Xamarin Forms MainActivity OnCreate LoadApplication System.NullReferenceException:对象引用未设置为对象的实例

Question

我正在使用VS2015与Xamarin一起创建一个多平台项目,该项目可以显示启动画面,然后在webview中加载网页.这是我的项目结构.我正在使用PCL项目类型如下:

TheApp(便携式)

-WebPageHoster.Xaml     //Contains a WebView control
   -WebPageHoster.Xaml.cs  //sets the WebView controls source property to load a webpage
   -App.Xaml
   -App.Xaml.cs

TheApp.Droid

/Resources/drawable/splashlogo.png
/Resources/drawable/icon3.png
/Resources/values/Styles.xml
-MainActivity.cs
-SplashActivity.cs     

TheApp.iOS

TheApp.WindowsPhone(Windows Phone 8.1)

这是代码 Styles.xml

<?xml version="1.0" encoding="utf-8" ?> 
<resources>
    <style name="Theme.Splash" parent="android:Theme">
        <item name="android:windowBackground">@drawable/splashlogo</item>
        <item name="android:windowNoTitle">true</item>
    </style>
</resources>

SplashActivity.cs

[Activity(MainLauncher = true, NoHistory = true, Theme = "@style/Theme.Splash", Icon = "@drawable/icon3")]
public class SplashActivity : AppCompatActivity
{
    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        var intent = new Intent(this, typeof(MainActivity));
        StartActivity(intent);
        Finish();
    }
}

MainActivity.cs

[Activity(Label = "Splash App",  ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsApplicationActivity
{
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        global::Xamarin.Forms.Forms.Init(this, bundle);
        LoadApplication(new App());  // << Problem here
    }

上述LoadApplication方法中的方法调用OnCreate加载App.Xaml.cs类的app构造函数,该类运行以下代码

public App(){InitializeComponent(); MainPage = new NavigationPage(new WebPageHoster(){Title ="Load Web-Page"}); }

这将显示启动画面,并在设置WebView的url后返回到OnCreate方法并提供此错误

System.NullReferenceException: Object reference not set to an instance of an object

我无法找到导致此错误的原因.这是Android Manifest文件.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="TheApp.TheApp" android:installLocation="auto" android:versionCode="1">
    <uses-sdk android:minSdkVersion="15" />
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE" />
    <application android:icon="@drawable/icon3" android:theme="@style/Theme.AppCompat" android:label="Splash Web Host"></application>
</manifest>

Answer 1

我测试了你的源代码,你的源代码MainActivity来源于FormsAppCompatActivity.在这种情况下,您必须提供使用的主题AppCompat.

为了解决这个问题,我AppCompat在你的风格中添加了一个主题:

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <item name="windowNoTitle">true</item>
    <item name="windowActionBar">false</item>
    <item name="windowActionModeOverlay">true</item>
</style> 

然后在你的上面使用它MainActivity:

[Activity(Label = "Southern Travel", Theme = "@style/AppTheme", ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity