runtimeclass Area
    event Windows.UI.Xaml.WindowSizeChangedEventHandler SizeChanged;

An event declaration implicitly adds two methods to the class: an add method, which a client calls to add an event handler to the source, and a remove method, which a client calls to remove a previously added event handler. Here are more examples.

// Instance event with no meaningful payload.
event Windows.Foundation.TypedEventHandler<BasicClass, Object> Changed;

// Instance event with event parameters.
event Windows.Foundation.TypedEventHandler<BasicClass, BasicClassSaveCompletedEventArgs> SaveCompleted;

// Static event with no meaningful payload.
static event Windows.Foundation.EventHandler<Object> ResetOccurred;

// Static event with event parameters.
static event Windows.Foundation.EventHandler<BasicClassDeviceAddedEventArgs> DeviceAdded;

By convention, two parameters are always passed to a Windows Runtime event handler: the identity of the sender, and an event arguments object. The sender is the object that raised the event, or null for static events. If the event has no meaningful payload, then the event arguments is an Object whose value is null.


A delegate type specifies a method with a particular parameter list and return type. A single instance of an event can contain any number of references to instances of its delegate type. The declaration is similar to that of a regular member method, except that it exists outside of a runtime class, and it's prefixed with the delegate keyword.

A delegate makes it possible to treat methods as entities that can be assigned to variables and passed as parameters. Delegates are similar to the concept of function pointers found in some other languages. But, unlike function pointers, delegates are object-oriented and type-safe.

If we don't want to use the WindowSizeChangedEventHandler delegate type from the platform, then we can define our own delegate type.

delegate void SizeChangedHandler(Object sender, Windows.UI.Core.WindowSizeChangedEventArgs args);

An instance of our SizeChangedHandler delegate type can reference any method that takes two arguments (an Object, and a WindowSizeChangedEventArgs), and returns void. After we've discussed structs, you'll also be able to replace the WindowSizeChangedEventArgs parameter with an event args type of your own.

An interesting and useful property of a delegate is that it doesn't know or care about the class of the method it references; all that matters is that the referenced method has the same parameters and return type as the delegate.

You can optionally attribute a delegate declaration with [uuid(...)].

Also see Delegates returning HRESULT.


A struct is a data structure that can contain data members (fields). But, unlike a class, a struct is a value type.

Structs are particularly useful for small data structures that have value semantics. Complex numbers, or points in a coordinate system, are good examples of structs. The use of structs rather than classes for small data structures can make a large difference in the number of memory allocations that an application performs.

Let's use an example to contrast classes and structs. Here's a version of Point first as a class.

runtimeclass Point
    Point(Int32 x, Int32 y);
    Int32 x;
    Int32 y;

This C# program creates and initializes an array of 100 instances of Point. With Point implemented as a class, 101 separate objects are instantiated: one for the array object itself; and one for each of the 100 Point elements.

class Test
    static Test()
        Point[] points = new Point[100];
        for (Int32 i = 0; i < 100; ++i) points[i] = new Point(i, i);

A more performant alternative is to make Point a struct, instead of a class.

struct Point
    Int32 x;
    Int32 y;

Now, only one object is instantiated—the array object itself. The Point elements are stored in line inside the array; a memory arrangement that processor caches are able to use to powerful effect.

Changing a struct is a binary breaking change. Therefore, structs implemented as part of Windows itself are not altered once introduced.


An interface defines a contract that can be implemented by classes. An interface can contain methods, properties, and events—just like classes.

Unlike a class, an interface doesn't provide implementations of the members it defines. It merely specifies the members that must be supplied by any class that implements the interface.

Interfaces may require a class that implements the interface to also implement other interfaces. In the following example, the interface IComboBox requires that any class implementing IComboBox, also implement both ITextBox and IListBox. Additionally, a class implementing IComboBox must also implement IControl. That's because both ITextBox and IListBox require it.

interface IControl
    void Paint();

interface ITextBox requires IControl
    void SetText(String text);

interface IListBox requires IControl
    void SetItems(String[] items);

interface IComboBox requires ITextBox, IListBox

A class can implement zero or more interfaces. In the next example, the class EditBox implements both IControl and IDataBound.

interface IDataBound
    void Bind(Binder b);

runtimeclass EditBox : IControl, IDataBound

For Windows Runtime types in the Windows platform, an interface is defined if developers who consume those types are expected to implement the interface. Another use case for defining an interface is when multiple runtime classes implement the interface, and developers consuming those runtime classes will access different types of object generically (and thus polymorphically) via that common interface.


