Xamarin.Forms 自定义控件中抛出序列异常

Question

我有一个非常基本的跨平台应用程序。该解决方案包含用于应用程序注入的 PCL 和一个 Android 应用程序。

我的MainPage.xaml:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:Crossplatform.app"
             xmlns:dragonfly="clr-namespace:Crossplatform.control;assembly=Crossplatform.control"
             x:Class="Crossplatform.app.MainPage">
    <ContentPage.Content>

        <dragonfly:Wizard DataItem="{Binding WizardData}" />

    </ContentPage.Content>
</ContentPage>

MainPage.xaml.cs

public partial class MainPage : ContentPage
{
    private string wizarddata;
    public string WizardData
    {
        get { return wizarddata; }
        set { wizarddata = value; }
    }

    public MainPage()
    {
        InitializeComponent();

        WizardData = "Hello from ViewModel";
    }
}

所有这些都是在定义一个字符串属性，然后我将其分配给自定义控件中的 BindableProperty。

自定义控件代码：

public class Wizard : StackLayout
{
    public static readonly BindableProperty DataContextProperty =
        BindableProperty.Create(
            propertyName: nameof(DataItem),
            returnType: typeof(string),
            declaringType: typeof(Wizard),
            defaultValue: "Show Me",
            defaultBindingMode: BindingMode.TwoWay,
            validateValue: null,
            propertyChanged: OnDataContextPropertyChanged);

    public string DataItem
    {
        get
        {
            return (string)GetValue(DataContextProperty);
        }
        set
        {
            SetValue(DataContextProperty, value);
        }
    }

    public Wizard()
    {

    }

    static void OnDataContextPropertyChanged(BindableObject obj, object oldvalue, object newvalue)
    {
        ;
    }
}

这里没什么疯狂的。我所做的只是定义一个名为 DataContextProperty 的可绑定属性，并使用此属性通知通过控件公开的 DataItem 属性的更改（根据上面的 main.xaml）。

代码成功命中 Wizard 控件的构造函数，但在 Android 的 MainActivity 中失败并失败，出现序列异常，堆栈跟踪如下。

过去 4 小时我一直在调试它，看不出它在给定堆栈上失败的原因。

任何帮助都将不胜感激，因为我确信我这里的内容是完全有效的。

堆栈跟踪：

  at System.Linq.Enumerable.First[TSource] (System.Collections.Generic.IEnumerable`1[T] source, System.Func`2[T,TResult] predicate) [0x00011] in <b5bd9d990a0b4733885e90ca5ec6c0fb>:0 
  at Xamarin.Forms.Xaml.ApplyPropertiesVisitor.TryAddToProperty (System.Object element, System.String localName, System.Object value, System.Xml.IXmlLineInfo lineInfo, Xamarin.Forms.Xaml.Internals.XamlServiceProvider serviceProvider, System.Exception& exception) [0x0005d] in C:\agent\_work\s\Xamarin.Forms.Xaml\ApplyPropertiesVisitor.cs:510 
  at Xamarin.Forms.Xaml.ApplyPropertiesVisitor.SetPropertyValue (System.Object xamlelement, Xamarin.Forms.Xaml.XmlName propertyName, System.Object value, System.Object rootElement, Xamarin.Forms.Xaml.INode node, Xamarin.Forms.Xaml.HydratationContext context, System.Xml.IXmlLineInfo lineInfo) [0x000a2] in C:\agent\_work\s\Xamarin.Forms.Xaml\ApplyPropertiesVisitor.cs:334 
  at Xamarin.Forms.Xaml.ApplyPropertiesVisitor.Visit (Xamarin.Forms.Xaml.ElementNode node, Xamarin.Forms.Xaml.INode parentNode) [0x00152] in C:\agent\_work\s\Xamarin.Forms.Xaml\ApplyPropertiesVisitor.cs:124 
  at Xamarin.Forms.Xaml.ElementNode.Accept (Xamarin.Forms.Xaml.IXamlNodeVisitor visitor, Xamarin.Forms.Xaml.INode parentNode) [0x000ab] in C:\agent\_work\s\Xamarin.Forms.Xaml\XamlNode.cs:149 
  at Xamarin.Forms.Xaml.ElementNode.Accept (Xamarin.Forms.Xaml.IXamlNodeVisitor visitor, Xamarin.Forms.Xaml.INode parentNode) [0x00043] in C:\agent\_work\s\Xamarin.Forms.Xaml\XamlNode.cs:143 
  at Xamarin.Forms.Xaml.RootNode.Accept (Xamarin.Forms.Xaml.IXamlNodeVisitor visitor, Xamarin.Forms.Xaml.INode parentNode) [0x00043] in C:\agent\_work\s\Xamarin.Forms.Xaml\XamlNode.cs:201 
  at Xamarin.Forms.Xaml.XamlLoader.Visit (Xamarin.Forms.Xaml.RootNode rootnode, Xamarin.Forms.Xaml.HydratationContext visitorContext) [0x0007b] in C:\agent\_work\s\Xamarin.Forms.Xaml\XamlLoader.cs:141 
  at Xamarin.Forms.Xaml.XamlLoader.Load (System.Object view, System.String xaml) [0x0004b] in C:\agent\_work\s\Xamarin.Forms.Xaml\XamlLoader.cs:89 
  at Xamarin.Forms.Xaml.XamlLoader.Load (System.Object view, System.Type callingType) [0x0002f] in C:\agent\_work\s\Xamarin.Forms.Xaml\XamlLoader.cs:68 
  at Xamarin.Forms.Xaml.Extensions.LoadFromXaml[TXaml] (TXaml view, System.Type callingType) [0x00000] in C:\agent\_work\s\Xamarin.Forms.Xaml\ViewExtensions.cs:36 
  at Crossplatform.app.MainPage.InitializeComponent () [0x00001] in C:\repos\Crossplatform.app\Crossplatform.app\Crossplatform.app\obj\Debug\Crossplatform.app.MainPage.xaml.g.cs:19 
  at Crossplatform.app.MainPage..ctor () [0x00008] in C:\repos\Crossplatform.app\Crossplatform.app\Crossplatform.app\MainPage.xaml.cs:21 
  at Crossplatform.app.App..ctor () [0x0000f] in C:\repos\Crossplatform.app\Crossplatform.app\Crossplatform.app\App.xaml.cs:16 
  at Crossplatform.app.Droid.MainActivity.OnCreate (Android.OS.Bundle bundle) [0x00028] in C:\repos\Crossplatform.app\Crossplatform.app\Crossplatform.app.Android\MainActivity.cs:25

Answer 1

The code successfully hits the ctor for the Wizard control, but falls through and fails in Android's MainActivity with a Sequence exception, the stack trace below.

I have been debugging this for the last 4 hours and cannot see why it's failing on the given stack.

你得到这个异常是因为你定义了一个 public 属性(DataItem) 与不同的 Xaml 属性名称 (DataContextProperty)，这是不允许。

要解决此问题，您需要将 DataContextProperty 更改为 DataItemProperty。

Xamarin.Forms 自定义控件中抛出序列异常

Sequence Exception thrown in Xamarin.Forms custom control

android

exception

xamarin.forms

custom-controls