I have a class which reads data from one file stream and writes to another. I am concerned about closing the streams after the processing has finished in closeFiles().
How would you handle the possibility that the dispose of one stream may throw an exception stopping the dispose of the other stream from being called.?
Should I be calling close and dispose on the streams or just one?
What happens if I catch any errors from the stream disposes and then continue with moving and deleting of the files as shown in lastOperation()?
In a perfect world I'd like to use a using statement in a c++ style initialisation list but I'm pretty sure that's not possible in c#.
EDIT : thanks for the quick responses guys. So what I should be doing is deriving from IDisposable and then change the constructor and add the two disposing methods like this?:
~FileProcessor()
{
Dispose(true);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
private void Dispose(bool disposing)
{
if (!this.disposed)
{
if (disposing)
{
sw.Flush();
}
closeFiles();
disposed = true;
}
}
This is basically what I'm doing:
class FileProcessor
{
private string in_filename;
private string out_filename;
private StreamReader sr;
private StreamWriter sw;
bool filesOpen = false;
public FileProcessor(string filename)
{
in_filename = filename;
out_filename = filename + ".out";
openFiles();
}
~FileProcessor()
{
closeFiles();
}
private void openFiles()
{
sr = new StreamReader(in_filename);
sw = new StreamWriter(out_filename);
filesOpen = true;
}
private void closeFiles()
{
if (filesOpen)
{
sr.Close();
sw.Close();
sr.Dispose();
sw.Dispose();
filesOpen = false;
}
}
/* various functions to read, process and write to the files */
public void lastOperation()
{
closeFiles();
File.Delete( in_filename );
Directory.Move(out_filename, outdir + out_filename);
}
}
Your FileProcessor class should not have a destructor. It is of no use but it is expensive.
It should have a Dispose() (and implement the IDisposable interface) to call closeFiles().
And like #marcelo answered, Stream.Dispose() should not throw. You can rely on this for BCL classes.
But you should check each Reader/Writer for null, in case the first one opened but the second one failed:
if (sr != null) sr.Dispose();
if (sw != null) sw.Dispose();
Your filesOpen can't cover both.
I think it is a good practise to have your class implements IDisposable interface if you are using IDisposable object inside it.
Then, you should make sure that, in your Dispose() implementation, don't throw exceptions. If every object you dispose makes this guarantee, your client will be safe.
Dispose methods should never throw exceptions. There's even a code analysis tool warning for this.
In C#, using does exist. The object provided to the using statement must implement the IDisposable interface. This interface provides the Dispose method, which should release the object's resources.
If your StreamReader and StreamWriter implement IDisposable, you can put them in a using block, and they will be disposed of cleanly when you have finished with them.
using(var sr = new StreamReader(in_filename)) {
// Perform reader actions
}
// Reader will now be disposed.
Related
Yesterday, after running Visual Studio code analysis on our codebase, the following code was highlighted as an issue:
using (var stringReader = new StringReader(someString))
{
using (var reader = XmlReader.Create(stringReader)) {
// Code
}
}
The warning that was returned was
Warning CA2202 Object 'stringReader' can be disposed more than once in
method '(method name)'. To avoid generating a
System.ObjectDisposedException you should not call Dispose more than
one time on an object.
After searching stack overflow, I've come to the general understanding that if I were to create a custom class containing IDisposable members, it should implement IDisposable itself, and call the dispose() method of the member.
My two questions are
In all cases where object X takes a reference to an IDisposable object Y as a parameter during creation, is it correct to assume that the object X will take ownership of Y and from that point onwards, calling X.dispose() will always result in calling Y.dispose()
This is an old piece of code and exceptions as described in the warning message have never been reported (to the best of my knowledge). If the above point is assumed, why does the double using block not result in calling stringReader.dispose() twice and therefore throwing an exception?
is it correct to assume that the object X will take ownership of Y and from that point onwards, calling X.dispose() will always result in calling Y.dispose()
No, it is never save to assume that. Let's check this specific case: XmlReader.Create(Stream).
