在外部SD卡上创建目录。 （写访问错误）

Question

我遵循了本教程：https://www.youtube.com/watch?v=Uzpy3qdYXmE，以使用/ storage / XXXX-XXXX /路径获得对外部SD卡进行写入和读取的权限。但是，即使代码似乎很乐意为我提供权限，但在写入SD卡时（例如，在创建目录时）仍然会出现拒绝访问错误。

这是在Android 7.0（自定义rom）上发生的。

只是让事情变得更困难，创建目录的过程是在一个单独的，应该跨平台运行的库中进行的。我使用编译符号来编译特定平台的库。

该应用程序本身是一个WebView应用程序，它通过由库启动的Websocket服务器与库中的代码连接。

我最初的想法是在MainActivity中请求所有必需的权限，事实就是这样。但显然我缺少了一些东西。

这是错误消息：

System.UnauthorizedAccessException: Access to the path "/storage/09F6-DC14/Test" is denied.
at System.IO.Directory.CreateDirectoriesInternal (System.String path) [0x0004b] in <fd78fee11d3a4b2b99a184d3a0faf63e>:0 
at System.IO.Directory.CreateDirectory (System.String path) [0x00075] in <fd78fee11d3a4b2b99a184d3a0faf63e>:0 
at Library.Handlers.DirectoryHandler.CreateDirectory (System.String path, System.String name) [0x000db] in

这是我的清单文件：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"     package="TestApp.TestApp" android:versionCode="1"     android:versionName="0.0.1 Preview Version -WIP"     android:installLocation="preferExternal">
    <uses-sdk android:targetSdkVersion="24" />
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.MANAGE_DOCUMENTS" />
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" />
    <uses-permission android:name="android.permission.MOUNT_FORMAT_FILESYSTEMS" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <application android:label="TestApp" android:hardwareAccelerated="true"></application>
</manifest>

这是AssemblyInfo.cs：

using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using Android.App;

// General Information about an assembly is controlled through the following 
// set of attributes. Change these attribute values to modify the information
// associated with an assembly.
[assembly: AssemblyTitle("TestApp")]
[assembly: AssemblyDescription("")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("")]
[assembly: AssemblyProduct("TestApp")]
[assembly: AssemblyCopyright("Copyright ©  2018")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture("")]
[assembly: ComVisible(false)]

// Version information for an assembly consists of the following four values:
//
//      Major Version
//      Minor Version 
//      Build Number
//      Revision
//
// You can specify all the values or you can default the Build and Revision Numbers 
// by using the '*' as shown below:
// [assembly: AssemblyVersion("1.0.*")]
[assembly: AssemblyVersion("1.0.0.0")]
[assembly: AssemblyFileVersion("1.0.0.0")]


[assembly: UsesPermission(Android.Manifest.Permission.WriteExternalStorage)]
[assembly: UsesPermission(Android.Manifest.Permission.ReadExternalStorage)]

这是请求权限的代码（与上述教程中使用的代码不同）：

MainActivity.cs：

    readonly string[] PermissionsGroupExternalStorage=
    {
                //TODO add more permissions
                Manifest.Permission.ReadExternalStorage,
                Manifest.Permission.WriteExternalStorage
    };


    async Task GetPermissionsAsync()
    {
        const string permissionRead = Manifest.Permission.ReadExternalStorage;
        const string permissionWrite = Manifest.Permission.WriteExternalStorage;

        if  ((CheckSelfPermission(permissionRead) == (int)Android.Content.PM.Permission.Granted) && (CheckSelfPermission(permissionWrite) == (int)Android.Content.PM.Permission.Granted))
        {
            //TODO change the message to show the permissions name
            Toast.MakeText(this, "Storage permissions granted.", ToastLength.Short).Show();
            return;
        } else 
        {

            Toast.MakeText(this, "Storage permissions not granted.", ToastLength.Short).Show();
            //set alert for executing the task
            AlertDialog.Builder alert = new AlertDialog.Builder(this);
            alert.SetTitle("Storage Permissions Needed");
            alert.SetMessage("The application need special permissions to read and write to (external) storage devices to continue");
            alert.SetPositiveButton("Request Permissions", (senderAlert, args) =>
            {
                RequestPermissions(PermissionsGroupExternalStorage, RequestLocationId);
            });

            alert.SetNegativeButton("Cancel", (senderAlert, args) =>
            {
                Toast.MakeText(this, "Cancelled!", ToastLength.Short).Show();
            });

            Dialog dialog = alert.Create();
            dialog.Show();


        }

        RequestPermissions(PermissionsGroupExternalStorage, RequestLocationId);

        return;
    }

然后我尝试使用以下代码创建一个目录（在crossplatform（ish）库中）：

我的库中的DirectoryHandler类：

Directory.CreateDirectory("/storage/09F6-DC14/Test")

很明显，可以对代码进行改进，但是我想让它首先工作...

烤面包机返回“已授予存储权限”，因此我认为我确实设置了正确的权限，这是阻止我解决此问题的原因，如果这不是由于权限所致...还有什么可能阻止我访问我的外部存储？

实际上，每次尝试从外部存储设备写入/读取数据时，实际上我是否都必须请求许可？这样做似乎有点hack。