Move to C++/WinRT from C#


If you've read this topic before, and you're returning to it with a particular task in mind, then you can jump to the Find content based on the task you're performing section of this topic.

This topic comprehensively catalogs the technical details involved in porting the source code in a C# project to its equivalent in C++/WinRT.

For a case study of porting one of the Universal Windows Platform (UWP) app samples, see the companion topic Porting the Clipboard sample to C++/WinRT from C#. You can gain porting practice and experience by following along with that walkthrough, and porting the sample for yourself as you go.

How to prepare, and what to expect

The case study Porting the Clipboard sample to C++/WinRT from C# illustrates examples of the kinds of software design decisions that you'll make while porting a project to C++/WinRT. So, it's a good idea to prepare for porting by gaining a solid understanding of how the existing code works. That way, you'll get a good overview of the app's functionality, and the code's structure, and then the decisions that you make will always take you forward, and in the right direction.

In terms of what kinds of porting changes to expect, you could group them into four categories.

  • Port the language projection. The Windows Runtime (WinRT) is projected into various programming languages. Each of those language projections is designed to feel idiomatic to the programming language in question. For C#, some Windows Runtime types are projected as .NET types. So for example you'll be translating System.Collections.Generic.IReadOnlyList<T> back to Windows.Foundation.Collections.IVectorView<T>. Also in C#, some Windows Runtime operations are projected as convenient C# language features. An example is that in C# you use the += operator syntax to register an event-handling delegate. So you'll be translating language features such as that back to the fundamental operation that's being performed (event registration, in this example).
  • Port language syntax. Many of these changes are simple mechanical transforms, replacing one symbol for another. For example, changing dot (.) to double-colon (::).
  • Port language procedure. Some of these can be simple, repetitive changes (such as myObject.MyProperty to myObject.MyProperty()). Others need deeper changes (for example, porting a procedure that involves the use of System.Text.StringBuilder to one that involves the use of std::wostringstream).
  • Porting-related tasks that are specific to C++/WinRT. Certain details of the Windows Runtime are taken care of impliclicly by C#, behind the scenes. Those details are done explicitly in C++/WinRT. An example is that you use an .idl file to define your runtime classes.

After the task-based index that follows, the rest of the sections in this topic are structured according to the taxonomy above.

Find content based on the task you're performing

Task Content
Author a Windows Runtime component (WRC) Certain functionality can be achieved (or certain APIs called) only with C++. You can factor that functionality into a C++/WinRT WRC, and then consume the WRC from (for example) a C# app. See Windows Runtime components with C++/WinRT and If you're authoring a runtime class in a Windows Runtime component.
Port an async method It's a good idea for the first line of an asynchronous method in a C++/WinRT runtime class to be auto lifetime = get_strong(); (see Safely accessing the this pointer in a class-member coroutine).

Porting from Task, see Async action.
Porting from Task<T>, see Async operation.
Porting from async void, see Fire-and-forget method.
Port a class First, determine whether the class needs to be a runtime class, or whether it can be an ordinary class. To help you decide that, see the very beginning of Author APIs with C++/WinRT. Then, see the next three rows below.
Port a runtime class A class that shares functionality outside of the C++ app, or a class that's used in XAML data binding. See If you're authoring a runtime class in a Windows Runtime component, or If you're authoring a runtime class to be referenced in your XAML UI.

