Combine Multiple Streams in a Single .NET Framework Stream Object

Abstract

This article represents a simple Stream class whose data is based on the number of another .NET Framework streams.

There are some tasks when you may want to provide the data combined from different parts via Stream interface. The most obvious way is to create the MemoryStream class instance and copy all data parts (streams in our case) into this single object. This solution has only one good thing - the simplicity and you can use it for sure for combining of small footprint streams without bothering about the reverse of the medal. While working with the large size streams or having big number of streams such primitive copying of data chunks may lead to bad application performance and irrational using of virtual memory.

MultiStream Class Declaration

In order to effectively combine number of streams into the single Stream class descendant you will need to organize access to streams data in appropriate order.

We will create new MultiStream inherited from .NET base abstract Stream class. Methods Read, Seek, Write and also Length property need to be overriden and implemented within MultiStream class. All remaining abstract Stream members have the obvious implementation and to be shortest we do not place them here. But you can find the full source code for this class at multistreamcs.zip

The code below shows the implementation of the Read method of MultiStream class:

public override int Read(byte[] buffer, int offset, int count)
{
long len = 0;
int result = 0;
int buf_pos = offset;
int bytesRead;
foreach(Stream stream in streamList)
{
if(position < (len + stream.Length))
{
stream.Position = position - len;
bytesRead = stream.Read(buffer, buf_pos, count);
result += bytesRead;
buf_pos += bytesRead;
position += bytesRead;
if(bytesRead < count)
{
count -= bytesRead;
}
else
{
break;
}
}
len += stream.Length;
}
return result;
}

This method walks through all streams being stored within the streamList member of MultiStream class and checks each of them for matching the MultiStream Position. If the current stream position plus total all of the previous streams sizes matches to the main Position then data from this stream will be transferred until requested buffer size extends up to the another stream items.

The Seek method implements locating of the TMultiStream Position pointer using the total size of collected stream items:

public override long Seek(long offset, SeekOrigin origin)
{
long len = Length;
switch(origin)
{
case SeekOrigin.Begin:
position = offset;
break;
case SeekOrigin.Current:
position += offset;
break;
case SeekOrigin.End:
position = len - offset;
break;
}
if(position > len)
{
position = len;
}
else if(position < 0)
{
position = 0;
}
return position;
}

Please note that the contents of the MultiStream can not be modified by calling of Write method. So implementation of this method will be very simple:

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

Indeed it is quite difficult to imagine such task when you might need to change the contents of the multistream object.

Using of MultiStream Class

With just few lines of code the new MultiStream class is ready to use. Lets go ahead and test it:

MultiStream stream = new MultiStream();
stream.AddStream(new StringStream("test string "));
stream.AddStream(new FileStream("testfile.txt", FileMode.Open, FileAccess.Read, FileShare.Read));
byte[] buf = new byte[1024];
int cnt = stream.Read(buf, 0, 1024);
Debug.Assert(new ASCIIEncoding().GetString(buf, 0, cnt).Equals("test string test file"));

This code is constantly being improved and your comments and suggestions are always welcome.