将大型虚拟文件从 C# 拖放到 Windows 资源管理器

问题描述

我有一个 C# WPF 应用程序，其中一个部分用作 FTP 客户端，列出远程服务器上的文件并允许用户下载它们.我希望用户能够将文件从文件列表拖放到他们自己的机器上(即到 Windows 资源管理器外壳中).

I have a C# WPF application with a section that serves as an FTP client, listing files on a remote server and allowing the user to download them. I want the user to be able to drag and drop files from the file listing onto their own machine (i.e. into a Windows Explorer shell).

为此，我使用了 Delay 博客中的 VirtualFileDataObject 代码，使用 SetData 的 Action 重载.这适用于较小的文件.

To accomplish this, I used the VirtualFileDataObject code from Delay's blog, using the Action<Stream> overload of SetData. This works great on smaller files.

我的问题是:我正在处理的一些文件非常大(2+ GB)，VirtualFileDataObject 类处理流的方式涉及将整个文件读入内存，这对于那些非常大的文件，最终可能会引发存储空间不足"错误.

My problem is: some of the files I'm dealing with are very large (2+ GB), and the way the VirtualFileDataObject class handles the stream involves reading the entire thing into memory, which can end up throwing a "not enough storage" error for those very large files.

VirtualFileDataObject 代码的相关部分如下.如何重写此代码以不要求整个流都在内存中?

The relevant section of the VirtualFileDataObject code is below. How can I rewrite this code to not require the entire stream to be in memory?

    public void SetData(short dataFormat, int index, Action<Stream> streamData) {
        _dataObjects.Add(
            new DataObject {
                FORMATETC = new FORMATETC {
                    cfFormat = dataFormat,
                    ptd = IntPtr.Zero,
                    dwAspect = DVASPECT.DVASPECT_CONTENT,
                    lindex = index,
                    tymed = TYMED.TYMED_ISTREAM
                },
                GetData = () => {
                    // Create IStream for data
                    var ptr = IntPtr.Zero;
                    var iStream = NativeMethods.CreateStreamOnHGlobal(IntPtr.Zero, true);
                    if (streamData != null) {
                        // Wrap in a .NET-friendly Stream and call provided code to fill it
                        using (var stream = new IStreamWrapper(iStream)) {
                            streamData(stream);
                        }
                    }
                    // Return an IntPtr for the IStream
                    ptr = Marshal.GetComInterfaceForObject(iStream, typeof(IStream));
                    Marshal.ReleaseComObject(iStream);
                    return new Tuple<IntPtr, int>(ptr, NativeMethods.S_OK);
                },
            });
    }

尤其是GetData这一段是罪魁祸首:

In particular, this section of GetData is the culprit:

// Wrap in a .NET-friendly Stream and call provided code to fill it
using (var stream = new IStreamWrapper(iStream)) {
    streamData(stream);
}

streamData 是我提供的 Action ，它将实际文件数据写入流.我的代表只是打开一个文件并将字节读取到提供的流中.

streamData is the Action<stream> I provide which writes the actual file data to the stream. My delegate is just opening a file and reading the bytes into the provided stream.

有没有办法避免这最后一步，也许以某种方式直接传递文件流以供 Explorer shell 读取?我正在考虑将 iStream 替换为指向我拥有的 .NET 文件流的指针......但我对 COM 互操作的了解还不够，甚至不知道这样做的语法.任何提示/方向将不胜感激！

Is there a way to avoid this last step, perhaps somehow passing the file stream directly to be read from by the Explorer shell? I'm thinking something like replacing iStream with a pointer to the .NET filestream I've got...but I don't know enough about COM interop to even know the syntax for doing that. Any tips/direction would be appreciated!

推荐答案

我遇到了同样的问题，不过很容易解决；)

I got the same issue, easy to fix though ;)

问题是我们正在创建一个新的内存流，而它并不需要，因为我们已经有了我们的.您可以在 c# 中创建一个实现 IStream 的 Stream 包装器:

The problem is that we are creating a new memory stream whereas it is not needed since we already have ours. You can create in c# an Stream wrapper that implements IStream:

    /// <summary>
/// Simple class that exposes a read-only Stream as a IStream.
/// </summary>
private class StreamWrapper : IStream
{

   private Stream _stream;

   public StreamWrapper(Stream stream)

   {
       _stream = stream;
   }

   public void Read(byte[] pv, int cb, System.IntPtr pcbRead)

   {
       Marshal.WriteInt32(pcbRead, _stream.Read(pv, 0, cb));
   }

   public void Seek(long dlibMove, int dwOrigin, System.IntPtr plibNewPosition)

   {
       Marshal.WriteInt32(plibNewPosition, (int)_stream.Seek(dlibMove, (SeekOrigin)dwOrigin));
   }

   public void Clone(out IStream ppstm)

   {
       throw new NotImplementedException();
   }

   public void Commit(int grfCommitFlags)

   {
       throw new NotImplementedException();
   }

   public void CopyTo(IStream pstm, long cb, IntPtr pcbRead, IntPtr pcbWritten)

   {
       throw new NotImplementedException();
   }

   public void LockRegion(long libOffset, long cb, int dwLockType)

   {
       throw new NotImplementedException();
   }

   public void Revert()

   {
       throw new NotImplementedException();
   }

   public void SetSize(long libNewSize)

   {
       throw new NotImplementedException();
   }

   public void Stat(out System.Runtime.InteropServices.ComTypes.STATSTG pstatstg, int grfStatFlag)

   {
       throw new NotImplementedException();
   }

   public void UnlockRegion(long libOffset, long cb, int dwLockType)

   {
       throw new NotImplementedException();
   }

   public void Write(byte[] pv, int cb, IntPtr pcbWritten)

   {
       throw new NotImplementedException();
   }
}

然后在 VirtualFileDataObject 类中，更改 SetData 方法的签名，以便现在传递一个 Stream:

And then in the VirtualFileDataObject class, change the signature of the SetData method so that you pass now a Stream:

public void SetData(short dataFormat, int index, Stream stream)
{
  ...
  var iStream = new StreamWrapper(stream);
  ...
  // Ensure the following line is commented out:
  //Marshal.ReleaseComObject(iStream);
  return new Tuple<IntPtr, int>(ptr, NativeMethods.S_OK);
 ...
}

现在，不会创建新的内存流.

Now, no new memory stream will be created.

有关更多信息，请访问 http://blogs.msdn.com/b/delay/archive/2009/11/04/creating-something-from-nothing-asynchronously-developer-friendly-virtual-file-implementation-for-net-improved.aspx#10496772 并阅读我的评论

For further info, go to http://blogs.msdn.com/b/delay/archive/2009/11/04/creating-something-from-nothing-asynchronously-developer-friendly-virtual-file-implementation-for-net-improved.aspx#10496772 and read my comments

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