We Keep Coding

Object instance valid only for the current method

Is it possible to create an object that can register whether the current thread leaves the method where it was created, or to check whether this has happened when a method on the instance gets called?
ScopeValid obj;
void Method1()
{
obj = new ScopeValid();
obj.Something();
}
void Method2()
{
Method1();
obj.Something(); //Exception
}
Can this technique be accomplished? I would like to develop a mechanism similar to TypedReference and ArgIterator, which can't "escape" the current method. These types are handled specially by the compiler, so I can't mimic this behavior exactly, but I hope it is possible to create at least a similar rule with the same results - disallow accessing the object if it has escaped the method where it was created.
Note that I can't use StackFrame and compare methods, because the object might escape and return to the same method.

Changing method behavior based upon the source of the call is a bad design choice.
Some example problems to consider with such a method include:
Testability - how would you test such a method?
Refactoring the calling code - What if the user of your code just does an end run around your error message that says you can't do that in a different method than it was created? "Okay, fine! I'll just do my bad thing in the same method, says the programmer."
If the user of your code breaks it, and it's their fault, let it break. Better to just document your code with something like:
IInvalidatable - Types which implement this member should be invalidated with Invalidate() when you are done working with this.
Ignoring the obvious point that this almost seems like is re-inventing IDisposible and using { } blocks (which have language support), if the user of your code doesn't use it right, it's not really your concern.
This is likely technically possible with AOP (I'm thinking PostSharp here), but it still depends on the user using your code correctly - they would have to have it in the build process, and failing to function if they aren't using a tool just because you're trying to make it easy on them is evil.
Another point - If you are just attempting to create an object which cannot be used outside of one method, and any attempted operation outside of the method would fail, just declare it a local inside the method.
Related: How to find out which assembly handled the request

Years laters, it seems this feature was finally added to C# 7.2: ref struct.
Another related language feature is the ability to declare a value type that must be stack allocated. In other words, these types can never be created on the heap as a member of another class. The primary motivation for this feature was Span and related structures. Span may contain a managed pointer as one of its members, the other being the length of the span. It's actually implemented a bit differently because C# doesn't support pointers to managed memory outside of an unsafe context. Any write that changes the pointer and the length is not atomic. That means a Span would be subject to out of range errors or other type safety violations were it not constrained to a single stack frame. In addition, putting a managed pointer on the GC heap typically crashes at JIT time.
This prevents the code from moving the value to the heap, which partly solves my original problem. I am not sure how returning a ref struct is constrained, though.

How to enforce the use of a method's return value in C#?

I have a piece of software written with fluent syntax. The method chain has a definitive "ending", before which nothing useful is actually done in the code (think NBuilder, or Linq-to-SQL's query generation not actually hitting the database until we iterate over our objects with, say, ToList()).
The problem I am having is there is confusion among other developers about proper usage of the code. They are neglecting to call the "ending" method (thus never actually "doing anything")!
I am interested in enforcing the usage of the return value of some of my methods so that we can never "end the chain" without calling that "Finalize()" or "Save()" method that actually does the work.
Consider the following code:
//The "factory" class the user will be dealing with
public class FluentClass
{
//The entry point for this software
public IntermediateClass<T> Init<T>()
{
return new IntermediateClass<T>();
}
}
//The class that actually does the work
public class IntermediateClass<T>
{
private List<T> _values;
//The user cannot call this constructor
internal IntermediateClass<T>()
{
_values = new List<T>();
}
//Once generated, they can call "setup" methods such as this
public IntermediateClass<T> With(T value)
{
var instance = new IntermediateClass<T>() { _values = _values };
instance._values.Add(value);
return instance;
}
//Picture "lazy loading" - you have to call this method to
//actually do anything worthwhile
public void Save()
{
var itemCount = _values.Count();
. . . //save to database, write a log, do some real work
}
}
As you can see, proper usage of this code would be something like:
new FluentClass().Init<int>().With(-1).With(300).With(42).Save();
The problem is that people are using it this way (thinking it achieves the same as the above):
new FluentClass().Init<int>().With(-1).With(300).With(42);
So pervasive is this problem that, with entirely good intentions, another developer once actually changed the name of the "Init" method to indicate that THAT method was doing the "real work" of the software.
Logic errors like these are very difficult to spot, and, of course, it compiles, because it is perfectly acceptable to call a method with a return value and just "pretend" it returns void. Visual Studio doesn't care if you do this; your software will still compile and run (although in some cases I believe it throws a warning). This is a great feature to have, of course. Imagine a simple "InsertToDatabase" method that returns the ID of the new row as an integer - it is easy to see that there are some cases where we need that ID, and some cases where we could do without it.
In the case of this piece of software, there is definitively never any reason to eschew that "Save" function at the end of the method chain. It is a very specialized utility, and the only gain comes from the final step.
I want somebody's software to fail at the compiler level if they call "With()" and not "Save()".
It seems like an impossible task by traditional means - but that's why I come to you guys. Is there an Attribute I can use to prevent a method from being "cast to void" or some such?
Note: The alternate way of achieving this goal that has already been suggested to me is writing a suite of unit tests to enforce this rule, and using something like http://www.testdriven.net to bind them to the compiler. This is an acceptable solution, but I am hoping for something more elegant.

