134

My root problem is that when using calls Dispose on a StreamWriter, it also disposes the BaseStream (same problem with Close).

I have a workaround for this, but as you can see, it involves copying the stream. Is there any way to do this without copying the stream?

The purpose of this is to get the contents of a string (originally read from a database) into a stream, so the stream can be read by a third party component.
NB: I cannot change the third party component.

public System.IO.Stream CreateStream(string value)
{
    var baseStream = new System.IO.MemoryStream();
    var baseCopy = new System.IO.MemoryStream();
    using (var writer = new System.IO.StreamWriter(baseStream, System.Text.Encoding.UTF8))
    {
        writer.Write(value);
        writer.Flush();
        baseStream.WriteTo(baseCopy); 
    }
    baseCopy.Seek(0, System.IO.SeekOrigin.Begin);
    return baseCopy;
}

Used as

public void Noddy()
{
    System.IO.Stream myStream = CreateStream("The contents of this string are unimportant");
    My3rdPartyComponent.ReadFromStream(myStream);
}

Ideally I'm looking for an imaginary method called BreakAssociationWithBaseStream, e.g.

public System.IO.Stream CreateStream_Alternate(string value)
{
    var baseStream = new System.IO.MemoryStream();
    using (var writer = new System.IO.StreamWriter(baseStream, System.Text.Encoding.UTF8))
    {
        writer.Write(value);
        writer.Flush();
        writer.BreakAssociationWithBaseStream();
    }
    return baseStream;
}
Palec
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Binary Worrier
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5 Answers5

138

If you are using .NET Framework 4.5 or later, there is a StreamWriter overload using which you can ask the base stream to be left open when the writer is closed.

In earlier versions of .NET Framework prior to 4.5, StreamWriter assumes it owns the stream. Options:

  • Don't dispose the StreamWriter; just flush it.
  • Create a stream wrapper which ignores calls to Close/Dispose but proxies everything else along. I have an implementation of that in MiscUtil, if you want to grab it from there.
Peter Duniho
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Jon Skeet
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    Clearly the 4.5 overload was a not thought out concession -- the overload requires the buffer size, which cannot be 0 nor null. Internally I know that 128 characters is the minimum size, so I just set it to 1. Otherwise this 'feature' makes me happy. – Gerard ONeill Sep 17 '14 at 14:52
  • Is there a way to set that `leaveOpen` parameter after `StreamWriter` was created? – c00000fd Oct 19 '18 at 05:16
  • @c00000fd: Not that I'm aware of. – Jon Skeet Oct 19 '18 at 06:50
  • The just flush it option seems dangerous... The current exemple is in the same method so the easiest option is to move the closing bracket but generally it is not that simple (opening a writer in a sub function) then dispose will probably be called at the end of the function automatically. – Yepeekai Jan 29 '19 at 15:19
  • @Yepeekai: Can you clarify the specific danger you're envisaging? – Jon Skeet Jan 29 '19 at 16:00
  • if I pass a stream to a sub method and that sub method create the StreamWriter, it will be disposed at the end of the execution of that sub method. So when the execution return to the calling method, the stream will be disposed unless the GC is lazy and do it later, but I don't think we should rely on that. – Yepeekai Jan 29 '19 at 16:12
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    @Yepeekai: "if I pass a stream to a sub method and that sub method create the StreamWriter, it will be disposed at the end of the execution of that sub method" No, that's simply not true. It will only be disposed if something calls `Dispose` on it. The method ending doesn't do that automatically. It may be *finalized* later if it has a finalizer, but that's not the same thing - and it's still not clear what danger you're anticipating. If you think it's unsafe to return a `StreamWriter` from a method because it could be automatically disposed by the GC, that's just not true. – Jon Skeet Jan 29 '19 at 16:14
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    @Yepeekai: And IIRC, `StreamWriter` doesn't have a finalizer - I wouldn't expect it to, for precisely this reason. – Jon Skeet Jan 29 '19 at 16:15
  • Thanks, I think that my understanding of dispose need more reading :) – Yepeekai Jan 29 '19 at 16:37
  • I think that a `StreamWriter` taking ownage of a stream is a hugh design flaw. I would like to pass the stream to a callback, so it can write it's data on it. I'd like to append some other data. Now i'm left to whoever implements the callback, that they don't close the stream _(by a streamwriter eg (not using the right overload))_. – Jeroen van Langen Nov 11 '21 at 20:25
49

.NET 4.5 has a new method for that:

StreamWriter(Stream, Encoding, Int32, Boolean)

public StreamWriter(
    Stream stream,
    Encoding encoding,
    int bufferSize,
    bool leaveOpen
)
Peter Duniho
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maliger
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  • Thanks mate! Did not know this, and if anything that would be a fine reason for me to start targeting .NET 4.5! – Robert Kaufmann Jan 15 '14 at 13:54
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    Shame there is no overload that doesn't require bufferSize to be set. I'm happy with the default there. I'm having to pass it myself. Not the end of the world. – Andy McCluggage Feb 27 '14 at 14:57
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    The default `bufferSize` is `1024`. [Details are here](https://stackoverflow.com/a/29413715/968003). – Alex Klaus Mar 16 '18 at 04:29
37

Simply don't call Dispose on the StreamWriter. The reason this class is disposable is not because it holds unmanaged resource but to allow the disposal of the stream which itself could hold unmanaged resources. If the life of the underlying stream is handled elsewhere, no need to dispose the writer.

