表达式必须具有 class 类型（C++ 初学者错误 Windows 8 App 使用 Win2D）

Question

我正在尝试开始在 Windows 8 应用程序中使用 Win2D。我只是尝试将库安装到我的新项目并使用其中一个函数绘制一个圆圈，但由于我不明白的错误而无法继续。

下面显示的是在 Mainpage.xaml.cpp 文件中自动创建的事件处理程序。我的错误是函数内部单行代码开头的'args'部分用红色下划线标出。将鼠标悬停在 'args' 下方的红线上会显示消息 "expression must have class type"。有人可以帮忙吗？

void AdditiveSignals::MainPage::CanvasAnimatedControl_Draw(Microsoft::Graphics::Canvas::UI::Xaml::ICanvasAnimatedControl^ sender, Microsoft::Graphics::Canvas::UI::Xaml::CanvasAnimatedDrawEventArgs^ args)
{
    args.DrawingSession.DrawCircle(640, 360, 30.0f, Colors->Blue);
}

如果需要，这里是完整的代码

在MainPage.xaml.h

//
// MainPage.xaml.h
// Declaration of the MainPage class.
//

#pragma once

#include "MainPage.g.h"

namespace AdditiveSignals
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public ref class MainPage sealed
    {
    public:
        MainPage();

    protected:
        virtual void OnNavigatedTo(Windows::UI::Xaml::Navigation::NavigationEventArgs^ e) override;
    private:
        void CanvasAnimatedControl_Draw(Microsoft::Graphics::Canvas::UI::Xaml::ICanvasAnimatedControl^ sender, Microsoft::Graphics::Canvas::UI::Xaml::CanvasAnimatedDrawEventArgs^ args);
    };
}

在MainPage.xaml

<Page
    x:Class="AdditiveSignals.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:AdditiveSignals"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:canvas="using:Microsoft.Graphics.Canvas.UI.Xaml"
    mc:Ignorable="d"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Grid>
        <canvas:CanvasAnimatedControl Draw="CanvasAnimatedControl_Draw" />
    </Grid>
</Page>

在MainPage.xaml.cpp（完整代码）

//
// MainPage.xaml.cpp
// Implementation of the MainPage class.
//

#include "pch.h"
#include "MainPage.xaml.h"

using namespace AdditiveSignals;

using namespace Platform;
using namespace Windows::Foundation;
using namespace Windows::Foundation::Collections;
using namespace Windows::UI;
using namespace Windows::UI::Xaml;
using namespace Windows::UI::Xaml::Controls;
using namespace Windows::UI::Xaml::Controls::Primitives;
using namespace Windows::UI::Xaml::Data;
using namespace Windows::UI::Xaml::Input;
using namespace Windows::UI::Xaml::Media;
using namespace Windows::UI::Xaml::Navigation;

// The Blank Page item template is documented at http://go.microsoft.com/fwlink/?LinkId=234238

MainPage::MainPage()
{
    InitializeComponent();
}

/// <summary>
/// Invoked when this page is about to be displayed in a Frame.
/// </summary>
/// <param name="e">Event data that describes how this page was reached.  The Parameter
/// property is typically used to configure the page.</param>
void MainPage::OnNavigatedTo(NavigationEventArgs^ e)
{
    (void) e;   // Unused parameter

    // TODO: Prepare page for display here.

    // TODO: If your application contains multiple pages, ensure that you are
    // handling the hardware Back button by registering for the
    // Windows::Phone::UI::Input::HardwareButtons.BackPressed event.
    // If you are using the NavigationHelper provided by some templates,
    // this event is handled for you.
}

//This event handler will be called 60 times per second
void AdditiveSignals::MainPage::CanvasAnimatedControl_Draw(Microsoft::Graphics::Canvas::UI::Xaml::ICanvasAnimatedControl^ sender, Microsoft::Graphics::Canvas::UI::Xaml::CanvasAnimatedDrawEventArgs^ args)
{


    args.DrawingSession.DrawCircle(640, 360, 30.0f, Colors->Blue);
}

Answer 1

关于语言的标准说明：C++/CLI 旨在作为允许 C# 或其他 .Net 代码调用 C++ 代码的桥梁。它不打算作为主要的开发语言。如果你想学习 C++，我会使用 C++ 和 MFC 作为你的 GUI 工具包。如果您想学习托管代码，我会通过 WinForms 或 WPF 学习 C#。

也就是说：

void CanvasAnimatedControl_Draw(ICanvasAnimatedControl^ sender, CanvasAnimatedDrawEventArgs^ args)
{
    args.DrawingSession.DrawCircle(640, 360, 30.0f, Colors->Blue);
}

args是引用类型，不是值类型。（它是通过引用 (^) 传递给您的，这大致类似于通过指针 (*) 传递 C++ 对象。）由于它是引用类型，因此可以使用 [=15 访问其成员=]，而不是 .，就像指向对象的 C++ 指针。

DrawingSession也是引用类型，所以它的成员也是用->来访问的。

Colors是一个class的名字；使用 :: 访问静态成员。

args->DrawingSession->DrawCircle(640, 360, 30.0f, Colors::Blue);

表达式必须具有 class 类型（C++ 初学者错误 Windows 8 App 使用 Win2D）

Expression must have a class type (Beginner error in C++ Windows 8 App using Win2D)

c++-cli

drawing

win2d