I have the following setup in an aspx page:
Object Original = obj;
System.Threading.Thread thread = new System.Threading.Thread(() => saveOriginalDetails(Original));
thread.Start();
The basic idea of this is that I have an object, and I want to save it exactly how it is before making any changes to it.
So I make a copy of the original object obj and store it as Original
I am starting a new thread because the saveOriginalDetails method is slowing the code down too much.
My question is, if I do this instead:
System.Threading.Thread thread = new System.Threading.Thread(() => saveOriginalDetails(obj));
thread.Start();
obj.name = "NewName";
Where I am now passing in the original object, and copy it inside the method that is running concurrently, like this:
private void saveOriginalDetails(object applicant)
{
object OriginalApplicant = applicant;
.....
}
Will the object passed in to the method:
saveOriginalDetails(obj));
Have the updated name value eg a name of newName ?
First, don't use Thread. Use the new Task classes instead (if possible - you didn't specify which .NET version you are using).
Secondly, you're only passing a parameter to the saveOriginalDetails.
Lastly, if your class is a model class (it sort of looks this way) and is serializable, you can relatively quickly create a perfect copy by serializing it and deserializing it (which has the benefit of working with any future changes you might make to your class). A faster-working solution (which, however, would require more actual programming work) would be to write your own code for cloning your class. That said, unless your class is really really large and complex, serializing it and deserializing it, while not the most optimal solution, should be fast enough.
Finally, unless you have an actual business need to store a copy of the DuoApplicant object, an in-memory copy, as described above, should suffice.
Related
I am tasked with writing a system to process result files created by a different process(which I have no control over) and and trying to modify my code to make use of Parallel.Foreach. The code works fine when just calling a foreach but I have some concerns about thread safety when using the parallel version. The base question I need answered here is "Is the way I am doing this going to guarantee thread safety?" or is this going to cause everything to go sideways on me.
I have tried to make sure all calls are to instances and have removed every static anything except the initial static void Main. It is my current understanding that this will do alot towards assuring thread safety.
I have basically the following, edited for brevity
static void Main(string[] args)
{
MyProcess process = new MyProcess();
process.DoThings();
}
And then in the actual process to do stuff I have
public class MyProcess
{
public void DoThings()
{
//Get some list of things
List<Thing> things = getThings();
Parallel.Foreach(things, item => {
//based on some criteria, take actions from MyActionClass
MyActionClass myAct = new MyActionClass(item);
string tempstring = myAct.DoOneThing();
if(somecondition)
{
MyAct.DoOtherThing();
}
...other similar calls to myAct below here
};
}
}
And over in the MyActionClass I have something like the following:
public class MyActionClass
{
private Thing _thing;
public MyActionClass(Thing item)
{
_thing = item;
}
public string DoOneThing()
{
return _thing.GetSubThings().FirstOrDefault();
}
public void DoOtherThing()
{
_thing.property1 = "Somenewvalue";
}
}
If I can explain this any better I'll try, but I think that's the basics of my needs
EDIT:
Something else I just noticed. If I change the value of a property of the item I'm working with while inside the Parallel.Foreach (in this case, a string value that gets written to a database inside the loop), will that have any affect on the rest of the loop iterations or just the one I'm on? Would it be better to create a new instance of Thing inside the loop to store the item i'm working with in this case?
There is no shared mutable state between actions in the Parallel.ForEach that I can see, so it should be thread-safe, because at most one thread can touch one object at a time.
But as it has been mentioned there is nothing shared that can be seen. It doesn't mean that in the actual code you use everything is as good as it seems here.
Or that nothing will be changed by you or your coworker that will make some state both shared and mutable (in the Thing, for example), and now you start getting difficult to reproduce crashes at best or just plain wrong behaviour at worst that can be left undetected for a long time.
So, perhaps you should try to go fully immutable near threading code?
Perhaps.
Immutability is good, but it is not a silver bullet, and it is not always easy to use and implement, or that every task can be reasonably expressed through immutable objects. And even that accidental "make shared and mutable" change may happen to it as well, though much less likely.
It should at least be considered as a possible option/alternative.
About the EDIT
If I change the value of a property of the item I'm working with while
inside the Parallel.Foreach (in this case, a string value that gets
written to a database inside the loop), will that have any affect on
the rest of the loop iterations or just the one I'm on?
