Xamarin拒绝路径访问

Question

我正在尝试在手机中保存位图图像，但无法做到。 此代码在主线程中正常工作，但在插入AsyncTask后它无效。

  

未处理的异常：System.UnauthorizedAccessException：访问   path /storage/emulated/0/Pictures/ATGImage.jpg“被拒绝。

    using (var os = new FileStream(newfile, FileMode.CreateNew))<--here i am getting Error
    {
        image.Compress(Bitmap.CompressFormat.Jpeg, 90, os);
    }

`

这是在AsyncTask中使用的代码

private class generateBarcode : AsyncTask<string,Java.Lang.Object,Bitmap>
        {
            generateActivity generate_activity;
            private Bitmap bmp_image;`


            public generateBarcode(generateActivity activity)
            {
                generate_activity = activity;
            }

            protected override void OnPreExecute()
            {
                generate_activity.progress_bar = new ProgressDialog(generate_activity);
                generate_activity.progress_bar.SetTitle("wait !");
                generate_activity.progress_bar.SetMessage("Generating image");
                generate_activity.progress_bar.Show();

            }


            protected override Bitmap RunInBackground(params string[] @params)
            {

                try
                {
                    long asset_number = Int64.Parse(@params[0]);
                    Debug("Thread _ Barcode", "asset_number " + asset_number.ToString());
                    BitMatrix matrix = new UPCAWriter().encode(asset_number.ToString(), ZXing.BarcodeFormat.UPC_A, 500, 300, null);
                    int width = matrix.Width;
                    int height = matrix.Height;
                    Bitmap image = Bitmap.CreateBitmap(width, height, Bitmap.Config.Argb8888);
                    for (int x = 0; x < width; x++)
                    {
                        for (int y = 0; y < height; y++)
                        {
                            image.SetPixel(x, y, (matrix[x, y] ? Android.Graphics.Color.Black : Android.Graphics.Color.White));

                        }
                    }

                var path = System.IO.Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath + "/" + Android.OS.Environment.DirectoryPictures, "ATGImage.jpg");
                var newfile = new Java.IO.File(path);

                    if (newfile.Exists())
                    {
                        newfile.Delete();

                        using (var os = new FileStream(newfile, FileMode.CreateNew)) <--here i am getting Error
                        {
                           image.Compress(Bitmap.CompressFormat.Jpeg, 90, os);
                        }
                    }
                    else
                    {
                         using (var os = new FileStream(newfile, FileMode.CreateNew)) <--here i am getting Error
                        {
                           image.Compress(Bitmap.CompressFormat.Jpeg, 90, os);
                        }
                    }
                    Log.Debug("Thread _ Barcode_created", "asset_number " + asset_number.ToString());
                    return image;
                }
                catch (ThreadAbortException ex)
                {
                    Log.Debug("Thread Aborted: ", ex.Message);
                    return null;

                }
                catch(ArgumentException ex)
                {
                    Log.Debug("Barcode number ", ex.Message);
                    return null;
                }

            }
            protected override void OnPostExecute(Bitmap result)
            {
                base.OnPostExecute(result);
                generate_activity.bmp = result;
                generate_activity.progress_bar.Dismiss();

            }
        }

这是Manifest.xml个文件

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:installLocation="auto" android:versionCode="1" android:versionName="1.0" package="atg.droid.com">
    <uses-sdk android:minSdkVersion="15" android:targetSdkVersion="24" />
  <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.BLUETOOTH" />
    <uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
    <uses-permission android:name="android.permission.BLUETOOTH_PRIVILEGED" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <application android:label="ATG" android:icon="@drawable/Icon" android:theme="@android:style/Theme.Material.Light.DarkActionBar"></application>
</manifest>

`