读取Windows Phone 8 SD卡上的文件时出现问题

Question

我在读取Windows Phone SD卡上的文件时遇到问题。我使用ExternalStorageFile.OpenForReadAsync获取有效的Stream对象。然而，尽管流CanSeek属性为真，任何查找操作都将被忽略，并且位置不会移动；

    private async void ReadFileOnSDCard(ExternalStorageFile file)
    {
        Stream stream = await file.OpenForReadAsync();
          using (stream)
          {
                 long curPos= stream.Seek(100, SeekOrigin.Begin);
                 long pos = stream.Position;

// curPos和pos都为0

Answer 1

我也在和同样的问题作斗争。Seek确实在Microsoft.Phone.Storage.NativeFileStream中被破坏了，这是SD卡上文件的流类型。最后，我用ILspy查看了这个类，就是这样：

public override long Seek(long offset, System.IO.SeekOrigin origin)
{
   ...
   uint num = (uint)((ulong)(offset & -4294967296L) >> 32);
   uint num2 = (uint)(offset & (long)((ulong)-1));
   uint num3 = NativeFileStream.SetFilePointer(this.m_handle, num, ref num2, 
   ...
}

和函数SetFilePointer：http://msdn.microsoft.com/en-us/library/windows/desktop/aa365541%28v=vs.85%29.aspx

要使寻道工作，偏移值应为长度的较高32位。

Answer 2

我猜您正在读取一个扩展名为reserved by Windows Phone的文件。不幸的是，你不能从Windows Phone上的SD卡中读取它们。如果你正在阅读的文件属于这些保留的扩展名，Windows Phone只需忽略它们即可。

假设你没有阅读这些保留的扩展，你可以按照以下两个步骤来解决你的问题。

1/首先检查您是否已经在WMAppManifest.xml中添加了<Capability Name="ID_CAP_REMOVABLE_STORAGE" />

2/如果没有注册文件关联扩展，则注册，例如.gpx

<FileTypeAssociation TaskID="_default" Name="GPX" NavUriFragment="fileToken=%s">
  <Logos>
    <Logo Size="small" IsRelative="true">Assets/Route_Mapper_Logo33x33.png</Logo>
    <Logo Size="medium" IsRelative="true">Assets/Route_Mapper_Logo69x69.png</Logo>
    <Logo Size="large" IsRelative="true">Assets/Route_Mapper_Logo176x176.png</Logo>
  </Logos>
  <SupportedFileTypes>
    <FileType ContentType="application/gpx">.gpx</FileType>
  </SupportedFileTypes>
</FileTypeAssociation>

Answer 3

如果文件很小，那么您可以简单地将流复制到MemoryStream。我已经测试过了，它是有效的：

Stream s = await file.OpenForReadAsync();
MemoryStream ms = new MemoryStream();
s.CopyTo(ms);

但是，如果文件太大，您将遇到内存问题，因此可以使用以下流包装器类来纠正Microsoft的错误(不过，在Windows Phone的未来版本中，一旦错误得到修复，您将需要禁用此修复)：

using System;
using System.IO;

namespace WindowsPhoneBugFix
{
    /// <summary>
    /// Stream wrapper to circumnavigate buggy Stream reading of stream returned by ExternalStorageFile.OpenForReadAsync()
    /// </summary>
    public sealed class ExternalStorageFileWrapper : Stream
    {
        private Stream _stream; // Underlying stream

        public ExternalStorageFileWrapper(Stream stream)
        {
            if (stream == null)
                throw new ArgumentNullException("stream");

            _stream = stream;
        }

        // Workaround described here - http://stackoverflow.com/a/21538189/250254
        public override long Seek(long offset, SeekOrigin origin)
        {
            ulong uoffset = (ulong)offset;
            ulong fix = ((uoffset & 0xffffffffL) << 32) | ((uoffset & 0xffffffff00000000L) >> 32);
            return _stream.Seek((long)fix, origin);
        }

        public override bool CanRead
        {
            get { return _stream.CanRead; }
        }

        public override bool CanSeek
        {
            get { return _stream.CanSeek; }
        }

        public override bool CanWrite
        {
            get { return _stream.CanWrite; }
        }

        public override void Flush()
        {
            _stream.Flush();
        }

        public override long Length
        {
            get { return _stream.Length; }
        }

        public override long Position
        {
            get
            {
                return _stream.Position;
            }
            set
            {
                _stream.Position = value;
            }
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            return _stream.Read(buffer, offset, count);
        }

        public override void SetLength(long value)
        {
            _stream.SetLength(value);
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            _stream.Write(buffer, offset, count);
        }
    }
}

此处提供了代码，可将其放入您的项目：https://github.com/gavinharriss/ExternalStorageFileWrapper-wp8