If you change a property and that object is not used anywhere else, and it doesn't rely on some global mutable state (for example, sort of a public static Int32 ChangesCount that increments with each state change), then you should be safe.
a string value that gets written to a database inside the loop - depending on the used data access technology and how you use it, you may be in trouble, because most of them are not designed for multithreaded environment, like EF DbContext, for example. And obviously do not forget that dealing with concurrent access in database is not always easy, though that is a bit away from our original theme.
Would it be better to create a new instance of Thing inside the loop to store the item i'm working with in this case - if there is no risk of external concurrent changes, then it is just an unnecessary work. And if there is a chance of another threads(not Parallel.For) making changes to those objects that are being persisted, then you already have bigger problems than Parallel.For.
Objects should always have observable consistent state (unlike when half of properties set by one thread, and half by another, while you try to persist that who-knows-what), and if they are used by many threads, then they should be already thread-safe - there should be no way to put them into inconsistent state.
And if they want to be persisted by external code, such objects should probably provide:
Either SyncRoot property to synchronize property reading code.
Or some current state snapshot DTO that is created internally by some thread-safe method like ThingSnapshot Thing.GetCurrentData() { lock() {} }.
Or something more exotic.
I have a doubt with the objects declarations in c#. I explain with this example
I can do this:
MyObject obj = New MyObject();
int a = obj.getInt();
Or I can do this
int a = new MyObject().getInt();
The result are the same, but, exists any diferences between this declarations? (without the syntax)
Thanks.
This isn't a declararation: it's a class instantiation.
There's no practical difference: it's all about readability and your own coding style.
I would add that there're few cases where you will need to declare reference to some object: when these objects are IDisposable.
For example:
// WRONG! Underlying stream may still be locked after reading to the end....
new StreamReader(...).ReadToEnd();
// OK! Store the whole instance in a reference so you can dispose it when you
// don't need it anymore.
using(StreamReader r = new StreamReader(...))
{
} // This will call r.Dispose() automatically
As some comment has added, there're a lot of edge cases where instantiating a class and storing the object in a reference (a variable) will be better/optimal, but about your simple sample, I believe the difference isn't enough and it's still a coding style/readability issue.
It's mostly syntax.
The main difference is that you can't use the instance of MyObject in the second example. Also, it may be nominated for Garbage Collection immediately.
No, technically they are the same.
The only thing I would suggest to consider in this case, as if the function does not actual need of instance creation, you may consider declare it static, so you can simply call it like:
int a = MyObject.getInt();
but this naturally depends on concrete implementation.
I am thinking about creating a persistent collection (lists or other) in C#, but I can't figure out a good API.
I use 'persistent' in the Clojure sense: a persistent list is a list that behaves as if it has value semantics instead of reference semantics, but does not incur the overhead of copying large value types. Persistent collections use copy-on-write to share internal structure. Pseudocode:
l1 = PersistentList()
l1.add("foo")
l1.add("bar")
l2 = l1
l1.add("baz")
print(l1) # ==> ["foo", "bar", "baz"]
print(l2) # ==> ["foo", "bar"]
# l1 and l2 share a common structure of ["foo", "bar"] to save memory
Clojure uses such datastructures, but additionally in Clojure all data structures are immutable. There is some overhead in doing all the copy-on-write stuff so Clojure provides a workaround in the form of transient datastructures that you can use if you are sure you're not sharing the datastructure with anyone else. If you have the only reference to a datastructure, why not mutate it directly instead of going through all the copy-on-write overhead.
One way to get this efficiency gain would be to keep a reference count on your datastructure (though I don't think Clojure works that way). If the refcount is 1, you're holding the only reference so do the updates destructively. If the refcount is higher, someone else is also holding a reference to it that's supposed to behave like a value type, so do copy-on-write to not disturb the other referrers.
In the API to such a datastructure, one could expose the refcounting, which makes the API seriously less usable, or one could not do the refcounting, leading to unnecessary copy-on-write overhead if every operation is COW'ed, or the API loses it's value type behaviour and the user has to manage when to do COW manually.
If C# had copy constructors for structs, this would be possible. One could define a struct containing a reference to the real datastructure, and do all the incref()/decref() calls in the copy constructor and destructor of the struct.
Is there a way to do something like reference counting or struct copy constructors automatically in C#, without bothering the API users?
Edit:
Just to be clear, I'm just asking about the API. Clojure already has an implementation of this written in Java.
