c# - What does Filestream.Read return value mean? How to read data in chunks and process it?

Question

I'm quite new to C# so please bear with me. I'm reading (using FileStream) data (fixed size) to small array, process the data and then read again and so on to the end of file.

I thought about using something like this:

            byte[] data = new byte[30];
            int numBytesToRead = (int)fStream.Length;
            int offset = 0;

            //reading
            while (numBytesToRead > 0)
            {
                fStream.Read(data, offset, 30);
                offset += 30;
                numBytesToRead -= 30;

                //do something with the data
            }

But I checked documentation and their examples and they stated that return value of the above read method is:

"Type: System.Int32 The total number of bytes read into the buffer. This might be less than the number of bytes requested if that number of bytes are not currently available, or zero if the end of the stream is reached."

What does it mean that they are not currently available, can this really happen when reading small amounts of data or is this just for large amounts? If only for large, how large approximately, because I'll be reading also in bigger chunks in some other places. If this can happen anytime how should I change my code so that the code will still execute efficiently?

Thank you for your time and answers.

Answer 1

The read method returns the number of bytes returned, which may be less than the number of bytes requested. Normally when you read a file, you will get all the bytes that you ask for (unless you reach the end of the file), however, you can't count on it always being that way.

It's possible that the system will make a difference between data that is immediately available and data that needs time to be retrieved, so that it will return the data currently available right away, start reading more data in the background and expect you to request the rest of the data in another call. AFAIK it doesn't do this currently, but it's a reasonable future scenario.

You should get the result of the Read method and use that to determine how much data you got. You shouldn't read it into the buffer at the location of offset, then you can't read a file that is larger than the buffer. Alternatively, you can declare an array to hold the entire stream, then you would read the data into the location of offset.

You should also handle the situation where the Read method returns zero, which means that there is no more data to read. This normally doesn't happen until you reach the end of the file, but if it would it would throw your code into an eternal loop.

byte[] data = new byte[30];
int numBytesToRead = (int)fStream.Length;
int offset = 0;

//reading
while (numBytesToRead > 0) {
  int len = fStream.Read(data, 0, data.Length);
  offset += len;
  numBytesToRead -= len;
  if (len == 0 && numBytesToRead > 0) {
    // error: unexpected end of file
  }
  //do something with the data (len bytes)
}