Those links describe this in more detail, but a runtime class must be declared in IDL. If your project already contains an an IDL file (for example, Project.idl), then we recommend that you declare any new runtime class in that file. In IDL, declare any methods and data members that will be used outside your app, or that will be used in XAML. After updating the IDL file, rebuild, and look at the generated stub files (.h and .cpp) in your project's Generated Files folder (In Solution Explorer, with the project node selected, make sure Show All Files is toggled on). Compare the stub files with the files already in your project, adding files or adding/updating function signatures as necessary. Stub file syntax is always correct, so we recommend that you use it in order to minimize build errors. Once the stubs in your project match those in the stub files, you can go ahead and implement them by porting the C# code over.
Port an ordinary classs See If you're not authoring a runtime class.
Author IDL Introduction to Microsoft Interface Definition Language 3.0
If you're authoring a runtime class to be referenced in your XAML UI
Consuming objects from XAML markup
Define your runtime classes in IDL
Port a collection Collections with C++/WinRT
Making a data source available to XAML markup
Associative container
Vector member access
Port an event Event handler delegate as class member
Revoke event handler delegate
Port a method From C#: private async void SampleButton_Tapped(object sender, Windows.UI.Xaml.Input.TappedRoutedEventArgs e) { ... }
To the C++/WinRT .h file: fire_and_forget SampleButton_Tapped(IInspectable const&, RoutedEventArgs const&);
To the C++/WinRT .cpp file: fire_and_forget OcrFileImage::SampleButton_Tapped(IInspectable const&, RoutedEventArgs const&) {...}
Port strings String handling in C++/WinRT
Boxing and unboxing a string
Type conversion (type casting) C#: o.ToString()
C++/WinRT: to_hstring(static_cast<int>(o))
Also see ToString.

C#: (Value)o
C++/WinRT: unbox_value<Value>(o)
Throws if unboxing fails. Also see Boxing and unboxing.

C#: o as Value? ?? fallback
C++/WinRT: unbox_value_or<Value>(o, fallback)
Returns fallback if unboxing fails. Also see Boxing and unboxing.

C#: (Class)o
Throws if conversion fails.

C#: o as Class
C++/WinRT: o.try_as<Class>()
Returns null if conversion fails.

Changes that involve the language projection

Category C# C++/WinRT See also
Untyped object object, or System.Object Windows::Foundation::IInspectable Porting the EnableClipboardContentChangedNotifications method
Projection namespaces using System; using namespace Windows::Foundation;
using System.Collections.Generic; using namespace Windows::Foundation::Collections;
Size of a collection collection.Count collection.Size() Porting the BuildClipboardFormatsOutputString method
Typical collection type IList<T>, and Add to add an element. IVector<T>, and Append to add an element. If you use a std::vector anywhere, then push_back to add an element.
Read-only collection type IReadOnlyList<T> IVectorView<T> Porting the BuildClipboardFormatsOutputString method
Event handler delegate as class member myObject.EventName += Handler; token = myObject.EventName({ get_weak(), &Class::Handler }); Porting the EnableClipboardContentChangedNotifications method
Revoke event handler delegate myObject.EventName -= Handler; myObject.EventName(token); Porting the EnableClipboardContentChangedNotifications method
Associative container IDictionary<K, V> IMap<K, V>
Vector member access x = v[i];
v[i] = x;
x = v.GetAt(i);
v.SetAt(i, x);

Register/revoke an event handler

In C++/WinRT, you have several syntactic options to register/revoke an event handler delegate, as described in Handle events by using delegates in C++/WinRT. Also see Porting the EnableClipboardContentChangedNotifications method.

Sometimes, for example when an event recipient (an object handling an event) is about to be destroyed, you'll want to revoke an event handler so that the event source (the object raising the event) doesn't call into a destroyed object. See Revoke a registered delegate. In cases like that, create an event_token member variable for your event handlers. For an example, see Porting the EnableClipboardContentChangedNotifications method.

You can also register an event handler in XAML markup.

<Button x:Name="OpenButton" Click="OpenButton_Click" />

In C#, your OpenButton_Click method can be private, and XAML will still be able to connect it to the ButtonBase.Click event raised by OpenButton.

