C# Casting MemoryStream to FileStream

Question

My code is this:

byte[] byteArray = Encoding.ASCII.GetBytes(someText);
MemoryStream stream = new MemoryStream(byteArray);
StreamReader reader = new StreamReader(stream);
FileStream file = (FileStream)reader.BaseStream;

Later I'm using file.Name.

I'm getting an InvalidCastException: it displays follows

Unable to cast object of type 'System.IO.MemoryStream' to type 'System.IO.FileStream'.

I read somewhere that I should just change FileStream to Stream. Is there something else I should do?

Answer 1

A MemoryStream is not associated with a file, and has no concept of a filename. Basically, you can't do that.

You certainly can't cast between them; you can only cast upwards an downwards - not sideways; to visualise:

        Stream
          |
   ---------------
   |             |
FileStream    MemoryStream

You can cast a MemoryStream to a Stream trivially, and a Stream to a MemoryStream via a type-check; but never a FileStream to a MemoryStream. That is like saying a dog is an animal, and an elephant is an animal, so we can cast a dog to an elephant.

You could subclass MemoryStream and add a Name property (that you supply a value for), but there would still be no commonality between a FileStream and a YourCustomMemoryStream, and FileStream doesn't implement a pre-existing interface to get a Name; so the caller would have to explicitly handle both separately, or use duck-typing (maybe via dynamic or reflection).

Another option (perhaps easier) might be: write your data to a temporary file; use a FileStream from there; then (later) delete the file.

Answer 2

You can compare Stream with animal, MemoryStream with dog and FileStream with cat. Although a dog is an animal, and a cat is an animal, a dog certainly is not a cat.

If you want to copy data from one stream to another, you will need to create both streams, read from one and write to the other.