It is certainly possible to make such an interface by using a struct with a reference to the real collection that is COW'ed on every operation. The use of refcounting would be an optimisation to avoid unnecessary COWing, but apparently isn't possible with a sane API.
What you're looking to do isn't possible, strictly speaking. You could get close by using static functions that do the reference counting, but I understand that that isn't a terrible palatable option.
Even if it were possible, I would stay away from this. While the semantics you describe may well be useful in Clojure, this cross between value type and reference type semantics will be confusing to most C# developers (mutable value types--or types with value type semantics that are mutable--are also usually considered Evil).
You may use the WeakReference class as an alternative to refcounting and achieve some of the benefits that refcounting gives you. When you hold the only copy to an object in a WeakReference, it will be garbage collected. WeakReference has some hooks for you to inspect whether that's been the case.
EDIT 3: While this approach does do the trick I'd urge you to stay away from persuing value semantics on C# collections. Users of your structure do not expect this kind of behavior on the platform. These semantics add confusion and the potential for mistakes.
EDIT 2: Added an example. #AdamRobinson: I'm afraid I was not clear how WeakReference can be of use. I must warn that performancewise, most of the time it might be even worse than doing a naive Copy-On-Write at every operation. This is due to the Garbage Collector call. Therefore this is merely an academic solution, and I cannot recommend it's use in production systems. It does do exactly what you ask however.
class Program
{
static void Main(string[] args)
{
var l1 = default(COWList);
l1.Add("foo"); // initialize
l1.Add("bar"); // no copy
l1.Add("baz"); // no copy
var l2 = l1;
l1.RemoveAt(0); // copy
l2.Add("foobar"); // no copy
l1.Add("barfoo"); // no copy
l2.RemoveAt(1); // no copy
var l3 = l2;
l3.RemoveAt(1); // copy
Trace.WriteLine(l1.ToString()); // bar baz barfoo
Trace.WriteLine(l2.ToString()); // foo baz foobar
Trace.WriteLine(l3.ToString()); // foo foobar
}
}
struct COWList
{
List<string> theList; // Contains the actual data
object dummy; // helper variable to facilitate detection of copies of this struct instance.
WeakReference weakDummy; // helper variable to facilitate detection of copies of this struct instance.
/// <summary>
/// Check whether this COWList has already been constructed properly.
/// </summary>
/// <returns>true when this COWList has already been initialized.</returns>
bool EnsureInitialization()
{
if (theList == null)
{
theList = new List<string>();
dummy = new object();
weakDummy = new WeakReference(dummy);
return false;
}
else
{
return true;
}
}
void EnsureUniqueness()
{
if (EnsureInitialization())
{
// If the COWList has been copied, removing the 'dummy' reference will not kill weakDummy because the copy retains a reference.
dummy = new object();
GC.Collect(2); // OUCH! This is expensive. You may replace it with GC.Collect(0), but that will cause spurious Copy-On-Write behaviour.
if (weakDummy.IsAlive) // I don't know if the GC guarantees detection of all GC'able objects, so there might be cases in which the weakDummy is still considered to be alive.
{
// At this point there is probably a copy.
// To be safe, do the expensive Copy-On-Write
theList = new List<string>(theList);
// Prepare for the next modification
weakDummy = new WeakReference(dummy);
Trace.WriteLine("Made copy.");
}
else
{
// At this point it is guaranteed there is no copy.
weakDummy.Target = dummy;
Trace.WriteLine("No copy made.");
}
}
else
{
Trace.WriteLine("Initialized an instance.");
}
}
public void Add(string val)
{
EnsureUniqueness();
theList.Add(val);
}
public void RemoveAt(int index)
{
EnsureUniqueness();
theList.RemoveAt(index);
}
public override string ToString()
{
if (theList == null)
{
return "Uninitialized COWList";
}
else
{
var sb = new StringBuilder("[ ");
foreach (var item in theList)
{
sb.Append("\"").Append(item).Append("\" ");
}
sb.Append("]");
return sb.ToString();
}
}
}
This outputs:
Initialized an instance.
No copy made.
No copy made.
Made copy.
No copy made.
No copy made.
No copy made.
Made copy.
[ "bar" "baz" "barfoo" ]
[ "foo" "baz" "foobar" ]
[ "foo" "foobar" ]
I read what you're asking for, and I'm thinking of a "terminal-server"-type API structure.
