Does the .NET Streams save memory?

Question

Example:

1. If I read a file and copy it to another file through .NET Streams will the total size of the file occupy the memory at any moment? Or will the bytes be discarded as soon as they are used?
2. If the naive approach doesn't saves memory would buffered Streams do it?

Answer 1

If you stream your copying, i.e. read a buffer, write a buffer, read a buffer, write a buffer etc until you've run out of data, it will only take as much memory as the buffer size. I expect File.Copy to do this (in the native Windows code, admittedly).

If you want to do it yourself, use something like this:

public void CopyData(Stream input, Stream output)
{
    byte[] buffer = new byte[32 * 1024];
    int read;
    while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
    {
        output.Write(buffer, 0, read);
    }
}

This will only take 32K however big the stream is.

EDIT: As noted in the comments, the streams may well have their own buffers as well, but the point is that you could still transfer a very large file without running out of memory.

Answer 2

If you call something like ReadToEnd() then yes, the contents of the file will be loaded into memory. You're correct in guessing that using a buffer is the appropriate approach to ensure that only a subset of the file's data is loaded into memory at any given time.