Question

I have a method that converts a file to bytes so that I can later send it over the internet. anyways because I plan to send large files I send chunks of files instead of sending the whole file. each chunk consist of an array of bytes (byte[]) . I am new to all this so I wanted to save each chunk in an List of chunks ( List ) before sending it . so my class looks like:

public class SomeClass
{

    public List<byte[]> binaryFileList;

    public void SendChunk(byte[] data, int index)
    {
        binaryFileList.Add(data);
        // later I will add code in here to do something with data
    }

    public void test(string path)
    {
        binaryFileList = new List<byte[]>();

        System.IO.FileStream stream = new System.IO.FileStream(path,
            System.IO.FileMode.Open, System.IO.FileAccess.Read);

        var MaxChunkSize = 10000;
        byte[] chunk = new byte[MaxChunkSize];
        while (true)
        {
            int index = 0;
            // There are various different ways of structuring this bit of code.
            // Fundamentally we're trying to keep reading in to our chunk until
            // either we reach the end of the stream, or we've read everything we need.
            while (index < chunk.Length)
            {
                int bytesRead = stream.Read(chunk, index, chunk.Length - index);

                if (bytesRead == 0)
                {
                    break;
                }
                index += bytesRead;
            }
            if (index != 0) // Our previous chunk may have been the last one
            {
                SendChunk(chunk, index); // index is the number of bytes in the chunk
            }
            if (index != chunk.Length) // We didn't read a full chunk: we're done
            {
                return;
            }
        }


    }
}

and when I execute:

SomeClass s = new SomeClass();
s.test(@"A:\Users\Tono\Desktop\t.iso");

binaryFileList List gets populated with chunks of the file: A:\Users\Tono\Desktop\t.iso

Now the problem came when I tied to create a file from that data. when debuging I noticed that the problem was because items in binaryFileList changed as I entered data. let me show you what I mean:

notice that in this debug it is the first time I add an item to binaryFileList. and also you can see each byte of that item in the array…

now I will let the method run more times adding more items to binaryFileList.

so now binaryFileList has 278 items instead of one like on the last picture:

so everything so far looks ok right? but did you guys recall that the first item of binaryFileList contained an array of bytes with almost all 0’s? take a look at the first item of binaryFileList:

and as I keep adding items to binaryFileList note how the first item changes:

In other words binaryFileList is a list of byte[]. and when I add a byte[] to binaryFileList other byte[] should not change. they do change! why!?

Answer 1

The following line has to go inside the loop:

byte[] chunk = new byte[MaxChunkSize];

You create the chunk only once and overwrite it each time with new data. What you store in you list, is just a reference to this chunk, not a copy of it.