英文:
Xamarin.Forms: Application.Current.MainPage.DisplayAlert() throws System.NullReferenceException
问题
我正在从一个单独的类中使用 `Application.Current.MainPage.DisplayAlert`，但是我得到了这个错误：
> System.NullReferenceException: '对象引用未设置到对象的实例。'
**App.xaml.cs**
```c#
public partial class App : Application
{
public App()
{
InitializeComponent();
MainPage = new NavigationPage(new MainPage());
}
}
MainPage.cs
public partial class MainPage : ContentPage
{
private static readonly DisplayMessage DisplayMessage = new();
public MainPage()
{
InitializeComponent();
DisplayMessage.Display();
}
}
DisplayMessage.cs
public class DisplayMessage
{
public async void Display()
{
await Application.Current.MainPage.DisplayAlert("Hello World", "Welcome", "OK");
}
}
我找到了这个文章提出了这个解决方案，但没有帮助：
public static Page RootPage;
public App()
{
InitializeComponent();
MainPage = new MainPage();
App.RootPage = this.MainPage;
}
App.RootPage.DisplayAlert(title, message, ok);
有人知道怎么解决吗？非常感谢。
<details>
<summary>英文:</summary>
I'm using `Application.Current.MainPage.DisplayAlert` from a separate class and I'm getting this error:
> System.NullReferenceException: 'Object reference not set to an instance of an object.'
**App.xaml.cs**
```c#
public partial class App : Application
{
public App()
{
InitializeComponent();
MainPage = new NavigationPage(new MainPage());
}
}
MainPage.cs
public partial class MainPage : ContentPage
{
private static readonly DisplayMessage DisplayMessage = new();
public MainPage()
{
InitializeComponent();
DisplayMessage.Display();
}
}
DisplayMessage.cs
public class DisplayMessage
{
public async void Display()
{
await Application.Current.MainPage.DisplayAlert("Hello World", "Welcome", "OK");
}
}
I found this article proposing this solution but it doesn't help
public static Page RootPage;
public App()
{
InitializeComponent();
MainPage = new MainPage();
App.RootPage = this.MainPage;
}
App.RootPage.DisplayAlert(title, message, ok);
Does someone have a direction to find out ? Thanks a lot.
答案1
得分: 1
问题在于你在MainPage实例完成构建之前尝试访问它。
不要这样做：
public partial class MainPage : ContentPage
{
private static readonly DisplayMessage DisplayMessage = new();
public MainPage()
{
InitializeComponent();
DisplayMessage.Display();
}
}
你可以这样做，应该可以工作：
public partial class MainPage : ContentPage
{
private static readonly DisplayMessage DisplayMessage = new();
public MainPage()
{
InitializeComponent();
}
public void ShowMessage()
{
DisplayMessage.Display();
}
}
然后，在构建完成之后，可以调用ShowMessage()方法，这将显示消息。
public partial class App : Application
{
public App()
{
InitializeComponent();
MainPage = new MainPage();
MainPage.ShowMessage();
}
}
现在，你的代码整体设计可能会有争议，但是进行这些更改应该会得到一个可用的解决方案。
更新：
你可能还希望在以下代码段中添加空值检查：
public class DisplayMessage
{
public async void Display()
{
//注意"?"
await Application.Current.MainPage?.DisplayAlert("Hello World", "Welcome", "OK");
}
}
注意?，它确保仅当MainPage设置为一个实例时才调用DisplayAlert()。
英文:
The problem is that you're trying to access the MainPage instance before it has finished construction.
Instead of doing this:
public partial class MainPage : ContentPage
{
private static readonly DisplayMessage DisplayMessage = new();
public MainPage()
{
InitializeComponent();
DisplayMessage.Display();
}
}
You can do the following, which should work:
public partial class MainPage : ContentPage
{
private static readonly DisplayMessage DisplayMessage = new();
public MainPage()
{
InitializeComponent();
}
public void ShowMessage()
{
DisplayMessage.Display();
}
}
Then, after construction finished, the ShowMessage() method can be called, which in turn will display the message.
public partial class App : Application
{
public App()
{
InitializeComponent();
MainPage = new MainPage();
MainPage.ShowMessage();
}
}
Now, the overall design of your code is debatable, but making these changes should lead to a usable solution.
Update:
You may also want to add a null-check to the following piece of code:
public class DisplayMessage
{
public async void Display()
{
//note the "?"
await Application.Current.MainPage?.DisplayAlert("Hello World", "Welcome", "OK");
}
}
Note the ? which serves to ensure that DisplayAlert() only gets called when MainPage is set to an instance.
答案2
得分: 1
🤔为了仅仅显示一个警告，为什么需要这么多的代码呢？
为了确保在页面加载后显示警告，我们可以通过Loaded事件处理程序传递警告代码，就像这样：
在MainPage.xaml:
<ContentPage
Loaded="AfterContentPageLoaded">
在MainPage.xaml.cs:
private void AfterContentPageLoaded(object sender, EventArgs e)
{
Application.Current.MainPage?.DisplayAlert("你好", "这真的很简单", "确定");
}
你也可以在MainPage.xaml.cs页面的Loaded事件处理程序中传递来自DisplayMessage.cs类的方法。
❤️
英文:
🤔 What is the Need of Such a Big Code for Just an Alert ?
To Make sure, Alert is Shown after Page is Loaded We Can Just Pass Alert Code through Loaded Event Handler , Just Do Like This:
On MainPage.xaml :
<ContentPage
Loaded="AfterContentPageLoaded">
On MainPage.xaml.cs :
private void AfterContentPageLoaded(object sender, EventArgs e)
{
Application.Current.MainPage?.DisplayAlert("Hello", "It was So Simple", "OK");
}
You can also pass your method from your DisplayMessage.cs class into Loaded event Handler on MainPage.xaml.cs page.
❤️
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