如何在C++/CX类中注册float值类型的DependencyProperty

I’ve been in your shoes—getting value-type DependencyProperties working in C++/CX can feel counterintuitive after dealing with reference types like Brush. The core issue with your attempts is that native C++ value types (like float) need to be boxed to work with WinRT’s DependencyProperty system, which expects a WinRT-compatible type reference.

The Correct Type Parameter #

Instead of typeid(float) or your other attempts, use Platform::Box<float>::typeid as the second argument to DependencyProperty::Register. This gives you the WinRT type information for the boxed float value, which is exactly what the method requires.

Full Example Code #

Header File (MyCustomClass.h)

#pragma once
#include "Windows.UI.Xaml.h"

namespace YourNamespace
{
    public ref class MyCustomClass sealed : Windows::UI::Xaml::DependencyObject
    {
    public:
        static property Windows::UI::Xaml::DependencyProperty^ MyFloatProperty { Windows::UI::Xaml::DependencyProperty^ get(); };
        property float MyFloat { float get(); void set(float value); };

    private:
        static Windows::UI::Xaml::DependencyProperty^ _myFloatProperty;
        static void OnMyFloatChanged(Windows::UI::Xaml::DependencyObject^ d, Windows::UI::Xaml::DependencyPropertyChangedEventArgs^ e);
    };
}

Implementation File (MyCustomClass.cpp)

#include "pch.h"
#include "MyCustomClass.h"

using namespace Windows::UI::Xaml;
using namespace Platform;

namespace YourNamespace
{
    DependencyProperty^ MyCustomClass::MyFloatProperty::get()
    {
        return _myFloatProperty;
    }

    float MyCustomClass::MyFloat::get()
    {
        // Unbox the stored value from the DependencyProperty
        return safe_cast<IBox<float>^>(GetValue(MyFloatProperty))->Value;
    }

    void MyCustomClass::MyFloat::set(float value)
    {
        // Box the float when passing it to SetValue
        SetValue(MyFloatProperty, Box<float>::Create(value));
    }

    DependencyProperty^ MyCustomClass::_myFloatProperty = DependencyProperty::Register(
        "MyFloat",
        Box<float>::typeid,
        MyCustomClass::typeid,
        ref new PropertyMetadata(
            // Default value is a boxed float (0.0f in this case)
            Box<float>::Create(0.0f),
            ref new PropertyChangedCallback(&MyCustomClass::OnMyFloatChanged)
        )
    );

    void MyCustomClass::OnMyFloatChanged(DependencyObject^ d, DependencyPropertyChangedEventArgs^ e)
    {
        MyCustomClass^ instance = safe_cast<MyCustomClass^>(d);
        float oldValue = safe_cast<IBox<float>^>(e->OldValue)->Value;
        float newValue = safe_cast<IBox<float>^>(e->NewValue)->Value;

        // Add your property change logic here
    }
}

Key Notes: #

• When retrieving the property value, you must unbox it with safe_cast<IBox<float>^> and access the Value property to get the native float.
• When setting the property or defining the default value in PropertyMetadata, you have to box the float using Box<float>::Create(value).
• This pattern works for all native value types—just replace float with int, double, bool, etc., and use the corresponding Platform::Box<T>::typeid.

内容的提问来源于stack exchange，提问作者ackh