Understanding the using statement

Question

I have written two codes:

code block 1:

Stream dataStream;
using (var response = (HttpWebResponse)req.GetResponse())
{
    dataStream = response.GetResponseStream();
}

//Open the stream using a StreamReader for easy access
using (var reader = new StreamReader(dataStream))
{
    data = reader.ReadToEnd();
}

code block 2:

Stream dataStream;
using (var response = (HttpWebResponse)req.GetResponse())
{
    dataStream = response.GetResponseStream();

    //Open the stream using a StreamReader for easy access
    using (var reader = new StreamReader(dataStream))
    {
        data = reader.ReadToEnd();
    }
}

Code block 1 is throwing error: stream is not reachable.
While progmatically i think both code will work same.
I added using block in to whole statment in code block 2 and it's working.
But I'm confused why is it throwing error in code block 1

Answer 1

Because post compilation (in IL), your code gets converted to the following

Code Block: 1

            HttpWebResponse response=null;
            Stream dataStream;
            try
            {
                response = (HttpWebResponse) req.GetResponse();
                dataStream = response.GetResponseStream();
            }
            finally
            {
                if(response!=null)
                    ((IDisposable)response).Dispose();
            }

            StreamReader reader = null;
            try
            {
                //DataStream is accessed AFTER response object is disposed 
                reader = new StreamReader(dataStream);
                data = reader.ReadToEnd();
            }
            finally
            {
                if(reader!=null)
                    reader.Dispose();
            }

Code Block: 2

  HttpWebResponse response=null;
            Stream dataStream;
            try
            {
                response = (HttpWebResponse) req.GetResponse();
                dataStream = response.GetResponseStream();

                StreamReader reader = null;
                try
                {
                    //DataStream is accessed while response object is alive, and connected (not disposed)
                    reader = new StreamReader(dataStream);
                    data = reader.ReadToEnd();
                }
                finally
                {
                    if (reader != null)
                        reader.Dispose();
                }
            }
            finally
            {
                if(response!=null)
                    ((IDisposable)response).Dispose();
            }

As you can see, in CodeBlock1, you are trying to access stream which is trying to read from a Disposed (and dis-connected) HttWebResponse object.