I have a static object that looks somewhat like this:
public static class MyStaticObject
{
public static void SomeMethod()
{
MyObject TheObject = new MyObject();
//some long runnning tasks
}
}
As you can see, when SomeMethod() runs, it creates a MyObject. What happens to this instantiated object after the method that created it returns? If SomeMethod() is called again while it's already executing in response to a previous method call, is there going to be a concurrency problem or does each method call instantiate its own MyObject?
TheObject is just a local variable, and it behaves the same way regardless of the fact if the method is static or not.
So after the method returns, the object (if there are no more references to it) is ready for the garbage collector.
And in case of recursion, just another local variable will be created.
If you have C/C++ background, the TheObject is not a static variable in C/C++ terms.
Related
In the following code:
public sealed class Switch
{
public static MyObj s_object = new MyObj();
private readonly SomeObject m_object = new SomeObject();
~Switch()
{
m_object?.Dispose();
}
}
public class Test()
{
Test()
{
Switch switch = new Switch();
switch = null;
...
}
}
When the Test ctor executes, a new Switch object is created then immediately set to null. At some point the GC will dispose of it, calling the ~Switch() destructor in the process. But will that happen when a class contains a static field like s_object and the calling app has not terminated (app domain still loaded)? Static objects persist for the lifetime of the application; does that mean the non-static class containing it will too?
This should be not a problem. Static fields are not related this way to the instance of the defining type in terms of memory representation.
Check this post for more detailed info: How exactly do static fields work internally?
Presence of static fields has no impact on timing when object will be garbage collected (and hence finalized). The instance will be finalized just fine at moment comparable to time it happens without static field.
The only impact static fields have on instances is that static initialization happens before first instance is created thus potentially making creation of first instance slower than the rest.
Note: code in the post shows invalid implementation of finalizer because it refers to other manged object and tries to call method on it. It results in undefined behavior for both cases (with/without static field).
Lets assume we have two class Foo and Bar as given below.
public class Foo
{
public static Bar BarInstance { get; set; }
public static void Main()
{
AssignBar("A");
AssignBar("B");
}
private static void AssignBar(string name)
{
BarInstance = new Bar(name);
}
}
public class Bar : IDisposable
{
public Bar(string name)
{
Name = name;
}
public string Name { get; set; }
protected virtual void Dispose(bool disposing)
{
if (!disposing)
{
return;
}
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
When object A gets replaced by object B. I expect Dispose to called because since there is no reference to Object A exists any more, but it doesn't get called. Could you please explain why?
Should I end up doing CurrentBar.Dispose like below if I want Object A to be disposed.
private static void AssignBar(string name)
{
if (BarInstance != null)
{
BarInstance.Dispose();
}
BarInstance = new Bar(name);
}
It depends on the ownership.
Is Bar given to Foo with the implication that Foo now owns Bar?
If so, then yes, you should call Dispose on the old before assigning a new instance.
Is Bar given to Foo with the implication that it's more of a loan, ie. you can reference and use this object, but it is owned by something else?
If so, then no, you should not call Dispose on the old before assigning a new instance.
Your code is not altogether clear on this point and I would suggest you clean that up, would make it easier to reason about this type in the future.
The way I see it you have 1 of 3 possible scenarios:
Foo always owns Bar. In this case I would put the disposal of the old instance in the setter of the property, or remove the public setter altogether if the only way to get a new Bar instance into it is through the method you have declared.
Foo never owns Bar. In this case I would not add any code that disposes of the old instance, but then I question the existence of that method. In this case it is likely nobody would really own the newly constructed instance.
Foo only owns Bar instances created through your method, but not instances given to it through the public setter. In this case I would add a new field that tracks whether Foo owns Bar or not, and set this to true in the method, and clear it in the property setter.
Dispose is not special in any way, from the C# standpoint it is just another method, as such there is nothing automatic that will call this method.
However, if the object references unmanaged resources, when the object at some point becomes eligible for collection, its finalizer will be executed releasing those unmanaged resources. However, to deterministically release them when you know they are no longer needed, you need to call Dispose.
Dispose is no special method. It will not be called automatically. You'll have to do it.
private static void AssignBar(string name)
{
if(BarInstance!=null)
{
BarInstance.Dispose();
}
BarInstance = new Bar(name);
}
I think you are confusing with finalizers and IDisposable. Finalizers are called at the time of garbage collection(not reliable though). If you have implemented DisposablePattern, then you'll have a Dispose(bool) method which is called from finalizer. Where finalizers are called by GC automatially during garbage collection.