After going to quite some code in the Reference Source, I have found that the Dispose method calls the Close method. That is quite obvious. Then notice this piece of code:
public override void Close() {
Close( closeInput );
}
So whether the backing stream will be closed and disposed depends on the value of the setting closeInput, which you can set through the XmlReaderSettings.CloseInput setting.
So the answer here is a definite no: you can't be sure it is disposed. You should always make sure yourself it is.
No, you cannot assume that another object will call Dispose() while disposing itself. The object having the disposable object as a reference might not even be using the disposable resource.
This warning is known to be somewhat weird. Look here to see some complaints about the warning. You should design your class so that it must be safe to call Dispose() more than once.
By the way, MSDN says:
A method implementation contains code paths that could cause multiple calls to IDisposable.Dispose or a Dispose equivalent, such as a Close() method on some types, on the same object.
So a path to the Close() method call can generate this warning too, which is why you see this warning in your case.
In all cases where object X takes a reference to an IDisposable object Y as a parameter during creation, is it correct to assume that the object X will take ownership of Y and from that point onwards, calling X.dispose() will always result in calling Y.dispose()
I think not and I will try to explain why.
There is a pattern called IDisposablePattern which looks something like that:
public class SimpleClass : IDisposable
{
// managed resources SqlConnection implements IDisposable as well.
private SqlConnection _connection;
private bool _disposed;
// implementing IDisposable
public void Dispose()
{
// Here in original Dispose method we call protected method with parameter true,
// saying that this object is being disposed.
this.Dispose(true);
// Then we "tell" garbage collector to suppress finalizer for this object because we are releasing
// its memory and doesnt need to be finalized. Calling finalizer(destructor) of a given type is expensive
// and tweaks like this help us improve performance of the application.
GC.SuppressFinalize(this);
}
// Following the best practices we should create another method in the class
// with parameter saying whether or not the object is being disposed.
// Its really important that this method DOES NOT throw exceptions thus allowing to be called multiple times
protected virtual void Dispose(bool disposing)
{
// another thing we may add is flag that tells us if object is disposed already
// and use it here
if (_disposed) { return; }
if (_connection != null)
{
_connection.Dispose();
_connection = null;
}
_disposed = true;
// call base Dispose(flag) method if we are using hierarchy.
}
}
Note that this can be extended to new level when your class uses unmanaged resources like this one:
public class SimpleClass2: IDisposable
{
// managed resources
private SqlConnection _connection;
private bool _disposed;
// unmanaged resources
private IntPtr _unmanagedResources;
// simple method for the demo
public string GetDate()
{
// One good practice that .NET Framework implies is that when object is being disposed
// trying to work with its resources should throw ObjectDisposedException so..
if(_disposed) { throw new ObjectDisposedException(this.GetType().Name);}
if (_connection == null)
{
_connection = new SqlConnection("Server=.\\SQLEXPRESS;Database=master;Integrated Security=SSPI;App=IDisposablePattern");
_connection.Open();
}
// allocation of unmanaged resources for the sake of demo.
if (_unmanagedResources == IntPtr.Zero)
{
_unmanagedResources = Marshal.AllocHGlobal(100 * 1024 * 1024);
}
using (var command = _connection.CreateCommand())
{
command.CommandText = "SELECT getdate()";
return command.ExecuteScalar().ToString();
}
}
public void Dispose()
{
// Here in original Dispose method we call protected method with parameter true,
// saying that this object is being disposed.
this.Dispose(true);
// Then we "tell" garbage collector to suppress finalizer for this object because we are releasing
// its memory and doesnt need to be finalized. Calling finalizer(destructor) of a given type is expensive
// and tweaks like this help us improve performance of the application.
// This is only when your class doesnt have unmanaged resources!!!
// Since this is just made to be a demo I will leave it there, but this contradicts with our defined finalizer.
GC.SuppressFinalize(this);
}
// Following the best practices we should create another method in the class
// with parameter saying wether or not the object is being disposed.
// Its really important that this method DOES NOT throw exceptions thus allowing to be called multiple times
protected virtual void Dispose(bool disposing)
{
// another thing we may add is flag that tells us if object is disposed already
// and use it here
if (_disposed) { return; }
// Thus Dispose method CAN NOT release UNMANAGED resources such as IntPtr structure,
// flag is also helping us know whether we are disposing managed or unmanaged resources
if (disposing)
{
if (_connection != null)
{
_connection.Dispose();
_connection = null;
}
_disposed = true;
}
// Why do we need to do that?
