Do you need to dispose of objects and set them to null?

Question

Do you need to dispose of objects and set them to null, or will the garbage collector clean them up when they go out of scope?

Solution 1

Objects will be cleaned up when they are no longer being used and when the garbage collector sees fit. Sometimes, you may need to set an object to null in order to make it go out of scope (such as a static field whose value you no longer need), but overall there is usually no need to set to null.

Regarding disposing objects, I agree with @Andre. If the object is IDisposable it is a good idea to dispose it when you no longer need it, especially if the object uses unmanaged resources. Not disposing unmanaged resources will lead to memory leaks.

You can use the using statement to automatically dispose an object once your program leaves the scope of the using statement.

using (MyIDisposableObject obj = new MyIDisposableObject())
{
    // use the object here
} // the object is disposed here

Which is functionally equivalent to:

MyIDisposableObject obj;
try
{
    obj = new MyIDisposableObject();
}
finally
{
    if (obj != null)
    {
        ((IDisposable)obj).Dispose();
    }
}

Solution 2

Objects never go out of scope in C# as they do in C++. They are dealt with by the Garbage Collector automatically when they are not used anymore. This is a more complicated approach than C++ where the scope of a variable is entirely deterministic. CLR garbage collector actively goes through all objects that have been created and works out if they are being used.

An object can go "out of scope" in one function but if its value is returned, then GC would look at whether or not the calling function holds onto the return value.

Setting object references to null is unnecessary as garbage collection works by working out which objects are being referenced by other objects.

In practice, you don't have to worry about destruction, it just works and it's great :)

Dispose must be called on all objects that implement IDisposable when you are finished working with them. Normally you would use a using block with those objects like so:

using (var ms = new MemoryStream()) {
  //...
}

EDIT On variable scope. Craig has asked whether the variable scope has any effect on the object lifetime. To properly explain that aspect of CLR, I'll need to explain a few concepts from C++ and C#.

Actual variable scope

In both languages the variable can only be used in the same scope as it was defined - class, function or a statement block enclosed by braces. The subtle difference, however, is that in C#, variables cannot be redefined in a nested block.

In C++, this is perfectly legal:

int iVal = 8;
//iVal == 8
if (iVal == 8){
    int iVal = 5;
    //iVal == 5
}
//iVal == 8

In C#, however you get a a compiler error:

int iVal = 8;
if(iVal == 8) {
    int iVal = 5; //error CS0136: A local variable named 'iVal' cannot be declared in this scope because it would give a different meaning to 'iVal', which is already used in a 'parent or current' scope to denote something else
}

This makes sense if you look at generated MSIL - all the variables used by the function are defined at the start of the function. Take a look at this function:

public static void Scope() {
    int iVal = 8;
    if(iVal == 8) {
        int iVal2 = 5;
    }
}

Below is the generated IL. Note that iVal2, which is defined inside the if block is actually defined at function level. Effectively this means that C# only has class and function level scope as far as variable lifetime is concerned.

.method public hidebysig static void  Scope() cil managed
{
  // Code size       19 (0x13)
  .maxstack  2
  .locals init ([0] int32 iVal,
           [1] int32 iVal2,
           [2] bool CS$4$0000)

//Function IL - omitted
} // end of method Test2::Scope

C++ scope and object lifetime

Whenever a C++ variable, allocated on the stack, goes out of scope it gets destructed. Remember that in C++ you can create objects on the stack or on the heap. When you create them on the stack, once execution leaves the scope, they get popped off the stack and gets destroyed.

if (true) {
  MyClass stackObj; //created on the stack
  MyClass heapObj = new MyClass(); //created on the heap
  obj.doSomething();
} //<-- stackObj is destroyed
//heapObj still lives

When C++ objects are created on the heap, they must be explicitly destroyed, otherwise it is a memory leak. No such problem with stack variables though.

C# Object Lifetime

In CLR, objects (i.e. reference types) are always created on the managed heap. This is further reinforced by object creation syntax. Consider this code snippet.

MyClass stackObj;

In C++ this would create an instance on MyClass on the stack and call its default constructor. In C# it would create a reference to class MyClass that doesn't point to anything. The only way to create an instance of a class is by using new operator:

MyClass stackObj = new MyClass();

In a way, C# objects are a lot like objects that are created using new syntax in C++ - they are created on the heap but unlike C++ objects, they are managed by the runtime, so you don't have to worry about destructing them.

Since the objects are always on the heap the fact that object references (i.e. pointers) go out of scope becomes moot. There are more factors involved in determining if an object is to be collected than simply presence of references to the object.

C# Object references

Jon Skeet compared object references in Java to pieces of string that are attached to the balloon, which is the object. Same analogy applies to C# object references. They simply point to a location of the heap that contains the object. Thus, setting it to null has no immediate effect on the object lifetime, the balloon continues to exist, until the GC "pops" it.