Read here for more info
The runtime won't automatically call Dispose for you, it's your responsibility to ensure it's called. Your object will be garbage collected, but this only ensures that memory is reclaimed. It's up to use to release resources.
You can do this using either a using statement:
using(var x = new BarInstance())
{
// Whatever
}
or you can call it yourself. In your example, since you're replacing the instance you'll need to call Dispose, otherwise you risk a resource leak.
You could opt to use a finalizer in your class, ~Bar. In the finalizer you can dispose of any resources you're holding. However, when the finalizer will be called in non-deterministic, and it's even possible that it will never be called if the garbage collector doesn't kick in.
I have a static container class that holds a handle to some class A:
public static class Container
{
private static A _a;
public static void Register(A a) { _a = a; }
public static void Run() { _a.DoIt(); }
}
Registration of the container A instance is performed in the A constructor:
public class A
{
public A() { Container.Register(this); }
public void DoIt() { Console.WriteLine("Running!"); }
}
Now, let's say that I register my A instance by calling a method that only contains an A instantiation:
public void Init() { var a = new A(); }
Theoretically, could compilation be optimized to ignore this assignment, or can I be 100% sure that A is always instantiated when I call the Init method?
Example When I run the following code:
Init();
...
Container.Run();
will Container._a always be defined and the output from the DoIt method written to the console?
The compiler doesn't in general know if the constructor of A has observable side-effects, so it will always call it. It may not keep the variable 'a' around though.
So, the constructor will be called, but the result may not be assigned to the variable; instead the A object may just be immediately registered for garbage collection if nothing else references it. (In your case, something else DOES reference it - namely, the Container class - so it WON'T be garbage-collected!)
In your case, the constructor manifestly DOES have side-effects in any case (so it would be a major error for the compiler to optimise away the constructor call).
In summary:
The constructor will always be called.
The assignment of the result to the local variable may not be done because the compiler knows that it has no observable side-effects.
In your code, something else retains a reference to the constructed object, so it won't be GCed.
Another question on SO inspired me to try this code in C#:
class Program
{
static Program()
{
new Program().Run();
}
static void Main(string[] args) { }
void Run()
{
System.Console.WriteLine("Running");
}
}
This prints "Running" when run.
I actually expected the compiler to complain about this. After all, if the class has not yet been initialized by the static constructor; how can we be sure that it is valid to call methods on it ?
So why does the compiler not restrict us from doing this ? Is there any important usage scenarios for this ?
Edit
I am aware of the Singleton pattern; the point in question is why I can call a method on the instance before my static constructor finishes. So far JaredPar's answer has some good reasoning about this.
It is allowed because not allowing it would be a lot worse. Code like this would deadlock badly:
class A {
public static readonly A a;
public static readonly B b;
static A() {
b = new B();
a = B.a;
}
}
class B {
public static readonly A a;
public static readonly B b;
static B() {
a = new A();
b = A.b;
}
}
You are of course pointing a loaded gun at your foot.
This behavior is documented in the CLI Spec (Ecma 335) Partition II, Chapter 10.5.3.2 "Relaxed guarantees":
A type can be marked with the attribute beforefieldinit (§10.1.6) to indicate that the guarantees specified in §10.5.3.1 are not necessarily required. In particular, the final requirement above need not be provided: the type initializer need not be executed before a static method is called or referenced.
[Rationale: When code can be executed in multiple application domains it becomes particularly expensive to ensure this final guarantee. At the same time, examination of large bodies of managed code have shown that this final guarantee is rarely required, since type initializers are almost always simple methods for initializing
static fields. Leaving it up to the CIL generator (and hence, possibly, to the programmer) to decide whether this guarantee is required therefore provides efficiency when it is desired at the cost of consistency guarantees.
end rationale]
The C# compiler indeed emits the beforefieldinit attribute on a class:
.class private auto ansi beforefieldinit ConsoleApplication2.Program
extends [mscorlib]System.Object
{
// etc...
}
Slightly different question.
How would the compiler prevent you from doing this?
Sure it's very easy to detect in your sample, but what about this sample?
class Program {
static void Fun() {
new Program();
}
static Program() {
Fun();
}
}
The ways in which you can trick the compiler to allow this are virtually endless. Even if the compiler got all of the answers you could still beat it with reflection.