// If consumer of this class forgets to call its Dispose method ( simply by not using the object in "using" statement
// Nevertheless garbage collector will fire eventually and it will invoke Dispose method whats the problem with that is if we didn't
// have the following code unmanaged resources wouldnt be disposed , because as we know GC cant release unmanaged code.
// So thats why we need destructor(finalizer).
if (_unmanagedResources != IntPtr.Zero)
{
Marshal.FreeHGlobal(_unmanagedResources);
_unmanagedResources = IntPtr.Zero;;
}
// call base Dispose(flag) method if we are using hierarchy.
}
~DatabaseStateImpr()
{
// At this point GC called our finalizer method , meaning
// that we don't know what state our managed resources are (collected or not) because
// our consumer may not used our object properly(not in using statement) so thats why
// we skip unmanaged resources as they may have been finalized themselves and we cant guarantee that we can
// access them - Remember? No exceptions in Dispose methods.
Dispose(false);
}
}
I want to compress a ProtoBuffer object on serialisation and decompress on deserialisation. Unfortunatly, C# stdlib offers only compression routines that work on streams rather than on byte[], that makes it a bit unesseray more verbose than a function call. My Code so far:
class MyObject{
public string P1 {get; set;}
public string P2 {get; set;}
// ...
public byte[] Serialize(){
var builder = new BinaryFormat.MyObject.Builder();
builder.SetP1(P1);
builder.SetP2(P2);
// ...
// object is now build, let's compress it.
var ms = new MemoryStream();
// Without this using, the serialisatoin/deserialisation Tests fail
using (var gz = new GZipStream(ms, CompressionMode.Compress))
{
builder.Build().WriteTo(gz);
}
return ms.ToArray();
}
public void Deserialize(byte[] data)
{
var ms = new MemoryStream();
// Here, Tests work, even when the "using" is left out, like this:
(new GZipStream(new MemoryStream(data), CompressionMode.Decompress)).CopyTo(ms);
var msg = BinaryFormat.MachineInfo.ParseFrom(ms.ToArray());
P1 = msg.P1;
P2 = msg.P2;
// ...
}
}
When dealing with streams, it seems one has to manually take care of the disposal of the objects. I wonder why that is, I'd expect GZipStream to be fully managed Code. And I wonder If Deserialize works only by accident and if I should dispose the MemoryStreams aswell.
I know I could probably solve this problem by simply using a thrid party compression library, but that's somewhat besides the point of this question.
GZipStream needs to be disposed so it flushes it's final blocks of compression out of its buffer to its underlying stream, it also calls dispose on the stream you passed in unless you use the overload that takes in a bool and you pass in false.
If you where using the overload that did not dispose of the MemoryStream it is not as critical to have the MemoryStream be disposed because it is not writing its internall buffer anywhere. The only thing it does is set some flags and set a Task object null so it can be GCed sooner if the stream lifetime is longer than the dispose point.
protected override void Dispose(bool disposing)
{
try {
if (disposing) {
_isOpen = false;
_writable = false;
_expandable = false;
// Don't set buffer to null - allow TryGetBuffer, GetBuffer & ToArray to work.
#if FEATURE_ASYNC_IO
_lastReadTask = null;
#endif
}
}
finally {
// Call base.Close() to cleanup async IO resources
base.Dispose(disposing);
}
}
Also, although the comment says "Call base.Close() to cleanup async IO resources" the base dispose function from the Stream class does nothing at all.
protected virtual void Dispose(bool disposing)
{
// Note: Never change this to call other virtual methods on Stream
// like Write, since the state on subclasses has already been
// torn down. This is the last code to run on cleanup for a stream.
