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I've been working on a data export program that's extracting a bunch of records from a database. One of the steps involves converting an RTF text string into plain text and this ended up causing a memory leak of User Objects when it ran. One of the columns that Task Manager will show is "USER objects" - when this hits ~10,000 the program will run out of allocation space and the program faults with "error creating window handle"

This was happening because I wasn't disposing of my object at the end of the method.

My question is, why didn't C#/.net dispose of it for me?

Here's a quick sample of code that will reproduce the leak. Put the code into a Winforms application and push the button to get it to loop through the memory waster.

private void wasteMemory()
{
    System.Windows.Forms.RichTextBox rtfBox = new System.Windows.Forms.RichTextBox();

    //RTF text that reads "Hello World"
    rtfBox.Rtf = "{\\rtf1\\ansi\\ansicpg1252\\deff0\\deflang1033{\\fonttbl{\\f0\\fnil\\fcharset0 Arial;}}  {\\colortbl ;\\red0\\green0\\blue0;}  \\viewkind4\\uc1\\pard\\cf1\\fs29 Hello World} ";

    //If line below is commented out, User Objects grow out of control.
    //rtfBox.Dispose();
}

private void button1_Click(object sender, EventArgs e)
{
    for (int i = 1; i < 100000; i++)
    {            
        wasteMemory();
    }
}

It's my understanding that the scope of a method any objects created in side of it are disposed of when the method completes. I expected the rtfBox to be disposed of, but it doesn't.

EtanSivad
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  • i suspect its because clr doesnt feel any memory pressure and so is not running GC yet, but windows runs out of handles – pm100 Sep 11 '13 at 17:14
  • Is it complete code? Do you attach rtfBox to other controls? – alexm Sep 11 '13 at 17:15
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    Windows handles are unmanaged resource (unlike memory) so GC does not "feel" the pressure from it. It is responsibility of the programmer to call dispose() in such cases. – alexm Sep 11 '13 at 17:18

7 Answers7

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The Dispose method is the .NET way of giving objects with native resources the chance to clean up. It is sort of like a Destructor/delete in C++--though not really. If you don't call Dispose on an object implementing IDisposable, it is a bug, and will most likely cause a memory leak. It would be best to do the following:

using(System.Windows.Forms.RichTextBox rtfBox = new System.Windows.Forms.RichTextBox())
{

  //RTF text that reads "Hello World"
  rtfBox.Rtf = "{\\rtf1\\ansi\\ansicpg1252\\deff0\\deflang1033{\\fonttbl{\\f0\\fnil\\fcharset0 Arial;}}  {\\colortbl ;\\red0\\green0\\blue0;}  \\viewkind4\\uc1\\pard\\cf1\\fs29 Hello World} ";
}

A using block will behave exactly like you are expecting it to. You can think of it like method scopes in C++ for an object on the stack.

It's my understanding that the scope of a method any objects created in side of it are disposed of when the method completes. I expected the rtfBox to be disposed of, but it doesn't.

No, this isn't true at all--or with most other garbage collected languages for that matter. If you recognize that your object here is dynamically allocated (i.e. very much like a pointer), then it isn't even true for languages like C++ either since dynamically allocated memory isn't cleaned up when its pointer goes out of scope: you have to explicitly call delete. In .NET, the object will be finalized,the destructor will be called, and dispose will be called when the garbage collector gets around to it, but not until then. Going out of scope will only signal to the garbage collector that the object in question is eligible to be collected. However, anything with resources, such as native code, file handles, or other IDisposable implementing objects, are supposed to be disposed of via. Dispose() as soon as the user is done with them in order to avoid memory leaks.

For more information, see http://msdn.microsoft.com/en-us/library/system.idisposable.aspx

Jonathan Henson
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    @gdoron and he explicity says he is not using it and that he sees the issue when he does not use it. The correct answer is "You are supposed to be using it and if you are not it is a bug in your program and you should expect bad stuff like this to happen" – Scott Chamberlain Sep 11 '13 at 17:18
  • So basically anything more complex then primitives I should be careful about disposing? That's good to know. – EtanSivad Sep 11 '13 at 17:21
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    @EtanSivad, no just anything implementing IDisposable. – Jonathan Henson Sep 11 '13 at 17:27
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Every answer here so far is incomplete. Yes, it's true that unmanaged resources have to be cleaned up, but the classes that implement IDisposable already do that. That's not the point.

In a class that properly implements IDisposable, then if the object is not explicitly or implicitly disposed, then it will be disposed during the finalizer stage of garbage collection. However, this process does not happen right away when the object goes out of scope. It might take minutes or even hours for the gc to run.

The issue here is that if you do not call Dispose() yourself (or implicitly call Dispose() by wrapping it in a using statement), then (if it's implemented correctly by the class) the object will not be disposed until the garbage collector runs, which could take a considerable amount of time.

This means you could run out of a non-managed resource before the garbage collector gets around to disposing of the unreferenced objects. And that's precisely the problem you are running into.

Calling Dispose() yourself ensures that unmanaged objects get cleaned up right after you are done with them, not whenever the GC gets around to it.

Think about it like a library. Someone checks out a book, and there are 5 copies on the shelf. As other people check out this library, some return them.. but they don't get put on the shelf right away, they sit in the return bin until someone gets around to checking them in and reshelving them.

Calling Dispose is like handing the book to the librarian, having them check it in right away, and putting it back on the shelf so the next person can get to it.