First, define an internal, thread-safe singleton class that will be your "server"; it actually holds the data you're looking at. It will expose a Get and Set method that will take the string of the value being set or gotten, controlled by a ReaderWriterLock to ensure that the value can be read by anyone, but not while anyone's writing and only one person can write at a time.
Then, provide a factory for a class that is your "terminal"; this class will be public, and contains a reference to the internal singleton (which otherwise cannot be seen). It will contain properties that are really just pass-throughs for the singleton instance. In this way, you can provide a large number of "terminals" that will all see the same data from the "server", and will be able to modify that data in a thread-safe way.
You could use copy constructors and a list of the values accessed by each instance to provide copy-type knowledge. You can also mashup the value names with the object's handle to support cases where L1 and L2 share an A, but L3 has a different A because it was declared seperately. Or, L3 can get the same A that L1 and L2 have. However you structure this, I would very clearly document how it should be expected to behave, because this is NOT the way things behave in basic .NET.
I'd like to have something like this on a flexible tree collection object of mine, though it wouldn't be by using value-type semantics (which would be essentially impossible in .net) but by having a clone generate a "virtual" deep clone instead of actually cloning every node within the collection. Instead of trying to keep an accurate reference count, every internal node would have three states:
Flexible
SharedImmutable
UnsharedMutable
Calling Clone() on a sharedImmutable node would simply yield the original object; calling Clone on a Flexible node would turn it into a SharedImmutable one. Calling Clone on an unshared mutable node would create a new node holding clones of all its descendents; the new object would be Flexible.
Before an object could be written, it would have to be made UnsharedMutable. To make an object UnsharedMutable if it isn't already, make its parent (the node via which it was accessed) UnsharedMutable (recursively). Then if the object was SharedImmutable, clone it (using a ForceClone method) and update the parent's link to point to the new object. Finally, set the new object's state to UnsharedMutable.
An essential aspect of this technique would be having separate classes for holding the data and providing the interface to it. A statement like MyCollection["this"]["that"]["theOther"].Add("George")needs to be evaluated by having the indexing operations return an indexer class which holds a reference to MyCollection. At that point, the "Add" method could then be able to act upon whatever intermediate nodes it had to in order to perform any necessary copy-on-write operations.
At first I assume I do need writerlock here but Im not sure (not much experience with that) what if I dont use it.
On the server side, there are client classes for each connected client. Each class contains public list which every other class can write to. Client requests are processed via threadpool workitems.
class client
{
public List <string> A;
someEventRaisedMethod(param)
{
client OtherClient=GetClientByID(param) //selects client class by ID sent by msg sender
OtherCLient.A.Add("blah");
}
}
What if two instances reference the same client and both try OtherCLient.A.Add("blah")? Isnt be here some writer lock? It works for me but I encounter some strange issues that I think are due to this.
Thank you!
(update: as always, Eric Lippert has a timely blog entry)
If you don't use a lock, you risk either missing data, state corruption, and probably the odd Exception - but only very occasionally, so very hard to debug.
Absolutely you need to synchronize here. I would expose a lock on the client (so we can span multiple operations):
lock(otherClient.LockObject) {
otherClient.A.Add("blah");
}
You could make a synchronous Add method on otherClient, but it is often useful to span multiple - perhaps to check Contains and then Add only if missing, etc.
Just to clarify 2 points:
all access to the list (even reads) must also take the lock; otherwise it doesn't work
the LockObject should be a readonly reference-type
for the second, perhaps:
private readonly object lockObject = new object();
public object LockObject {get {return lockObject;}}
From my point of view you should do the following:
Isolate the list into a separate class which implements either the IList Interface or only the subset which you require
Either add locking on a private object in the methods of your list class or use the ReaderWriterSlim implementation - as it is isolated there is only one place needed for changing in one single class
I don't know the C# internals, but I do remember reading awhile back about java example that could cause a thread to endlessly loop if it was reading a collection whilst an insert was being done on the collection (I think it was a hashtable), so make sure if you are using multiple threads that you lock on both read and write. Marc Gravell is correct that you should just create a global lock to handle this since it sounds like you have fairly low volume.
ReaderWriterLockSlim is also a good option if you do alot of reading and only a few write / update actions.
Short Version
For those who don't have the time to read my reasoning for this question below:
Is there any way to enforce a policy of "new objects only" or "existing objects only" for a method's parameters?