Darin Dimitrov
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  • Of course, there is then an implementation detail: does it buffer any data? Personally I'd rather use `NonClosingStreamWrapper` from MiscUtil – Marc Gravell Apr 19 '10 at 11:23
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    @Marc, wouldn't calling `Flush` do the job in case it buffers data? – Darin Dimitrov Apr 19 '10 at 11:26
  • @Darin - maybe yes, but actually I know of several `Stream` implementations that don't respect `Flush` (only fully flushing on `Close`; so again you'd have to treat it as an implementation detail, IMO. – Marc Gravell Apr 19 '10 at 11:28
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    Fine, but once we exit CreateStream, the StreamWrtier is collectable, forcing the third part reader to race against the GC, which is not a situation I want to be left in. Or am I missing something? – Binary Worrier Apr 19 '10 at 11:33
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    @BinaryWorrier: No, there's no race condition: StreamWriter doesn't have a finalizer (and indeed shouldn't). – Jon Skeet Apr 19 '10 at 12:15
  • @Jon Skeet: Really? I *am* surprised. I assumed anything that implements Dispose had a finalizer. One shall look in future and be done with assuming. @Darin +1 apologies for casting aspersions upon your answer. – Binary Worrier Apr 19 '10 at 14:22
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    @Binary Worrier: You should only have a finalizer if you *directly* own the resource. In this case, the StreamWriter should assume that the Stream will clean itself up if it needs to. – Jon Skeet Apr 19 '10 at 14:34
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    It seems that the 'close' method of StreamWriter also closes and disposes of the stream. So one must flush, but not close or dispose the streamwriter, so it doesn't close the stream, which would do the equivalent of dispose the stream. Way too much "help" from the API here. – Gerard ONeill Sep 17 '14 at 13:41
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    @JonSkeet but _knowing_ that `StreamWriter` doesn't have a finalizer is an implementation detail - I'm not supposed to know that, or depend on it. – KFL Sep 23 '17 at 19:21
8

Memory stream has a ToArray property that can be used even when stream is closed. To Array writes the stream contents to a byte array, regardless of the Position property. You can create a new stream based on the stream you wrote in.

public System.IO.Stream CreateStream(string value)
{
    var baseStream = new System.IO.MemoryStream();
    var baseCopy = new System.IO.MemoryStream();
    using (var writer = new System.IO.StreamWriter(baseStream, System.Text.Encoding.UTF8))
    {
        writer.Write(value);
        writer.Flush();
        baseStream.WriteTo(baseCopy); 
    }
    var returnStream = new System.IO.MemoryStream( baseCopy.ToArray());
    return returnStream;
}
Tudor
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  • Does that correctly limit the returned array to the content size? Because `Stream.Position` can _not_ be called after it's disposed. – Nyerguds Mar 15 '16 at 11:56
2

You need to create a descendant of the StreamWriter and override its dispose method, by always passing false to the disposing parameter, it will force the stream writer NOT to close, the StreamWriter just calls dispose in the close method, so there is no need to override it (of course you can add all the constructors if you want, i just have one):

public class NoCloseStreamWriter : StreamWriter
{
    public NoCloseStreamWriter(Stream stream, Encoding encoding)
        : base(stream, encoding)
    {
    }

    protected override void Dispose(bool disposing)
    {
        base.Dispose(false);
    }
}
Aaron Murgatroyd
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    I believe this does not do what you think it does. The `disposing` flag is part of [the `IDisposable` pattern](http://msdn.microsoft.com/en-us/magazine/cc163392.aspx). Always passing `false` to the `Dispose(bool)` method of the base class basically signals to the `StreamWriter` that it is being called from the finalizer (which is not so when you call `Dispose()` explicitly), and should thus not access any managed objects. *This* is why it won't dispose the base stream. However, the way you've achieved this is a hack; it would be much simpler to simply not call `Dispose` in the first place! – stakx - no longer contributing Aug 11 '12 at 14:59
  • Thats Symantec's really, anything you do apart from rewriting the streaming entirely from scratch is going to be a hack. Definitely though, you could simply not call base.Dispose(false) at all, but there would be no functional difference, and I like the clarity of my example. However, keep this in mind, a future version of the StreamWriter class may do more than just close the stream when it disposes, so calling dispose(false) future proofs it as well. But to each his own. – Aaron Murgatroyd Aug 12 '12 at 11:59
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    Another way to do it would be to create your own stream wrapper which contains another stream where the Close method simply does nothing instead of closing the underlying stream, this is less of a hack but is a more work. – Aaron Murgatroyd Aug 12 '12 at 12:03
  • Amazing timing: I was just about to suggest the same thing (decorator class, possibly named `OwnedStream`, that ignores `Dispose(bool)` and `Close`). – stakx - no longer contributing Aug 12 '12 at 12:04
  • Yeah, the code above is how I do it for a quick and dirty method, but if I was making a commercial application or something that actually mattered to me I would do it correctly using the Stream wrapper class. Personally I think Microsoft made a mistake here, the streamwriter should have had a boolean property to close the underlying stream instead, but then I dont work at Microsoft so they do what they like I guess :D – Aaron Murgatroyd Aug 12 '12 at 12:16
  • @AaronMurgatroyd Excellent solution for pre-4.5. Can be called simply as `using (StreamWriter sw = new StreamWriter(new NonDisposingStream(outputStream), encoding)) { ... }` – Nyerguds Mar 15 '16 at 12:33