In C++/WinRT, your OpenButton_Click method must be public in your implementation type if you want to register it in XAML markup. If you register an event handler only in imperative code, then the event handler doesn't need to be public.

namespace winrt::MyProject::implementation
    struct MyPage : MyPageT<MyPage>
        void OpenButton_Click(
            winrt::Windows:Foundation::IInspectable const& sender,
            winrt::Windows::UI::Xaml::RoutedEventArgs const& args);

Alternatively, you can make the registering XAML page a friend of your implementation type, and OpenButton_Click private.

namespace winrt::MyProject::implementation
    struct MyPage : MyPageT<MyPage>
        friend MyPageT;
        void OpenButton_Click(
            winrt::Windows:Foundation::IInspectable const& sender,
            winrt::Windows::UI::Xaml::RoutedEventArgs const& args);

One final scenario is where the C# project that you're porting binds to the event handler from markup (for more background on that scenario, see Functions in x:Bind).

<Button x:Name="OpenButton" Click="{x:Bind OpenButton_Click}" />

You could just change that markup to the more simple Click="OpenButton_Click". Or, if you prefer, you can keep that markup as it is. All you have to do to support it is to declare the event handler in IDL.

void OpenButton_Click(Object sender, Windows.UI.Xaml.RoutedEventArgs e);


Declare the function as void even if you implement it as Fire and forget.

Changes that involve the language syntax

Category C# C++/WinRT See also
Access modifiers public \<member\> public:
Porting the Button_Click method
Access a data member this.variable this->variable  
Async action async Task ... IAsyncAction ... IAsyncAction interface, Concurrency and asynchronous operations with C++/WinRT
Async operation async Task<T> ... IAsyncOperation<T> ... IAsyncOperation interface, Concurrency and asynchronous operations with C++/WinRT
Fire-and-forget method (implies async) async void ... winrt::fire_and_forget ... Porting the CopyButton_Click method, Fire and forget
Access an enumerated constant E.Value E::Value Porting the DisplayChangedFormats method
Cooperatively wait await ... co_await ... Porting the CopyButton_Click method
Collection of projected types as a private field private List<MyRuntimeClass> myRuntimeClasses = new List<MyRuntimeClass>(); std::vector
GUID construction private static readonly Guid myGuid = new Guid("C380465D-2271-428C-9B83-ECEA3B4A85C1"); winrt::guid myGuid{ 0xC380465D, 0x2271, 0x428C, { 0x9B, 0x83, 0xEC, 0xEA, 0x3B, 0x4A, 0x85, 0xC1} };
Namespace separator A.B.T A::B::T
Null null nullptr Porting the UpdateStatus method
Obtain a type object typeof(MyType) winrt::xaml_typename<MyType>() Porting the Scenarios property
Parameter declaration for a method MyType MyType const& Parameter-passing
Parameter declaration for an async method MyType MyType Parameter-passing
Call a static method T.Method() T::Method()
Strings string, or System.String winrt::hstring String handling in C++/WinRT
String literal "a string literal" L"a string literal" Porting the constructor, Current, and FEATURE_NAME
Inferred (or deduced) type var auto Porting the BuildClipboardFormatsOutputString method
Using-directive using A.B.C; using namespace A::B::C; Porting the constructor, Current, and FEATURE_NAME
Verbatim/raw string literal @"verbatim string literal" LR"(raw string literal)" Porting the DisplayToast method


If a header file doesn't contain a using namespace directive for a given namespace, then you'll have to fully-qualify all type names for that namespace; or at least qualify them sufficiently for the compiler to find them. For an example, see Porting the DisplayToast method.

Porting classes and members

You'll need to decide, for each C# type, whether to port it to a Windows Runtime type, or to a regular C++ class/struct/enumeration. For more info, and detailed examples illustrating how to make those decisions, see Porting the Clipboard sample to C++/WinRT from C#.

A C# property typically becomes an accessor function, a mutator function, and a backing data member. For more info, and an example, see Porting the IsClipboardContentChangedEnabled property.

For non-static fields, make them data members of your implementation type.

A C# static field becomes a C++/WinRT static accessor and/or mutator function. For more info, and an example, see Porting the constructor, Current, and FEATURE_NAME.