Long Version
There are plenty of methods which take objects as parameters, and it doesn't matter whether the method has the object "all to itself" or not. For instance:
var people = new List<Person>();
Person bob = new Person("Bob");
people.Add(bob);
people.Add(new Person("Larry"));
Here the List<Person>.Add method has taken an "existing" Person (Bob) as well as a "new" Person (Larry), and the list contains both items. Bob can be accessed as either bob or people[0]. Larry can be accessed as people[1] and, if desired, cached and accessed as larry (or whatever) thereafter.
OK, fine. But sometimes a method really shouldn't be passed a new object. Take, for example, Array.Sort<T>. The following doesn't make a whole lot of sense:
Array.Sort<int>(new int[] {5, 6, 3, 7, 2, 1});
All the above code does is take a new array, sort it, and then forget it (as its reference count reaches zero after Array.Sort<int> exits and the sorted array will therefore be garbage collected, if I'm not mistaken). So Array.Sort<T> expects an "existing" array as its argument.
There are conceivably other methods which may expect "new" objects (though I would generally think that to have such an expectation would be a design mistake). An imperfect example would be this:
DataTable firstTable = myDataSet.Tables["FirstTable"];
DataTable secondTable = myDataSet.Tables["SecondTable"];
firstTable.Rows.Add(secondTable.Rows[0]);
As I said, this isn't a great example, since DataRowCollection.Add doesn't actually expect a new DataRow, exactly; but it does expect a DataRow that doesn't already belong to a DataTable. So the last line in the code above won't work; it needs to be:
firstTable.ImportRow(secondTable.Rows[0]);
Anyway, this is a lot of setup for my question, which is: is there any way to enforce a policy of "new objects only" or "existing objects only" for a method's parameters, either in its definition (perhaps by some custom attributes I'm not aware of) or within the method itself (perhaps by reflection, though I'd probably shy away from this even if it were available)?
If not, any interesting ideas as to how to possibly accomplish this would be more than welcome. For instance I suppose if there were some way to get the GC's reference count for a given object, you could tell right away at the start of a method whether you've received a new object or not (assuming you're dealing with reference types, of course--which is the only scenario to which this question is relevant anyway).
EDIT:
The longer version gets longer.
All right, suppose I have some method that I want to optionally accept a TextWriter to output its progress or what-have-you:
static void TryDoSomething(TextWriter output) {
// do something...
if (output != null)
output.WriteLine("Did something...");
// do something else...
if (output != null)
output.WriteLine("Did something else...");
// etc. etc.
if (output != null)
// do I call output.Close() or not?
}
static void TryDoSomething() {
TryDoSomething(null);
}
Now, let's consider two different ways I could call this method:
string path = GetFilePath();
using (StreamWriter writer = new StreamWriter(path)) {
TryDoSomething(writer);
// do more things with writer
}
OR:
TryDoSomething(new StreamWriter(path));
Hmm... it would seem that this poses a problem, doesn't it? I've constructed a StreamWriter, which implements IDisposable, but TryDoSomething isn't going to presume to know whether it has exclusive access to its output argument or not. So the object either gets disposed prematurely (in the first case), or doesn't get disposed at all (in the second case).
I'm not saying this would be a great design, necessarily. Perhaps Josh Stodola is right and this is just a bad idea from the start. Anyway, I asked the question mainly because I was just curious if such a thing were possible. Looks like the answer is: not really.
No, basically.
There's really no difference between:
var x = new ...;
Foo(x);
and
Foo(new ...);
and indeed sometimes you might convert between the two for debugging purposes.
Note that in the DataRow/DataTable example, there's an alternative approach though - that DataRow can know its parent as part of its state. That's not the same thing as being "new" or not - you could have a "detach" operation for example. Defining conditions in terms of the genuine hard-and-fast state of the object makes a lot more sense than woolly terms such as "new".
Yes, there is a way to do this.
Sort of.
If you make your parameter a ref parameter, you'll have to have an existing variable as your argument. You can't do something like this:
DoSomething(ref new Customer());
If you do, you'll get the error "A ref or out argument must be an assignable variable."
Of course, using ref has other implications. However, if you're the one writing the method, you don't need to worry about them. As long as you don't reassign the ref parameter inside the method, it won't make any difference whether you use ref or not.
