Xamarin.Forms在文件系统中保存文件 [英] Xamarin.Forms Saving file in filesystem

问题描述

我尝试从Web下载文件并保存到Environment.SpecialFolder的任何位置.不管我尝试哪种方法-我总是得到

System.UnauthorizedAccessException:'对路径'我尝试的任何可能路径(甚至超出Environment.SpecialFolder)的访问都被拒绝."

我尝试在UWP和Android上访问文件系统-两者都是相同的异常.

到目前为止，我没有碰到什么尝试:

我尝试使用DependencyService通过PCL和每个平台分别进行此操作.

检查文件夹是否为只读

以管理员权限启动Visual Studio

将调试更改为发布

尝试过Xam.Plugins.DownloadManager

代码示例:

I try to download file from web and save to any of Environment.SpecialFolder locations. No matter of what approach I try - I always get

System.UnauthorizedAccessException: 'Access to the path 'any possible path I try (even beyond Environment.SpecialFolder)' is denied.'

I tried accessing filesystem on UWP and Android - both same exception.

What I tried with no luck so far:

I tried to do this via PCL and via each platform individually using DependencyService.

Checked if folders are read-only

Started Visual Studio with administrator privileges

Changed Debug to Release

Tried Xam.Plugins.DownloadManager

Code sample:

var webClient = new WebClient();
webClient.DownloadDataCompleted += (s, e) => {
   var bytes = e.Result;
   string documentsPath = Environment.GetFolderPath(Environment.SpecialFolder.MyPictures);
   string localFilename = "downloaded.jpg";
   string localPath = Path.Combine(documentsPath, localFilename);
   File.WriteAllBytes(localPath, bytes);
};
webClient.DownloadDataAsync(new Uri(url));

推荐答案

您正面临权限问题.

首先，您必须添加AndroidManifest:

First, you will have to add in your AndroidManifest:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

并且由于Android Marshmallow，您需要向用户询问权限，因此，我建议使用软件包权限插件

And since Android Marshmallow, you need to ask the user for the permissions, so I advise to use the package Permissions.Plugin

并添加您的MainActivity:

And add in your MainActivity:

public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
{
    PermissionsImplementation.Current.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
}

您可以通过以下方式签入运行时是否具有权限:

You can check in runtime if you have the permissions by:

var status = await CrossPermissions.Current.CheckPermissionStatusAsync(Permission.Storage);
    if (status != PermissionStatus.Granted)
    {
        if(await CrossPermissions.Current.ShouldShowRequestPermissionRationaleAsync(Permission.Storage))
        {
            await DisplayAlert("Need storage, "Request storage permission", "OK");
        }

        var results = await CrossPermissions.Current.RequestPermissionsAsync(Permission.Storage);
        //Best practice to always check that the key exists
        if(results.ContainsKey(Permission.Storage))
            status = results[Permission.Storage];
    }

有关更多信息，您可以查看此博客文章，其中解释了Android中的所有权限- https://devblogs.microsoft.com/xamarin/requesting-runtime-permissions-in-android-marshmallow/

For further information you can check this blog post explaining all the permissions in Android - https://devblogs.microsoft.com/xamarin/requesting-runtime-permissions-in-android-marshmallow/

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