Copying Stream from XmlTextWriter to a FileStream

Question

I have an HTTP Handler that is binding an XmlTextWriter instance to Response.Output like so...

Sub GenerateXml(ByRef Response As HttpWebResponse)
    Using Writer As New XmlTextWriter(Response.Output)
        ' Build XML
    End Using
End Sub

And now I also want the same XML to be saved to the local hard drive (in addition to being streamed to the response). How can I do it?

Things I have tried

1) The obvious, copying the Response.OutputStream to a new FileStream...

    Sub GenerateXml(ByRef Response As HttpWebResponse)
        Using Writer As New XmlTextWriter(Response.Output)
            Build XML Here
        End Using

        // Also tried this inside the Using, with and without .Flush()
        CopyStream(Response.OutputStream, File.Create("C:\test.xml"))
    End Sub

    Sub CopyStream(ByRef Input As Stream, ByRef Output As Stream)
        Dim Buffer(1024) As Byte
        Dim Read As Integer

        Using Input
            Using Output
                While (Read = Input.Read(Buffer, 0, Buffer.Length)) > 0
                    Output.Write(Buffer, 0, Read)
                End While
            End Using
        End Using
    End Sub

I get errors like "Method not supported" and "NullReference"

2) Using a Memory Stream and then copying it...

    Using Memory As New MemoryStream()
        Using Writer As New XmlTextWriter(Memory, Encoding.UTF8)
            Build XML Here
        End Using

        // Tried outside of the Using, with and without Flush()
        CopyStream(Memory, Response.OutputStream)
        CopyStream(Memory, File.Create("C:\test.xml"))
    End Using

I get errors like "Cannot access a closed Stream."

Why is something so simple such a PITA?!

Do I really have to resort to writing the file to disk and then using Response.TransmitFile()? — Josh Stodola

user7116 user7116 · Accepted Answer · 2010-01-11T15:32:27

You can't wire up Response.OutputStream in that manner. However, you could do one of two things:

Create a method that calls Stream.Write for both your test file and Response.OutputStream.

using (var writerResponse = new XmlTextWriter(Response.OutputStream))
using (var writerFile = new XmlTextWriter(File.Create(@"c:\test.xml"))
{
    StartElement(writerResponse, writerFile, @"test");
}

Create a wrapper class which encapsulates two streams to be written to and provides all of the stream methods you require (Write etc).

public class DualStream : Stream
{
    public DualStream(Stream first, Stream second)
    {
    }

    public override bool CanRead { get { return false; } }
    public override bool CanWrite { get { return true; } }

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

// ...

using (var writerFile = new XmlTextWriter(File.Create(@"c:\test.xml")))
using (var writer = new XmlTextWriter(
      new DualStream(Response.OutputStream, writerFile)), null))
{
     // use writer
}

Copying Stream from XmlTextWriter to a FileStream

4 Answers