For member functions, again, you'll need to decide for each one whether or not it belongs in the IDL, or whether it's a public or private member function of your implementation type. For more info, and examples of how to decide, see IDL for the MainPage type.

Porting XAML markup, and asset files

In the case of Porting the Clipboard sample to C++/WinRT from C#, we were able to use the same XAML markup (including resources) and asset files across the C# and the C++/WinRT project. In some cases, edits to markup will be necessary to achieve that. See Copy the XAML and styles necessary to finish up porting MainPage.

Changes that involve procedures within the language

Category C# C++/WinRT See also
Lifetime management in an async method N/A auto lifetime{ get_strong() }; or
auto lifetime = get_strong();
Porting the CopyButton_Click method
Disposal using (var t = v) auto t{ v };
t.Close(); // or let wrapper destructor do the work
Porting the CopyImage method
Construct object new MyType(args) MyType{ args } or
Porting the Scenarios property
Create uninitialized reference MyType myObject; MyType myObject{ nullptr }; or
MyType myObject = nullptr;
Porting the constructor, Current, and FEATURE_NAME
Construct object into variable with args var myObject = new MyType(args); auto myObject{ MyType{ args } }; or
auto myObject{ MyType(args) }; or
auto myObject = MyType{ args }; or
auto myObject = MyType(args); or
MyType myObject{ args }; or
MyType myObject(args);
Porting the Footer_Click method
Construct object into variable without args var myObject = new T(); MyType myObject; Porting the BuildClipboardFormatsOutputString method
Object initialization shorthand var p = new FileOpenPicker{
    ViewMode = PickerViewMode.List
FileOpenPicker p;
Bulk vector operation var p = new FileOpenPicker{
    FileTypeFilter = { ".png", ".jpg", ".gif" }
FileOpenPicker p;
p.FileTypeFilter().ReplaceAll({ L".png", L".jpg", L".gif" });
Porting the CopyButton_Click method
Iterate over collection foreach (var v in c) for (auto&& v : c) Porting the BuildClipboardFormatsOutputString method
Catch an exception catch (Exception ex) catch (winrt::hresult_error const& ex) Porting the PasteButton_Click method
Exception details ex.Message ex.message() Porting the PasteButton_Click method
Get a property value myObject.MyProperty myObject.MyProperty() Porting the NotifyUser method
Set a property value myObject.MyProperty = value; myObject.MyProperty(value);
Increment a property value myObject.MyProperty += v; myObject.MyProperty(thing.Property() + v);
For strings, switch to a builder
ToString() myObject.ToString() winrt::to_hstring(myObject) ToString()
Language string to Windows Runtime string N/A winrt::hstring{ s }
String-building StringBuilder builder;
std::wostringstream builder;
builder << ...;
String interpolation $"{i++}) {s.Title}" winrt::to_hstring, and/or winrt::hstring::operator+ Porting the OnNavigatedTo method
Empty string for comparison System.String.Empty winrt::hstring::empty Porting the UpdateStatus method
Create empty string var myEmptyString = String.Empty; winrt::hstring myEmptyString{ L"" };
Dictionary operations map[k] = v; // replaces any existing
v = map[k]; // throws if not present
map.Insert(k, v); // replaces any existing
v = map.Lookup(k); // throws if not present
Type conversion (throw on failure) (MyType)v<MyType>() Porting the Footer_Click method
Type conversion (null on failure) v as MyType v.try_as<MyType>() Porting the PasteButton_Click method
XAML elements with x:Name are properties MyNamedElement MyNamedElement() Porting the constructor, Current, and FEATURE_NAME
Switch to the UI thread CoreDispatcher.RunAsync CoreDispatcher.RunAsync, or winrt::resume_foreground Porting the NotifyUser method, and Porting the HistoryAndRoaming method
UI element construction in imperative code in a XAML page See UI element construction See UI element construction

The following sections go into more detail regarding some of the items in the table.