Continuing down the balloon analogy, it would seem logical that once the balloon has no strings attached to it, it can be destroyed. In fact this is exactly how reference counted objects work in non-managed languages. Except this approach doesn't work for circular references very well. Imagine two balloons that are attached together by a string but neither balloon has a string to anything else. Under simple ref counting rules, they both continue to exist, even though the whole balloon group is "orphaned".

.NET objects are a lot like helium balloons under a roof. When the roof opens (GC runs) - the unused balloons float away, even though there might be groups of balloons that are tethered together.

.NET GC uses a combination of generational GC and mark and sweep. Generational approach involves the runtime favouring to inspect objects that have been allocated most recently, as they are more likely to be unused and mark and sweep involves runtime going through the whole object graph and working out if there are object groups that are unused. This adequately deals with circular dependency problem.

Also, .NET GC runs on another thread(so called finalizer thread) as it has quite a bit to do and doing that on the main thread would interrupt your program.

Solution 3

As others have said you definitely want to call Dispose if the class implements IDisposable. I take a fairly rigid position on this. Some might claim that calling Dispose on DataSet, for example, is pointless because they disassembled it and saw that it did not do anything meaningful. But, I think there are fallacies abound in that argument.

Read this for an interesting debate by respected individuals on the subject. Then read my reasoning here why I think Jeffery Richter is in the wrong camp.

Now, on to whether or not you should set a reference to null. The answer is no. Let me illustrate my point with the following code.

public static void Main()
{
  Object a = new Object();
  Console.WriteLine("object created");
  DoSomething(a);
  Console.WriteLine("object used");
  a = null;
  Console.WriteLine("reference set to null");
}

So when do you think the object referenced by a is eligible for collection? If you said after the call to a = null then you are wrong. If you said after the Main method completes then you are also wrong. The correct answer is that it is eligible for collection sometime during the call to DoSomething. That is right. It is eligible before the reference is set to null and perhaps even before the call to DoSomething completes. That is because the JIT compiler can recognize when object references are no longer dereferenced even if they are still rooted.

Solution 4

You never need to set objects to null in C#. The compiler and runtime will take care of figuring out when they are no longer in scope.

Yes, you should dispose of objects that implement IDisposable.

Solution 5

If the object implements IDisposable, then yes, you should dispose it. The object could be hanging on to native resources (file handles, OS objects) that might not be freed immediately otherwise. This can lead to resource starvation, file-locking issues, and other subtle bugs that could otherwise be avoided.

See also Implementing a Dispose Method on MSDN.

View more solutions

Comments

[CJ7]

But won't the garabage collector call Dispose()? If so, why would you need to call it?

[Chris Schmich]

Unless you call it explicitly, there is no guarantee that Dispose will be called. Also, if your object is holding on to a scarce resource or is locking some resource (e.g. a file), then you'll want to free it up as soon as possible. Waiting for the GC to do that is suboptimal.

[Randy Levy]

If obj is a reference type then the finally block is equivalent to: if (obj != null) ((IDisposable)obj).Dispose();

[Zach Johnson]

@Tuzo: Thanks! Edited to reflect that.

[adrianm]

GC will never call Dispose(). GC might call a finalizer which by convention should clean up resources.

[CJ7]

@Igor: By going 'out of scope' I mean that the object reference is out of context and cannot be referred to in the current scope. Surely this still happens in C#.

[CJ7]

What do you mean about 'events and delegates' - what should be 'cleaned up' with these?

[Randy Levy]

@Craig Johnston, don't confuse variable scoping used by the compiler with variable lifetime which is determined by the runtime -- they are different. A local variable may not be "live" even though it is still in scope.

[CJ7]

@Tuzo: you will need to elaborate. The only kind of scoping I am talking about is the scoping that makes a variable local to a function go out of scope when the functions finishes. What is the other kind of scoping?

[Marnix van Valen]

@Craig - I edited my answer. Hopefully this clarifies it a bit.

[leppie]

@adrianm: Not might call, but will call.

[adrianm]

@leppie: finalizers are not deterministic and might not be called ( e.g. when the appdomain is unloaded). If you need deterministic finalization you must implement what i think is called a critical handler. The CLR has special handling of these objects to guarantee that they are finalized (e.g it pre-jits the finalization code to handle low memory)

[Steven Sudit]

If you have a long-lived (or even static) reference to a large object, you want to null it out as soon as you're done with it so that it's free to be reclaimed.

[Randy Levy]

@Craig Johnston: See blogs.msdn.com/b/ericgu/archive/2004/07/23/192842.aspx : "there is no guarantee that a local variable will remain live until the end of a scope if it isn't used. The runtime is free to analyze the code that it has and determine what there are no further usages of a variable beyond a certain point, and therefore not keep that variable live beyond that point (ie not treat it as a root for the purposes of GC)."

[Steven Sudit]

@adrianm: I've also found a few odd cases, such as locker objects, where Dispose does things that Finalize does not.

[Steven Sudit]

@Tuzo: True. That's what GC.KeepAlive is for.

[EMP]

If you're ever "done with it" it shouldn't be static. If it's not static, but "long-lived" then it should still go out of scope soon after you're done with it. The need to set references to null indicates a problem with the structure of the code.

