Finalization on StreamWriter and StreamReader

Question

If I have this:

StreamWriter cout = new StreamWriter("test.txt");
cout.WriteLine("XXX");

// here IOException...
StreamReader cin = new StreamReader("test.txt");
string text = cin.ReadLine();

the clr throws an IOException because I haven't close the cout.

In fact If I do this:

StreamWriter cout = new StreamWriter("test.txt");
cout.WriteLine("XXX");

cout.Close();

StreamReader cin = new StreamReader("test.txt");
string text = cin.ReadLine();

I have no exception.

But If I do this and then exit from the application:

StreamReader cin = new StreamReader("test.txt");
string text = cin.ReadLine();

without closing cin the file can from the OS opened and written.

However reading the source code of StreamReader.cs I didn't' find a destructor method (i.e. ~StreamReader(...)). So who does free that file if the garbage collector doesn't invoke Dispose and there is no finalization method?

Answer 1

Internally, the StreamReader uses a FileStream:

 Stream stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, DefaultFileStreamBufferSize, FileOptions.SequentialScan, Path.GetFileName(path), false, false, checkHost);

The FileStream class, being the class that ultimately accesses the file and therefore needs to guarantee cleanup, does have a finalizer, which closes the actual underlying stream. The Dispose method on the StreamReader just calls the Close on the underlying FileStream.

Answer 2

StreamReader and StreamWriter uses a FileStream to access the file. FileStream uses SafeFileHandle to store the underlying OS file handle. As the SafeFileHandle class controls an unmanaged resource it correctly has a finalizer (what you call a destructor) that closes the file handle.

But If I do this and then exit from the application: [...] without closing cin the file can from the OS opened and written

When a process terminates all resources used by that process is released to the operating system. It doesn't matter if your application forgot to close a file handle (even though SafeFileHandle will not "forget"). You will always observe the described behavior no matter how badly written your application is.

I just want to point out that the best way to work with StreamReader and StreamWriter and similar classes is using:

using (StreamWriter cout = new StreamWriter("test.txt")) {
  cout.WriteLine("XXX");
}

using (StreamReader cin = new StreamReader("test.txt")) {
  string text = cin.ReadLine();
}

This deterministically closes the files when the using block ends even if an exception is thrown while processing the file.