Returning a stream from File.OpenRead()

Question

I'm in the process of writing a WCF service that will allow an ASP.Net web site to retrieve files (based on this article). My problem is that when I return the stream, it's blank.

For simplicity, I've isolated the code into a simple winforms app to try and find what the problem is with returning a stream and this is the code:

    private Stream TestStream()
    {
        Stream fs = File.OpenRead(@"c:\testdocument.docx");
        return fs;
    }

    // This method converts the filestream into a byte array so that when it is 
    // used in my ASP.Net project the file can be sent using response.Write
    private void Test()
    {            
        System.IO.MemoryStream data = new System.IO.MemoryStream();
        System.IO.Stream str = TestStream();

        str.CopyTo(data);
        byte[] buf = new byte[data.Length];
        data.Read(buf, 0, buf.Length);                       
    }

The result of this code is that buf is 12,587 bytes long (the correct length of the file) but it just contains 0's.

The Word document opens without problems if I try it, am I missing something obvious?

Answer 1

You forgot to seek:

str.CopyTo(data);
data.Seek(0, SeekOrigin.Begin); // <-- missing line
byte[] buf = new byte[data.Length];
data.Read(buf, 0, buf.Length);

Answer 2

Options:

• Use data.Seek as suggested by ken2k

• Use the somewhat simpler Position property:

  data.Position = 0;
  

• Use the ToArray call in MemoryStream to make your life simpler to start with:

  byte[] buf = data.ToArray();
  

The third option would be my preferred approach.

Note that you should have a using statement to close the file stream automatically (and optionally for the MemoryStream), and I'd add a using directive for System.IO to make your code cleaner:

byte[] buf;
using (MemoryStream data = new MemoryStream())
{
    using (Stream file = TestStream())
    {
        file.CopyTo(data);
        buf = data.ToArray();
    }
}

// Use buf

You might also want to create an extension method on Stream to do this for you in one place, e.g.

public static byte[] CopyToArray(this Stream input)
{
    using (MemoryStream memoryStream = new MemoryStream())
    {
        input.CopyTo(memoryStream);
        return memoryStream.ToArray();
    }
}

Note that this doesn't close the input stream.

Answer 3

You forgot to reset the position of the memory stream:

private void Test()
{            
    System.IO.MemoryStream data = new System.IO.MemoryStream();
    System.IO.Stream str = TestStream();

    str.CopyTo(data);
    // Reset memory stream
    data.Seek(0, SeekOrigin.Begin);
    byte[] buf = new byte[data.Length];
    data.Read(buf, 0, buf.Length);                       
}

Update:

There is one more thing to note: It usually pays not to ignore the return values of methods. A more robust implementation should check how many bytes have been read after the call returns:

private void Test()
{            
    using(MemoryStream data = new MemoryStream())
    {
        using(Stream str = TestStream())
        {
           str.CopyTo(data);
        }
        // Reset memory stream
        data.Seek(0, SeekOrigin.Begin);
        byte[] buf = new byte[data.Length];
        int bytesRead = data.Read(buf, 0, buf.Length);

        Debug.Assert(bytesRead == data.Length, 
                    String.Format("Expected to read {0} bytes, but read {1}.",
                        data.Length, bytesRead));
    }                     
}

Answer 4

You need

    str.CopyTo(data);
    data.Position = 0; // reset to beginning
    byte[] buf = new byte[data.Length];
    data.Read(buf, 0, buf.Length);  

And since your Test() method is imitating the client it ought to Close() or Dispose() the str Stream. And the memoryStream too, just out of principal.

Answer 5

Try changing your code to this:

private void Test()
{            
    System.IO.MemoryStream data = new System.IO.MemoryStream(TestStream());

    byte[] buf = new byte[data.Length];
    data.Read(buf, 0, buf.Length);                       
}