UI element construction

These code examples show the construction of a UI element in the imperative code of a XAML page.

var myTextBlock = new TextBlock()
    Text = "Text",
    Style = (Windows.UI.Xaml.Style)this.Resources["MyTextBlockStyle"]
TextBlock myTextBlock;


C# types provide the Object.ToString method.

int i = 2;
var s = i.ToString(); // s is a System.String with value "2".

C++/WinRT doesn't directly provide this facility, but you can turn to alternatives.

int i{ 2 };
auto s{ std::to_wstring(i) }; // s is a std::wstring with value L"2".

C++/WinRT also supports winrt::to_hstring for a limited number of types. You'll need to add overloads for any additional types you want to stringify.

Language Stringify int Stringify enum
C# string result = "hello, " + intValue.ToString();
string result = $"hello, {intValue}";
string result = "status: " + status.ToString();
string result = $"status: {status}";
C++/WinRT hstring result = L"hello, " + to_hstring(intValue); // must define overload (see below)
hstring result = L"status: " + to_hstring(status);

In the case of stringifying an enum, you will need to provide the implementation of winrt::to_hstring.

namespace winrt
    hstring to_hstring(StatusEnum status)
        switch (status)
        case StatusEnum::Success: return L"Success";
        case StatusEnum::AccessDenied: return L"AccessDenied";
        case StatusEnum::DisabledByPolicy: return L"DisabledByPolicy";
        default: return to_hstring(static_cast<int>(status));

These stringifications are often consumed implicitly by data binding.

You have <Run Text="{Binding FlowerCount}"/> flowers.
Most recent status is <Run Text="{x:Bind LatestOperation.Status}"/>.

These bindings will perform winrt::to_hstring of the bound property. In the case of the second example (the StatusEnum), you must provide your own overload of winrt::to_hstring, otherwise you'll get a compiler error.

Also see Porting the Footer_Click method.


For string building, C# has a built-in StringBuilder type.

Category C# C++/WinRT
String-building StringBuilder builder;
std::wostringstream builder;
builder << ...;
Append a Windows Runtime string, preserving nulls builder.Append(s); builder << std::wstring_view{ s };
Add a newline builder.Append(Environment.NewLine); builder << std::endl;
Access the result s = builder.ToString(); ws = builder.str();

Also see Porting the BuildClipboardFormatsOutputString method, and Porting the DisplayChangedFormats method.

Running code on the main UI thread

This example is taken from the Barcode scanner sample.

When you want to do work on the main UI thread in a C# project, you typically use the CoreDispatcher.RunAsync method, like this.

private async void Watcher_Added(DeviceWatcher sender, DeviceInformation args)
    await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
        // Do work on the main UI thread here.

It's much simpler to express that in C++/WinRT. Notice that we're accepting parameters by value on the assumption we'll want to access them after the first suspension point (the co_await, in this case). For more info, see Parameter-passing.

winrt::fire_and_forget Watcher_Added(DeviceWatcher sender, winrt::DeviceInformation args)
    co_await Dispatcher();
    // Do work on the main UI thread here.

If you need to do the work at a priority other than the default, then see the winrt::resume_foreground function, which has an overload that takes a priority. For code examples showing how to await a call to winrt::resume_foreground, see Programming with thread affinity in mind.

Define your runtime classes in IDL

See IDL for the MainPage type, and Consolidate your .idl files.

Include the C++/WinRT Windows namespace header files that you need

In C++/WinRT, whenever you want to use a type from a Windows namespaces, you need to include the corresponding C++/WinRT Windows namespace header file. For an example, see Porting the NotifyUser method.

Boxing and unboxing

C# automatically boxes scalars into objects. C++/WinRT requires you to call the winrt::box_value function explicitly. Both languages require you to unbox explicitly. See Boxing and unboxing with C++/WinRT.

In the tables that follows, we'll use these definitions.