}
All that being said, when decompressing a GZipStream you can likely get away with not disposing it for the same reason as not disposing the MemoryStream, when decompressing it does not buffer bytes anywhere so there is no need to flush any buffers.
how can you implement the Dispose method after using "using" on a MemoryStream object in a class that implements IDisposable?
public class ControlToByte :IDisposable
{
public static byte[] GetByte(object control)
{
using (System.IO.MemoryStream memoryStream = new System.IO.MemoryStream())
{ //do something here }
public void Dispose()
{
//how do you dispose of memorystream object?
}
You don't have to implement IDispose, nor do you have to call Dispose explicitly. The using block will ensure that, after completion, the MemoryStream will be disposed. using blocks actually work like try/finally blocks. Something like:
{
System.IO.MemoryStream memoryStream = null;
try
{
memoryStream = new System.IO.MemoryStream();
//....your code
}
finally
{
if (memoryStream != null) //this check may be optmized away
memoryStream.Dispose();
}
}
using C#
The using statement ensures that Dispose is called even if an
exception occurs while you are calling methods on the object. You
can achieve the same result by putting the object inside a try block
and then calling Dispose in a finally block; in fact, this is how
the using statement is translated by the compiler.
In your current code, you are implementing IDisposable on your class ControlToByte, This would be useful if you want to dispose resourses in ControlToByte. As illustrated, you don't need to implement IDisposable for your class.
I'm having a real strange issue while working with streams in mono.
I open a stream like this:
_stream = new FileStream(filename, FileMode.Open, FileAccess.ReadWrite, FileShare.Read);
Where _stream of course is a FileStream. I added a dispose method for this class like that:
public override void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
private void Dispose(bool disposing)
{
if(!_disposed)
{
if(disposing)
{
_stream.Flush(true);
_stream.Dispose();
_items.Clear();
}
_disposed = true;
}
}
Now i have this simple test:
[Test]
public void SimpleTest()
{
SyncFileWriter s = new SyncFileWriter("file");
s.Save("a", "b");
s.Dispose();
SyncFileWriter s2 = new SyncFileWriter("file");
s2.Dispose();
}
But if fails with a IOException: Sharing violation. [edit]It looks like there was an error with monodevelop. Restarted it and now it fails everytime[/edit]
How to avoid this and close the stream properly?
It looks like it was my fault. The SyncFileWriter is derived from a abstract class that checkes whether the file exist, or not. If not, it will initialize this file with some default things and that's why I did the following
File.Create(filename);
As you see, I create a file, but File.Create opens a stream and does not close it after leaving it's scope, whatever.
Now I do this:
Filestream f = File.Create(filename);
...
f.Dispose();
and everything is fine.
I think here lies also the problem for the first nondeterministic behavoir, because it does this only, when a parameter is null, and it is only null, when you create a new file.
Anyway, Thanks for your help!
Not quite sure what you're trying to achieve, but I guess this may help: http://www.blackwasp.co.uk/UsingStatement.aspx
Code is:
using (MemoryStream memorystream = new MemoryStream(bytes))
{
using (BinaryWriter writer = new BinaryWriter(memorystream))
{
writer.Write((double)100.0);
}
return memorystream.ToArray();
}
Isn't the above code appropriate to properly dispose of both object?
Is the code analysis at all useful? Other than garbage information about variables names and namespaces it seems to complain about a lot of things that are not reality. I am really thinking that maybe it is useful and I am just missing the point.
OKAY to address concerns whether the MemoryStream is disposed of or not (its not) here is an example where the VS code analysis gives me the exact same warning. Clearly nothing is getting disposed of here
public class MyClass : IDisposable
{
public void DoSomethingElse()
{
}
#region IDisposable Members
public void Dispose()
{
throw new NotImplementedException();
}
#endregion
}
public class MyOtherClass : IDisposable
{
public MyOtherClass(MyClass mc)
{
}
public void DoSomething() { }
}
public void Foo()
{
using (MyClass mc = new MyClass())
{
using (MyOtherClass otherclass = new MyOtherClass(mc))
{
otherclass.DoSomething();
}
mc.DoSomethingElse();
}
}
On "If the stream were disposed then how can I call memorystream.ToArray()" part of the question:
MemeoryStream.Dispose does not release internal buffer. The only thing that it does is blocking all Stream methods (like Read) by throwing "Object Disposed" exception. I.e. following code is perfectly valid (usually similar code accessing lower level storage can be written for other other Stream objects that manage some sort of storage):
MemoryStream memoryStream = new MemoryStream();
using (memoryStream)
{
// write something to the stream
}
// Note memoryStream.Write will fail here with an exception,
// only calls to get storage (ToArray and GetBuffer) make sense at this point.
var data = memoryStream.ToArray();
BinaryWriter does not dispose the MemoryStream. YOU NEED TO DISPOSE OF IT. Get the results of .ToArray in a variable, then dispose the MemoryStream, then return the result. Or use a try/finally to dispose of the MemoryStream.
