Problem
Do you need to get rid of objects and set them to null, or will the garbage collector take care of it when they’re no longer needed?
Asked by CJ7
Solution #1
When objects are no longer in use and the garbage collector sees fit, they will be cleaned up. You may need to set an object to null to make it go out of scope (for example, a static field whose value you no longer want), but setting to null is rarely necessary.
In terms of item disposal, I agree with @Andre. It’s a good practice to dispose of IDisposable objects after you’re done with them, especially if they require unmanaged resources. Memory leaks will occur if unmanaged resources are not properly disposed of.
When your application exits the scope of the using statement, you can use the using statement to automatically dispose of an object.
using (MyIDisposableObject obj = new MyIDisposableObject())
{
// use the object here
} // the object is disposed here
Which is equal in terms of functionality to:
MyIDisposableObject obj;
try
{
obj = new MyIDisposableObject();
}
finally
{
if (obj != null)
{
((IDisposable)obj).Dispose();
}
}
Answered by Zach Johnson
Solution #2
Objects in C# do not go out of scope like they do in C++. When they are no longer in use, the Garbage Collector automatically disposes of them. This is a more complex method than C++, where a variable’s scope is completely deterministic. The CLR trash collector actively examines all newly generated objects to see if they are being used.
In one function, an object can fall “out of scope,” but if its value is returned, the GC will check whether the calling function keeps the return value.
Garbage collection works by determining which objects are referred by other objects, so setting object references to null isn’t necessary.
In practice, there is no need to be concerned about damage; it simply works and is fantastic:)
When you’re done working with an object that implements IDisposable, you must call dispose on it. With those objects, you’d normally use a using block like this:
using (var ms = new MemoryStream()) {
//...
}
EDIT: I’ve made a change to the variable scope. Craig inquired about the effect of variable scope on object lifespan. I’ll need to discuss a few C++ and C# concepts to adequately describe that component of CLR.
The variable can only be used in the same scope as it was defined in both languages – a class, function, or a statement block surrounded by braces. The only distinction is that variables in C# cannot be redefined in nested blocks.
This is fully legal in C++:
int iVal = 8;
//iVal == 8
if (iVal == 8){
int iVal = 5;
//iVal == 5
}
//iVal == 8
However, in C#, you’ll get the following 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
}
When you look at the resulting MSIL, you’ll notice that all of the variables utilized by the function are defined at the beginning. Take a peek at the following feature:
public static void Scope() {
int iVal = 8;
if(iVal == 8) {
int iVal2 = 5;
}
}
The produced IL is shown below. It’s worth noting that the variable iVal2, which is defined within the if block, is actually defined at the function level. In terms of variable lifetime, this effectively means that C# only has class and function level scope.
.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
Whenever a stack-allocated C++ variable passes out of sight, it is destructed. Keep in mind that you can build things on the stack or the heap in C++. When you build them on the stack, they are popped off the stack and destroyed whenever execution exits the scope.
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 deleted, or else a memory leak will occur. Stack variables, on the other hand, are not a concern.
Objects (reference types) are always produced on the managed heap in the CLR. This is reinforced even further by the object creation syntax. Take a look at the following code snippet.
MyClass stackObj;
This would build a MyClass object on the stack and call its default constructor in C++. It would create a reference to class MyClass in C#, but it would point to nothing. The only way to create a class instance is to use the new operator:
MyClass stackObj = new MyClass();
C# objects are similar to C++ new syntax objects in that they are created on the heap, but unlike C++ objects, they are handled by the runtime, so you don’t have to worry about destructing them.
The fact that object references (i.e. pointers) move out of scope is irrelevant because the objects are constantly on the heap. There are additional elements to consider than the presence of references to an object when deciding whether or not it should be collected.
Simply point to a heap location that contains the object. As a result, changing it to null has no immediate impact on the object’s lifetime; the balloon will continue to exist until the GC “pops” it.
Continuing with the balloon analogy, it would seem obvious that the balloon can be destroyed if the strings are removed. In non-managed languages, reference counted objects work exactly like this. Except that this method fails miserably when dealing with circular references. Consider two balloons connected by a string, but neither balloon has a string attached to anything else. Even though the entire balloon group is “orphaned,” they both continue to exist under simple ref counting rules.
.NET objects are similar to helium balloons hidden behind a ceiling. Even though there are groups of balloons linked together, when the ceiling opens (GC runs), the unused balloons float away.
.NET GC employs a hybrid of generational and mark-and-sweep GC. The generational technique favours inspecting objects that have been allocated most recently, as they are more likely to be underutilised, while the mark and sweep approach entails the runtime running over the entire object graph and determining whether there are any unused object groups. This effectively solves the problem of circular dependency.
.NET GC also runs on a separate thread (the finalizer thread) because it has a lot of work to do, and doing so on the main thread might cause your program to crash.
Answered by Igor Zevaka
Solution #3
If the class implements IDisposable, you should definitely use Dispose, as others have suggested. On this, I take a rather firm stance. Some may argue that calling Dispose on a DataSet is unnecessary because they dismantled it and discovered that it accomplished nothing useful. However, I believe that argument has numerous errors.
This article has an interesting debate about the subject amongst well-known people. Then read why I believe Jeffery Richter is in the wrong camp here.
Now it’s up to you to decide whether or not to set a reference to null. No, that is not the case. Let me demonstrate my point with the code below.
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 suppose the object referred to by an is ready to be collected? You are incorrect if you mentioned after the call to a = null. You’re also incorrect if you said after the Main procedure completes. The correct answer is that it is collectible at any point during the DoSomething call. That’s correct. It’s eligible before the reference is set to null, and maybe even before the DoSomething call is finished. This is because, even though object references are still rooted, the JIT compiler may detect when they are no longer dereferenced.
Answered by Brian Gideon
Solution #4
In C#, there’s no need to set objects to null. When they are no longer in scope, the compiler and runtime will figure it out.
Objects that implement IDisposable should be disposed away.
Answered by EMP
Solution #5
Yes, you should dispose of the object if it implements IDisposable. The item may be clinging on native resources (file handles, OS objects) that would otherwise not be released. This can result in resource depletion, file locking issues, and other minor faults that could have been prevented otherwise.
On MSDN, see Implementing a Dispose Method.
Answered by Chris Schmich
Post is based on https://stackoverflow.com/questions/2926869/do-you-need-to-dispose-of-objects-and-set-them-to-null