Question

As per title is there any difference passing a reference type to a method such as:

public void GetStream(Stream outputStream)
{
   outputStream.Write(data);
}

VS

public Stream GetStream()
{
   MemoryStream ms = new MemoryStream();
   ms.Write(data);
   return ms; 
}

I noticed alot of Java code passes class by reference (not sure of the exact reasons for this). However in .NET is this just a matter of preference?

When would you choose one over the other?

Answer 1

The two are very different. In your first example, the caller is responsible for creating a stream (of whatever kind they prefer), and may pass a stream that is already positioned at some arbitrary position.

In the second, the callee has determined the type of stream, and is always writing at position 0.

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If your first example was instead:

public void GetStream(out stream outputStream)
{
   outputStream = new MemoryStream();
   outputStream.Write(data);
}

they would at least be closer to comparable.

Here, the major difference is that the caller has to have a declared variable to capture the outputStream, whereas in the other case, the caller could ignore the returned stream value.

However, the second from (returning the value) is more commonly seen – if a method has a single value to return, it’s by far preferred that that value is returned by the method, rather than a void method with an out parameter – mostly, in .NET, out parameters should be used sparingly (and only if the return value from the method is already something useful). An example of such a method would be the TryParse methods on various types, that return a bool (indicating success), and the parsed value is passed back as an out parameter.