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?!

Answer 1

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

1. 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");
   }
   

2. 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
   }