Erik Funkenbusch
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  • That makes a lot of sense, thank you. In the original app I was calling the method that was using the rtfBox thousands of times a second in a pretty tight For loop, so I can see why GC wasn't occurring here. It caught me off guard because I had assumed that "sense it's C# / .net, I don't need to worry about memory management." It would be interesting to see how long until GC gets around to disposing it if I don't do it manually. – EtanSivad Sep 11 '13 at 17:39
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    You're assuming that every class that implements IDisposable also implements a finalizer. Unfortunately, that is not true (look at streams). I.e. cleaning up resources like handles is not necessarily done automatically as a side effect of GC. – Brian Rasmussen Sep 11 '13 at 17:56
  • @BrianRasmussen - I'm not sure what you mean about streams, as they have finalizers, and they clean up their streams, unless you have explicitly told them not to. In that case, you know what you're doing. – Erik Funkenbusch Sep 11 '13 at 20:47
  • `StreamWriter` doesn't have a finalizer. The problem is that the finalization order is undefined, so some scenarios cannot be solved by relying on finalizers for cleanup. See http://dotnet.dzone.com/news/net-internals-guaranteed-final – Brian Rasmussen Sep 11 '13 at 20:54
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    @BrianRasmussen - A StreamWriter is not a Stream, that's a different issue. A StreamWriter has no unmanaged resources. The Stream does, and the Stream does have a finalizer. – Erik Funkenbusch Sep 11 '13 at 23:12
  • You're right. It is not a Stream. I'm sorry for not being more specific in my initial comment. However, it doesn't change the fact that relying on finalization for cleanup is not always an option as illustrated by the `FileStream` / `StreamWriter` example. – Brian Rasmussen Sep 11 '13 at 23:42
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    @BrianRasmussen - I would say that yes, you should be able to rely on the fact that any class that disposes of unmanaged resources will have a finalizer, and if it doesn't, it's a bug, and falls under my "properly implemented" qualifier. StreamWriter has no unmanaged resources, so it's a non-issue in this regard. – Erik Funkenbusch Sep 11 '13 at 23:50
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.NET offers automatic garbage collection of memory only. It doesn't know anything about handles, so you have to clean those up yourself. That's what the IDisposable pattern and the finalizer are for.

Brian Rasmussen
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  • While this is true, he is not implementing IDisposable himself. It's already implemented by the class he's using. So your answer is more or less pointless as he already knows about it. He just doesn't understand why it's not being disposed of automatically when it goes out of scope, and why he has to call Dispose himself. – Erik Funkenbusch Sep 11 '13 at 17:32
  • @MystereMan He asks why .NET didn't dispose of the handles. The reason for this is that the GC is only concerned about memory. It doesn't know anything about the handles and thus cannot automatically dispose these. – Brian Rasmussen Sep 11 '13 at 17:41
  • Again, while true, it's irrelevant.. because the handles will get disposed when the object is garbage collected via the finalizer calling Dispose. The problem here has nothing whatsoever to do with .net knowing about any handles. Pretend, in a magic world, that .net was able to dispose the handles, the problem would still be the same because the gc would not have run. The class he's using already implements IDisposable, so telling him that he has to implement it is pointless. – Erik Funkenbusch Sep 11 '13 at 17:45
  • I'm not telling him he has to implement IDisposable. I'm telling him that the way .NET handles these resources is through the IDisposable pattern and the finalizer. – Brian Rasmussen Sep 11 '13 at 17:53
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The .NET runtime and its Garbage Collector will only dispose of an object if it's a managed object. Unmanaged objects must be disposed by you. That's because an unmanaged object can make use of resources which are not controlled by the runtime. If the runtime offed those objects itself, you'd have a lot of garbage in the system memory that no other process would be able to handle.

You might want to check this question out: What is meant by "managed" vs "unmanaged" resources in .NET?

Community
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Geeky Guy
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The garbage collector only responds to memory pressure. If you have other constrained resources, like GUI handles, you need to make sure you're handling them appropriately. The garbage collector makes no claim to deal with that for you.

That's precisely what IDisposable is for.

recursive
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0

There's a difference between garbage collection and the dispose method that you are calling in your code. The garbage collector will collect objects that no longer have any references to, but the collection is within the domain of the .NET runtime. If there are any native resources associated with an object, the garbage collector will have no way of cleaning those.

The IDisposable interface is designed to tackle this problem. The user of an IDisposable object is expected to call the Dispose method manually to give the object a chance to clean up after itself (any native resources it may use for example).

Take a look at the "using" statement in C# which will make the usage of IDisposable objects easier.

Some further reading:

IDisposable: http://msdn.microsoft.com/en-us/library/system.idisposable.aspx

Using statement: http://msdn.microsoft.com/en-us/library/yh598w02.aspx

Ameen
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.NET only knows about what memory .NET allocates. This means that if any code calls "unmanaged" code that allocates memory .NET has no idea that this memory exists.

This is why .Dispose() exists, so you can dispose of "unmanaged" memory ahead of time. Somebody who makes a .NET object that uses unmanaged memory should implement IDisposable.

If the unmanaged object is implemented correctly then it will also contain a "finalizer" ~ClassName() which "should" get called whenever the GC gets round to a big tidy-up. However you should never rely on the finalizers being called. If anything the finalizers are just there for clean up if a process if its abruptly closed or as a crutch for developers who don't understand the dispose pattern.

Quibblesome
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