Think twice about using the requires keyword in MIDL 3.0. It can lead to messy designs, especially when versioning is taken into account.


An enum type (or enumerated type, or enumeration) is a distinct value type with a set of named constants. The following example defines and uses an enum type named Color with three constant values: Red, Green, and Blue.

enum Color
    Blue, // Trailing comma is optional, but recommended to make future changes easier.

Each enum type has a corresponding integral type called the underlying type of the enum type. The underlying type of an enum is either Int32 or UInt32.

The Windows Runtime supports two kinds of enums: normal enums, and flags enums. An enum of the normal kind expresses a set of exclusive values; while one of the flags kind represents a set of Boolean values. To enable bitwise operators for a flags enum, the MIDL 3.0 compiler generates C++ operator overloads.

A flags enum has the [flags] attribute applied. In that case, the underlying type of the enum is UInt32. When the [flags] attribute is not present (a normal enum), the underlying type of the enum is Int32. It's not possible to declare an enum as any other type.

enum SetOfBooleanValues
    None   = 0x00000000,
    Value1 = 0x00000001,
    Value2 = 0x00000002,
    Value3 = 0x00000004,

An enum type's storage format and range of possible values are determined by its underlying type. The set of values that an enum type can take on is not limited by its declared enum members.

The following example defines an enum type named Alignment, with an underlying type of Int32.

enum Alignment
    Left = -1,
    Center = 0,
    Right = 1

As is also true for C and C++, a MIDL 3.0 enum can include a constant expression that specifies the value of the member (as seen above). The constant value for each enum member must be in the range of the underlying type of the enum. When an enum member declaration doesn't explicitly specify a value, the member is given the value zero (if it is the first member in the enum type), or the value of the textually preceding enum member plus one.

The following example defines an enum type named Permissions, with an underlying type of UInt32.

enum Permissions
    None = 0x0000,
    Camera = 0x0001,
    Microphone = 0x0002


Types, members, and other entities in MIDL 3.0 source code support modifiers that control certain aspects of their behavior. For example, the accessibility of a method is controlled using the protected access modifier. MIDL 3.0 generalizes this capability such that user-defined types of declarative information can be attached to program entities, and retrieved at run-time from the metadata.

Programs specify this additional declarative information by defining and using attributes.

The next example defines a HelpAttribute attribute, which can be placed on program entities to provide links to their associated documentation. As you can see, an attribute is essentially a struct type, so it doesn't have a constructor, and contains only data members.

[attributeusage(target_runtimeclass, target_event, target_method, target_property)]
attribute HelpAttribute
    String ClassUri;
    String MemberTopic;

An attribute can be applied by giving its name, along with any arguments, inside square brackets just before the associated declaration. If an attribute's name ends in Attribute, then that part of the name can be omitted when the attribute is referenced. For example, the HelpAttribute attribute can be used like this.

[Help("", "BookSku class")]
runtimeclass BookSku : Windows.UI.Xaml.Data.INotifyPropertyChanged
    [Help("", "Title method")]
    String Title;

You can apply the same attribute to multiple declarations by using a scope block following the attribute. That is, an attribute immediately followed by braces surrounding the declarations to which the attribute applies.

runtimeclass Widget
    [Help("", "Widget members")]
        void Display(String text);
        void Print();
        Single Rate;

Attributes implemented as part of Windows itself are usually in the Windows.Foundation namespace.

As shown in the first example, you use the [attributeusage(<target>)] attribute on your attribute definition. Valid target values are target_all, target_delegate, target_enum, target_event, target_field, target_interface, target_method, target_parameter, target_property, target_runtimeclass, and target_struct. You can include multiple targets within the parentheses, separated by commas.

Other attributes you can apply to an attribute are [allowmultiple] and [attributename("<name>")].

Parameterized types

The example below produces error MIDL2025: [msg]syntax error [context]: expecting > or, near ">>".

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

Instead, insert a space between the two > characters so that the pair of template-closing characters is not misinterpreted as a right-shift operator.

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

The example below produces error MIDL2025: [msg]syntax error [context]: expecting > or, near "[". This is because it's invalid to use an array as a parameter type argument to a parameterized interface.

Windows.Foundation.IAsyncOperation<Int32[]> RetrieveArrayAsync();

For the solution, see Returning an array asynchronously.