Xamarin.Android app doesn't request “WriteExternalStorage” (but code works fine with “ReadExternalStorage”)

问题

I already have some experiences with C#, but I'm new to Android-Development and wanted to start with a simple project app, that can create a new directory with a file on the external storage ("/storage/emulated/0/" [Android.OS.Environment.ExternalStorageDirectory.AbsolutePath]) and read it.

In the documentation I read that in the newer SDKs it's not enough to mention the permissions (in this case "ANDROID.PERMISSION.WRITE_EXTERNAL_STORAGE") in the AndroidManifest.xml. So I added a method to request them at runtime.

My problem is that the emulator [Android 8.1 API 27] (and my phone with USB debugging [Android 8.0]) doesn't open the permission dialogue, if I request "Manifest.Permission.WriteExternalStorage". If I use my code to request "Manifest.Permission.ReadExternalStorage" the dialogue appears, you can accept and decline and the action visible in the settings.

First, I tried to write my own code to the documentation (https://docs.microsoft.com/de-de/xamarin/android/platform/files/external-storage), which doesn't open the dialogue. I tried to let a NuGet-Package (Karamunting.Android.PermissionManager) manage the request, same problem: It just asked for the read permission, not write. Now I'm using code that I copied from a GitHub project (https://github.com/xamarin/monodroid-samples/tree/master/LocalFiles), but still the same problem.

This is the method that should request the permission

const int RC_WRITE_EXTERNAL_STORAGE_PERMISSION = 1000;
static readonly string[] PERMISSIONS_TO_REQUEST = { Manifest.Permission.WriteExternalStorage };

bool RequestExternalStoragePermissionIfNecessary(int requestCode)
        {
            if (Android.OS.Environment.MediaMounted.Equals(Android.OS.Environment.ExternalStorageState))
            {
                if (CheckSelfPermission(Manifest.Permission.WriteExternalStorage) == Permission.Granted)
                {
                    Log.Debug(TAG, "Neccessary permissions granted");
                    return false;
                }

                Log.Debug(TAG, "Permissions denied --> requesting");
                if (ShouldShowRequestPermissionRationale(Manifest.Permission.WriteExternalStorage))
                {
                    Snackbar.Make(FindViewById(Android.Resource.Id.Content),
                                  Resource.String.write_external_permissions_rationale,
                                  Snackbar.LengthIndefinite)
                            .SetAction(Resource.String.ok, delegate { RequestPermissions(PERMISSIONS_TO_REQUEST, requestCode); });
                }
                else
                {
                    RequestPermissions(PERMISSIONS_TO_REQUEST, requestCode);
                }

                return true;
            }

            Log.Warn(TAG, "External storage is not mounted; cannot request permission");
            return true;
        }

It is called on a button click , but I also tried to use it in the OnCreate() method

bool useIntern = RequestExternalStoragePermissionIfNecessary(RC_WRITE_EXTERNAL_STORAGE_PERMISSION);

I've also mentioned the permissions in the AndroidManifest

<uses-sdk android:minSdkVersion="23" android:targetSdkVersion="27" />
<uses-permission android:name="ANDROID.PERMISSION.WRITE_EXTERNAL_STORAGE" />

I expect that the application will check on startup/ button click if the permission is granted (It does check for permission and writes in the console "Permissions denied --> requesting").

But after that the app should show the dialogue "Allow App to […] on your device? - Deny - Allow". This doesn't happen if I request the write permission, but happens if I request read.

Thanks for any kind of help in Advance

Phil

回答1:

It seems that the problem was not the code, but the configuration.
In the AndroidManifest.xml I changed the minSdk from 23 to 21 and the capitalization of the permission:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1" android:versionName="1.0" package="com.companyname.testapp" android:installLocation="auto">
    <uses-sdk android:minSdkVersion="21" android:targetSdkVersion="27" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <application android:allowBackup="true" android:icon="@mipmap/ic_launcher" android:label="@string/app_name" android:roundIcon="@mipmap/ic_launcher_round" android:supportsRtl="true" android:theme="@style/AppTheme"></application>
</manifest>

Thanks for your help @LeoZhu-MSFT

来源：https://stackoverflow.com/questions/56209650/xamarin-android-app-doesnt-request-writeexternalstorage-but-code-works-fine

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xamarin.android

android-permissions