I'm not saying it's good style, necessarily. You shouldn't use ref or out unless you really, really need to and have no other way to do what you're doing. But using ref will make what you want to do work.
No. And if there is some reason that you need to do this, your code has improper architecture.
Short answer - no there isn't
In the vast majority of cases I usually find that the issues that you've listed above don't really matter all that much. When they do you could overload a method so that you can accept something else as a parameter instead of the object you are worried about sharing.
// For example create a method that allows you to do this:
people.Add("Larry");
// Instead of this:
people.Add(new Person("Larry"));
// The new method might look a little like this:
public void Add(string name)
{
Person person = new Person(name);
this.add(person); // This method could be private if neccessary
}
I can think of a way to do this, but I would definitely not recommend this. Just for argument's sake.
What does it mean for an object to be a "new" object? It means there is only one reference keeping it alive. An "existing" object would have more than one reference to it.
With this in mind, look at the following code:
class Program
{
static void Main(string[] args)
{
object o = new object();
Console.WriteLine(IsExistingObject(o));
Console.WriteLine(IsExistingObject(new object()));
o.ToString(); // Just something to simulate further usage of o. If we didn't do this, in a release build, o would be collected by the GC.Collect call in IsExistingObject. (not in a Debug build)
}
public static bool IsExistingObject(object o)
{
var oRef = new WeakReference(o);
#if DEBUG
o = null; // In Debug, we need to set o to null. This is not necessary in a release build.
#endif
GC.Collect();
GC.WaitForPendingFinalizers();
return oRef.IsAlive;
}
}
This prints True on the first line, False on the second.
But again, please do not use this in your code.
Let me rewrite your question to something shorter.
Is there any way, in my method, which takes an object as an argument, to know if this object will ever be used outside of my method?
And the short answer to that is: No.
Let me venture an opinion at this point: There should not be any such mechanism either.
This would complicate method calls all over the place.
If there was a method where I could, in a method call, tell if the object I'm given would really be used or not, then it's a signal to me, as a developer of that method, to take that into account.
Basically, you'd see this type of code all over the place (hypothetical, since it isn't available/supported:)
if (ReferenceCount(obj) == 1) return; // only reference is the one we have
My opinion is this: If the code that calls your method isn't going to use the object for anything, and there are no side-effects outside of modifying the object, then that code should not exist to begin with.
It's like code that looks like this:
1 + 2;
What does this code do? Well, depending on the C/C++ compiler, it might compile into something that evaluates 1+2. But then what, where is the result stored? Do you use it for anything? No? Then why is that code part of your source code to begin with?
Of course, you could argue that the code is actually a+b;, and the purpose is to ensure that the evaluation of a+b isn't going to throw an exception denoting overflow, but such a case is so diminishingly rare that a special case for it would just mask real problems, and it would be really simple to fix by just assigning it to a temporary variable.
In any case, for any feature in any programming language and/or runtime and/or environment, where a feature isn't available, the reasons for why it isn't available are:
It wasn't designed properly
It wasn't specified properly
It wasn't implemented properly
It wasn't tested properly
It wasn't documented properly
It wasn't prioritized above competing features
All of these are required to get a feature to appear in version X of application Y, be it C# 4.0 or MS Works 7.0.
Nope, there's no way of knowing.
All that gets passed in is the object reference. Whether it is 'newed' in-situ, or is sourced from an array, the method in question has no way of knowing how the parameters being passed in have been instantiated and/or where.
One way to know if an object passed to a function (or a method) has been created right before the call to the function/method is that the object has a property that is initialized with the timestamp passed from a system function; in that way, looking at that property, it would be possible to resolve the problem.
Frankly, I would not use such method because
I don't see any reason why the code should now if the passed parameter is an object right created, or if it has been created in a different moment.
The method I suggest depends from a system function that in some systems could not be present, or that could be less reliable.
With the modern CPUs, which are a way faster than the CPUs used 10 years ago, there could be the problem to use the right value for the threshold value to decide when an object has been freshly created, or not.
The other solution would be to use an object property that is set to a a value from the object creator, and that is set to a different value from all the methods of the object.
In this case the problem would be to forget to add the code to change that property in each method.
Once again I would ask to myself "Is there a really need to do this?".
As a possible partial solution if you only wanted one of an object to be consumed by a method maybe you could look at a Singleton. In this way the method in question could not create another instance if it existed already.