If I were to guess I would say that the logic of the analysis rule is identifying a possiblity that goes someting like this:
"You are passing a disposable object into the constructor of another disposable object (thus semantically implying a transfer of ownership). If ownership is truly being trasnsferred to the second object, it's likely that it will dispose of the object passed to its constructor in its own Dispose method but you're disposing of it as well."
Your second example proves that it is not actually analysing whether or not the second object takes ownership of and disposes the first, but rather this rule is saying this pattern has some semantic ambiguity, as the code responsible for disposal of the first object is no longer clear.
The dispose pattern has all of the inherent pitfalls of alloc/free and new/delete as only one class should own and thus control the lifetime of a disposable instance.
All pure conjecture of course but that's how I'd read it.
MemoryStream wraps a managed buffer that will be garbage collected at some point in the future regardless of whether its disposed of.
The IDisposable implementation is part of the Stream contract. It closes the stream preventing you from using the Read and Write methods but you are still permitted to access the underlying bytes. (Preventing calls to ToArray would probably make it difficult to use with StreamWriters.) A Stream implementation that does use unmanaged resources like a FileStream would release the file handle when it is disposed of.
The code analysis is noting that both you and the StreamWriter are calling Dispose on the same object and warning about it. This is pretty meaningless for a MemoryStream but might be dangerous with some other IDisposable implementations.
You appear to be confusing the IDisposable interface with garbage collection. Calling Dispose does not release managed objects, it just gives your class the opportunity to release unmanaged resources in a deterministic manner.
I think BinaryWriter.Dispose() will call its underlying MemoryStream's .Dispose(), and then your using will re-dispose it.
At least I think that's what will happen. I haven't opened the source for BinaryWriter to verify, but I was always under the impression it closed and disposed its underlying stream.
Edit:
This is the source for the classes. You can see where your memory stream is being disposed:
BinaryStream:
public BinaryWriter(Stream output) : this(output, new UTF8Encoding(false, true))
{
}
public BinaryWriter(Stream output, Encoding encoding)
{
if (output==null)
throw new ArgumentNullException("output");
if (encoding==null)
throw new ArgumentNullException("encoding");
if (!output.CanWrite)
throw new ArgumentException(Environment.GetResourceString("Argument_StreamNotWritable"));
Contract.EndContractBlock();
OutStream = output;
_buffer = new byte[16];
_encoding = encoding;
_encoder = _encoding.GetEncoder();
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
OutStream.Close();
}
MemoryStream:
public class MemoryStream : Stream
{
....
}
Stream
public virtual void Close()
{
Dispose(true);
GC.SuppressFinalize(this);
}
To answer one of your further questions, "why can you still call .ToArray() if MemoryStream has been disposed", well, this test passes just fine:
[TestMethod]
public void TestDispose()
{
var m = new MemoryStream();
m.WriteByte(120);
m.Dispose();
var a = m.ToArray();
Assert.AreEqual(1, a.Length);
}
So the .ToArray() method is still accessible after dispose.
The bytes are still available. MemoryStream dispose just sets some flags internally that prevent you from further modifying the stream:
protected override void Dispose(bool disposing)
{
try {
if (disposing) {
_isOpen = false;
_writable = false;
_expandable = false;
// Don't set buffer to null - allow GetBuffer & ToArray to work.
}
}
finally {
// Call base.Close() to cleanup async IO resources
base.Dispose(disposing);
}
}
In fact, note that the source has the comment in it:
// Don't set buffer to null - allow GetBuffer & ToArray to work.
so that really answers that question :)