Xamarin Android中FileProvider.GetUriForFile空引用异常求助

在安装下载的APK文件时触发System.NullReferenceException：“对象引用未设置到对象的实例”，以下是问题排查过程及最终解决办法：

报错代码 #

var file = new Java.IO.File(filePath);
if (Build.VERSION.SdkInt >= Android.OS.BuildVersionCodes.N)
{
    System.Diagnostics.Debug.WriteLine(MainActivity.Instance.ApplicationContext.PackageName + ".fileProvider");
//此处报错
    Uri apkUri = AndroidX.Core.Content.FileProvider.GetUriForFile(MainActivity.Instance,
        MainActivity.Instance.ApplicationContext.PackageName + ".fileProvider", file);
    Intent intentS = new Intent(Intent.ActionInstallPackage);
    intentS.SetData(apkUri);
    intentS.SetFlags(ActivityFlags.GrantReadUriPermission);
    MainActivity.Instance.StartActivity(intentS);
}

MainActivity代码 #

[Activity(Label = "Dyspozycja Rewizor",
        Icon = "@mipmap/icon",
        Theme = "@style/MainTheme",
        MainLauncher = true,
        //Name ="Dysp.Rew.MainActivity",
        ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize)]
    [ContentProvider(
        new[] { "${applicationId}.provider" },
        Name = "AndroidX.Core.Content.FileProvider",
        Exported = false,
        GrantUriPermissions = true)]
    [MetaData(
        "android.support.FILE_PROVIDER_PATHS",
        Resource = "@xml/file_paths")]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        internal static MainActivity Instance { get; private set; }

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            Instance = this;
            Xamarin.Essentials.Platform.Init(this, savedInstanceState);
            global::Xamarin.Forms.Forms.Init(this, savedInstanceState);

            DependencyService.Register<IInstallerService, InstallerService>();

            LoadApplication(new App());
        }
        public override void OnRequestPermissionsResult(int requestCode, string[] permissions,
            [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
        {
            Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);

            base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }

尝试过的FileProvider配置 #

特性配置（MainActivity上标注） #

已在MainActivity类上添加[ContentProvider]和[MetaData]特性，如上述代码所示。

AndroidManifest.xml配置 #

<application android:debuggable="true" android:label="Dyspozycja Rewizor">
    <provider android:name="androidx.core.content.FileProvider"
                    android:authorities="${applicationId}.provider"
                    android:exported="false"
                    android:grantUriPermissions="true">
      <meta-data android:name="android.support.FILE_PROVIDER_PATHS" android:resource="@xml/file_paths"></meta-data>
    </provider>
  </application>

file_paths.xml配置 #

<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
  <files-path name="my_docs" path="docs/"/>
  <external-path name="external_files" path="."/>
</paths>

另一种Intent启动方式（同样触发异常） #

Intent intent = new Intent(Intent.ActionView);
 intent.SetDataAndType(Uri.FromFile(new Java.IO.File(filePath)), "application/vnd.android.packagearchive");
 intent.SetFlags(ActivityFlags.NewTask); 
 //此处报错
 MainActivity.Instance.StartActivity(intent);

最终问题根源 #

AndroidManifest.xml中存在重复的<application>节点，导致配置失效：

<application android:label="JakubStegienko.rewizor" android:theme="@style/MainTheme" android:icon="@mipmap/icon"></application>

解决方案 #

删除AndroidManifest.xml中重复的<application>节点，确保整个配置文件仅保留一个<application>节点，并将FileProvider的配置正确嵌套在该节点内部。

内容的提问来源于stack exchange，提问作者Axlii50