C# C++/WinRT
int i; int i;
string s; winrt::hstring s;
object o; IInspectable o;
Operation C# C++/WinRT
Boxing o = 1;
o = "string";
o = box_value(1);
o = box_value(L"string");
Unboxing i = (int)o;
s = (string)o;
i = unbox_value<int>(o);
s = unbox_value<winrt::hstring>(o);

C++/CX and C# raise exceptions if you try to unbox a null pointer to a value type. C++/WinRT considers this a programming error, and it crashes. In C++/WinRT, use the winrt::unbox_value_or function if you want to handle the case where the object is not of the type that you thought it was.

Scenario C# C++/WinRT
Unbox a known integer i = (int)o; i = unbox_value<int>(o);
If o is null System.NullReferenceException Crash
If o is not a boxed int System.InvalidCastException Crash
Unbox int, use fallback if null; crash if anything else i = o != null ? (int)o : fallback; i = o ? unbox_value<int>(o) : fallback;
Unbox int if possible; use fallback for anything else i = as int? ?? fallback; i = unbox_value_or<int>(o, fallback);

For an example, see Porting the OnNavigatedTo method, and Porting the Footer_Click method.

Boxing and unboxing a string

A string is in some ways a value type, and in other ways a reference type. C# and C++/WinRT treat strings differently.

The ABI type HSTRING is a pointer to a reference-counted string. But it doesn't derive from IInspectable, so it's not technically an object. Furthermore, a null HSTRING represents the empty string. Boxing of things not derived from IInspectable is done by wrapping them inside an IReference<T>, and the Windows Runtime provides a standard implementation in the form of the PropertyValue object (custom types are reported as PropertyType::OtherType).

C# represents a Windows Runtime string as a reference type; while C++/WinRT projects a string as a value type. This means that a boxed null string can have different representations depending how you got there.

Behavior C# C++/WinRT
Declarations object o;
string s;
IInspectable o;
hstring s;
String type category Reference type Value type
null HSTRING projects as "" hstring{}
Are null and "" identical? No Yes
Validity of null s = null;
s.Length raises NullReferenceException
s = hstring{};
s.size() == 0 (valid)
If you assign null string to object o = (string)null;
o == null
o = box_value(hstring{});
o != nullptr
If you assign "" to object o = "";
o != null
o = box_value(hstring{L""});
o != nullptr

Basic boxing and unboxing.

Operation C# C++/WinRT
Box a string o = s;
Empty string becomes non-null object.
o = box_value(s);
Empty string becomes non-null object.
Unbox a known string s = (string)o;
Null object becomes null string.
InvalidCastException if not a string.
s = unbox_value<hstring>(o);
Null object crashes.
Crash if not a string.
Unbox a possible string s = o as string;
Null object or non-string becomes null string.


s = o as string ?? fallback;
Null or non-string becomes fallback.
Empty string preserved.
s = unbox_value_or<hstring>(o, fallback);
Null or non-string becomes fallback.
Empty string preserved.

Making a class available to the {Binding} markup extension

If you intend to use the {Binding} markup extension to data bind to your data type, then see Binding object declared using {Binding}.

Consuming objects from XAML markup

In a C# project, you can consume private members and named elements from XAML markup. But in C++/WinRT, all entities consumed by using the XAML {x:Bind} markup extension must be exposed publicly in IDL.

Also, binding to a Boolean displays true or false in C#, but it shows Windows.Foundation.IReference`1<Boolean> in C++/WinRT.

For more info, and code examples, see Consuming objects from markup.

Making a data source available to XAML markup

In C++/WinRT version 2.0.190530.8 or later, winrt::single_threaded_observable_vector creates an observable vector that supports both IObservableVector<T> and IObservableVector<IInspectable>. For an example, see Porting the Scenarios property.

You can author your Midl file (.idl) like this (also see Factoring runtime classes into Midl files (.idl)).

namespace Bookstore
    runtimeclass BookSku { ... }

    runtimeclass BookstoreViewModel
        Windows.Foundation.Collections.IObservableVector<BookSku> BookSkus{ get; };

    runtimeclass MainPage : Windows.UI.Xaml.Controls.Page
        BookstoreViewModel MainViewModel{ get; };

And implement like this.

// BookstoreViewModel.h
struct BookstoreViewModel : BookstoreViewModelT<BookstoreViewModel>
        m_bookSkus = winrt::single_threaded_observable_vector<Bookstore::BookSku>();
        m_bookSkus.Append(winrt::make<Bookstore::implementation::BookSku>(L"To Kill A Mockingbird"));
	Windows::Foundation::Collections::IObservableVector<Bookstore::BookSku> BookSkus();
        return m_bookSkus;

    Windows::Foundation::Collections::IObservableVector<Bookstore::BookSku> m_bookSkus;

For more info, see XAML items controls; bind to a C++/WinRT collection, and Collections with C++/WinRT.

Making a data source available to XAML markup (prior to C++/WinRT 2.0.190530.8)

XAML data binding requires that an items source implements IIterable<IInspectable>, as well as one of the following combinations of interfaces.

  • IObservableVector<IInspectable>
  • IBindableVector and INotifyCollectionChanged
  • IBindableVector and IBindableObservableVector
  • IBindableVector by itself (will not respond to changes)
  • IVector<IInspectable>
  • IBindableIterable (will iterate and save elements into a private collection)

A generic interface such as IVector<T> can't be detected at runtime. Each IVector<T> has a different interface identifier (IID), which is a function of T. Any developer can expand the set of T arbitrarily, so clearly the XAML binding code can never know the full set to query for. That restriction isn't a problem for C# because every CLR object that implements IEnumerable<T> automatically implements IEnumerable. At the ABI level, that means that every object that implements IObservableVector<T> automatically implements IObservableVector<IInspectable>.

C++/WinRT doesn't offer that guarantee. If a C++/WinRT runtime class implements IObservableVector<T>, then we can't assume that an implementation of IObservableVector<IInspectable> is somehow also provided.

Consequently, here's how the previous example will need to look.

runtimeclass BookstoreViewModel
    // This is really an observable vector of BookSku.
    Windows.Foundation.Collections.IObservableVector<Object> BookSkus{ get; };

And the implementation.

// BookstoreViewModel.h
struct BookstoreViewModel : BookstoreViewModelT<BookstoreViewModel>
        m_bookSkus = winrt::single_threaded_observable_vector<Windows::Foundation::IInspectable>();
        m_bookSkus.Append(winrt::make<Bookstore::implementation::BookSku>(L"To Kill A Mockingbird"));
    // This is really an observable vector of BookSku.
	Windows::Foundation::Collections::IObservableVector<Windows::Foundation::IInspectable> BookSkus();
        return m_bookSkus;

    Windows::Foundation::Collections::IObservableVector<Windows::Foundation::IInspectable> m_bookSkus;

If you need to access objects in m_bookSkus, then you'll need to QI them back to Bookstore::BookSku.

Widget MyPage::BookstoreViewModel(winrt::hstring title)
    for (auto&& obj : m_bookSkus)
        auto bookSku =<Bookstore::BookSku>();
        if (bookSku.Title() == title) return bookSku;
    return nullptr;

Derived classes

In order to derive from a runtime class, the base class must be composable. C# doesn't require that you take any special steps to make your classes composable, but C++/WinRT does. You use the unsealed keyword to indicate that you want your class to be usable as a base class.

unsealed runtimeclass BasePage : Windows.UI.Xaml.Controls.Page
runtimeclass DerivedPage : BasePage

In the header file for your implementation type, you must include the base class header file before you include the autogenerated header for the derived class. Otherwise you'll get errors such as "Illegal use of this type as an expression".

// DerivedPage.h
#include "BasePage.h"       // This comes first.
#include "DerivedPage.g.h"  // Otherwise this header file will produce an error.

namespace winrt::MyNamespace::implementation
    struct DerivedPage : DerivedPageT<DerivedPage>

Important APIs