[causa prima]

You can have a static item that you get done with. Consider: A static resource that's read from disk in a user-friendly format and then parsed into a format suitable for program use. You can end up with a private copy of the raw data that serves no further purpose. (Real world example: The parsing is a two-pass routine and thus can't simply process the data as it's read.)

[CJ7]

@Tuzo: Is the 'life' of a variable of any relevance to the programmer? It sounds like something that happens well after the programmer has written and compiled the code.

[EMP]

Then it should not store any raw data in a static field if it's only used temporarily. Sure, you can do that, it's just not good practice for exactly this reason: you then have to manage its lifetime manually.

[Randy Levy]

@Craig Johnston: No and Yes. No because the .NET runtime manages that for you and does a good job. Yes because the job of the programmer is not to write code that (just) compiles but to write code that runs. Sometimes it helps to know what the runtime is doing under the covers (e.g. troubleshooting). One could argue that it's the type of knowledge that helps to separate good programmers from great programmers.

[Igor Zevaka]

Well, just like the rest of the program, variable lifetime is inherently a run-time characteristic of a program. Yes it happens after the programmer wrote the code, but just like any code, the programmer has some control over the runtime behaviour.

[causa prima]

How in the world do you avoid it? You have an object that obtains raw data and processes it. The processed data persists for the life of the program. Unless you manually dispose of the raw data the object is still going to retain a reference to it. Whether the object is static or dynamic is irrelevant.

[EMP]

You avoid it by storing the raw data in a local variable in the method that processes it. The method returns the processed data, which you keep, but the local for the raw data goes out of scope when the method exits and is automatically GCed.

[Kevin P. Rice]

What if a is a private member field in a class? If a is not set to null the GC has no way of knowing if a will be used again in some method, right? Thus a will not be collected until the entire containing class is collected. No?

[Brian Gideon]

@Kevin: Correct. If a were a class member and the class containing a was still rooted and in-use then it too would hang around. That is one scenario where setting it to null could be beneficial.

[James]

"you never need to set objects to null in C#" - bit of a wild statement do you have evidence to back that up? If I have a lazily loaded/singleton object and I subsequently call Dispose on it there is the potential for me to attempt to use that object after it's been disposed as there is no guarantee that the GC will null out the field before it's used again.

[supercat]

Your point ties in with a reason why Dispose is important--it's not possible to invoke Dispose (or any other non-inlineable method) on an object without a rooted reference to it; calling Dispose after one is done using an object will ensure that a rooted reference will continue to exist throughout the duration of the last action performed on it. Abandoning all references to an object without calling Dispose may ironically have the effect of causing the object's resources to occasionally get released too early.

[Aidiakapi]

One remark regarding IDisposable. Failing to Dispose an object will generally not cause a memory leak on any well designed class. When working with unmanaged resources in C# you should have a finalizer that will still release the unmanaged resources. This means that instead of deallocating the resources when it should be done, it will be deferred to when the garbage collector finalizes the managed object. It can still cause many other issues though (such as unreleased locks). You should Dispose an IDisposable though!

[Basic]

@RandyLevy Do you have a reference for that? Thanks

[Mihail Georgescu]

But my question is Dispose() needs to implement any logic? Does it have to do something? Or internally when Dispose() is called signals GC that is good to go? I checked the source code for the TextWriter for example and Dispose has no implementation.

[Zach Johnson]

@Mihail I haven't been doing C# lately so I could be a bit rusty, but IIRC there's nothing special about Dispose and the garbage collector, it's just a pattern in C# to release resources before the garbage collector gets around to finalizing your object. If you have a finalizer, make sure to call GC.SuppressFinalize(this) inside dispose so GC doesn't have to do more work than necessary. See msdn.microsoft.com/library/b1yfkh5e(v=vs.100).aspx

[Mihail Georgescu]

@ZachJohnson thank you for your answer. Yes I know Microsoft's pattern , I know that the destructor aka finalizer is used to to get rid of unmanaged resoures , and Dispose() is used to do both using Microsoft's pattern, but I was wondering if we acually NEED to do something about the managed resources , we all know , if a reference is out of scope the object is good to go and GC will collect it at some time. I am a bit confused , do I have to nullify the references or something , does that even help at all? I am a bit confused. Thank you again.

[Zach Johnson]

@Mihail you shouldn't need to worry about managed resources, or nulling things out (unless they're being referenced directly or indirectly via a static field - a memory profiler can help out here)

[Mihail Georgescu]

@ZachJohnson makes sense! Thank you very much sir! Cheers!

[dathompson]

This example and explanation doesn't seem definitive on the hard suggestion to never set references to null. I mean, except for Kevin's comment, a reference set to null after it is disposed seems pretty benign, so what is the harm? Am I missing something?

[StayOnTarget]

@dathompson setting it to null twice would also be "benign" but that isn't a reason to do so. (ad absurdum...)

[variable]

Is there any code analyzer that goes through the code and warns when things that should have been disposed aren't disposed?