Question

This seems like a fairly straightforward question, but I couldn’t find this particular use-case after some searching around.

Suppose I have a simple method that, say, determines if a file is opened by some process. I can do this (not 100% correctly, but fairly well) with this:

public bool IsOpen(string fileName)
{
  try
  {
    File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None);
  }
  catch
  {
    // if an exception is thrown, the file must be opened by some other process
    return true;
  } 
}

(obviously this isn’t the best or even correct way to determine this – File.Open throws a number of different exceptions, all with different meanings, but it works for this example)

Now the File.Open call returns a FileStream, and FileStream implements IDisposable. Normally we’d want to wrap the usage of any FileStream instantiations in a using block to make sure they’re disposed of properly. But what happens in the case where we don’t actually assign the return value to anything? Is it still necessary to dispose of the FileStream, like so:

try
{
  using (File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None));
  { /* nop */ }
}
catch 
{
  return true;
}

Should I create a FileStream instance and dispose of that?

try
{
  using (FileStream fs = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None));
}
...

Or are these totally unnecessary? Can we simply call File.Open and not assign it to anything (first code example), and let the GC dispose of it right away?

Answer 1

Yes, you should definitely dispose of the FileStream. Otherwise the stream will remain open and the file won’t be usable until a finalizer happens to clean it up.

The important thing here is ownership: for File.Open, the caller is assumed to “own” the stream returned to it – and if you own something which implements IDisposable, it’s your responsibility to dispose of it.

Compare this with the situation of Image.FromStream: in that case, you pass in a stream and the Image then assumes that it owns that stream. You mustn’t close the stream yourself, in that case – you have to dispose of the image when you’re done, and it will dispose of the stream.

Calling a static method which returns something disposable almost always assumes that the caller takes ownership of the resource. Ditto constructors (which are effectively static methods.)

The irony in this case is that if you don’t dispose of the stream returned by File.Open, you’ll have found that the file is usable – at the same time as making it unusable until some indeterminate time.