In the end though this is actually legal, if a bit dangerous, code in both C# and IL. It is safe to do this as long as you are careful about accessing static's from within this code. It also is helpful / possibly necessary for certain patterns like Singleton's
Static constructor can initialize only static class members, this is not related to class instances and regular non-static class members.
What you may not realize is that for every class that does not have a non-static constructor, the compiler will generate one. This is different from your static constructor, which when you boil it down into MSIL is little more than a flag telling the CLR "Hey, run this code before you run what's in main()". So, the code of your static constructor is executed first. It instantiates a locally-scoped Program object using the NON-static constructor generated behind the scenes, and once instantiated, Run() is called on the object. Then, because you haven't stored this new object anywhere, it is disposed of when the constructor finishes executing. The main() function then runs (and does nothing).
Try this expansion:
class Program
{
static Program()
{
new Program().Run();
}
public Program()
{
Console.WriteLine("Instantiating a Program");
}
public override void Finalize()
{
Console.WriteLine("Finalizing a Program");
}
static void Main(string[] args) { Console.WriteLine("main() called"); }
void Run()
{
System.Console.WriteLine("Running");
}
}
See what the output is. My guess is that it will look something like this:
Instantiating a Program
Running
Finalizing a Program
main() called
The last two lines may be swapped because garbage collection may not get the chance to destroy the instance before main starts running (GC runs in a seperate managed thread, and so it works on its own time within the lifetime of the process), but the instance IS local to the static constructor in scope, and so is marked for collection before main() starts running. So, if you called Thread.Sleep(1000) in main() before printing the message, GC should collect the object in that time.
How does C#, or other languages for that matter, handle memory allocation (and memory de-allocation) between these two scenarios:
1.) A method on a static class is invoked.
public Program {
Foo foo = Loader.load();
}
public static Loader {
public static Foo load() {
return new Foo();
}
}
2.) A method is invoked on an instance, which then falls out of scope.
public Program {
Foo foo = new Loader().load();
}
public Loader {
public Foo load() {
return new Foo();
}
}
I suppose the static class is loaded, and remains, in memory; whereas the class instance succumbs to garbage collection at C#'s leisure. Are there any pros or cons to these two paradigms? Is there ever a time when you have a class that never needs to be instantiated (i.e. some sort of resource loader or factory), but you use the second methodology anyway to take advantage of garbage collection?
The important part of my question is whether or not the first paradigm, while being conceptually correct in some circumstances, may suffer from holding on to memory unnecessarily.
Your second example doesn't work, so let's explore the real options:
1.) A method on a static class is invoked.
public Program {
Foo foo = Loader.Load();
}
public static Loader {
public static Foo Load() {
return new Foo();
}
}
2.) A static method in a non-static class is invoked.
public Program {
Foo foo = Loader.Load();
}
public Loader {
public static Foo Load() {
return new Foo();
}
}
3.) An instance method is invoked on an instance
public Program {
Foo foo = new Loader().Load();
}
public Loader {
public Foo Load() {
return new Foo();
}
}
The two first are the same. Calling a static method is the same regardless if the class is static or not.
The third option will create an instance of the class on the heap. As the class has no data members, it will only be something like 16 bytes. It will be garbage collected eventually, but due to the small size it doesn't matter much when that happens.
Calling an instance method is also slightly different from a static method. A reference to the class instance is sent along, that you can access through the this keyword. It makes little difference in this case as there is no real data in the object to access.
The second form creates a temporary Loader object (which is very cheap). You will always have to load the Loader class, no matter which approach you choose.
There is very little performance (memory saving) to gain here. You would normally choose for a static member in a static class if there is no 'state' needed outside the methods local vars.
A static method, field, property, or event is callable on a class even when no instance of the class has been created.
http://msdn.microsoft.com/en-us/library/79b3xss3(VS.80).aspx
So in that sense your static methods behaves just as it would if you used it from within a class instance: it is scoped to the type.
I cannot find any sources for this, but from my knowledge of programming, when you refernce a class(non static), it's structure is loaded into memory
Creating an instance of a class just to call a method, would waste a lot of processing power(due to creating an instance, assigning it memory, and the garbage collecting).
Instead of keeping the definition, and then on top of it, an instance. Why not just keep the definition(static).
As long as you don't store any data in static variables, your static method should take up the same amount of memory as your non static method definition. But using a static method, only the method will be kept in memory and be ready to be called whenever you need without creating instances. Where as, if the method is non static, it will need to be instantiated(using up memory and processing power) and the garbage collected(freeing memory and using up cpu) therefore it is definitely better using a static member. Thats what they are there for.