I don't know of a way to enforce this at a compiler level. It's often requested for objects which implement IDisposable as well, but isn't really enforceable.
One potential option which can help, however, is to set up your class, in DEBUG only, to have a finalizer that logs/throws/etc. if Save() was never called. This can help you discover these runtime problems while debugging instead of relying on searching the code, etc.
However, make sure that, in release mode, this is not used, as it will incur a performance overhead since the addition of an unnecessary finalizer is very bad on GC performance.

You could require specific methods to use a callback like so:
new FluentClass().Init<int>(x =>
{
x.Save(y =>
{
y.With(-1),
y.With(300)
});
});
The with method returns some specific object, and the only way to get that object is by calling x.Save(), which itself has a callback that lets you set up your indeterminate number of with statements. So the init takes something like this:
public T Init<T>(Func<MyInitInputType, MySaveResultType> initSetup)

I can think of three a few solutions, not ideal.
AIUI what you want is a function which is called when the temporary variable goes out of scope (as in, when it becomes available for garbage collection, but will probably not be garbage collected for some time yet). (See: The difference between a destructor and a finalizer?) This hypothetical function would say "if you've constructed a query in this object but not called save, produce an error". C++/CLI calls this RAII, and in C++/CLI there is a concept of a "destructor" when the object isn't used any more, and a "finaliser" which is called when it's finally garbage collected. Very confusingly, C# has only a so-called destructor, but this is only called by the garbage collector (it would be valid for the framework to call it earlier, as if it were partially cleaning the object immediately, but AFAIK it doesn't do anything like that). So what you would like is a C++/CLI destructor. Unfortunately, AIUI this maps onto the concept of IDisposable, which exposes a dispose() method which can be called when a C++/CLI destructor would be called, or when the C# destructor is called -- but AIUI you still have to call "dispose" manually, which defeats the point?
Refactor the interface slightly to convey the concept more accurately. Call the init function something like "prepareQuery" or "AAA" or "initRememberToCallSaveOrThisWontDoAnything". (The last is an exaggeration, but it might be necessary to make the point).
This is more of a social problem than a technical problem. The interface should make it easy to do the right thing, but programmers do have to know how to use code! Get all the programmers together. Explain simply once-and-for-all this simple fact. If necessary have them all sign a piece of paper saying they understand, and if they wilfully continue to write code which doesn't do anythign they're worse than useless to the company and will be fired.
Fiddle with the way the operators are chained, eg. have each of the intermediateClass functions assemble an aggregate intermediateclass object containing all of the parameters (you mostly do it this was already (?)) but require an init-like function of the original class to take that as an argument, rather than have them chained after it, and then you can have save and the other functions return two different class types (with essentially the same contents), and have init only accept a class of the correct type.
The fact that it's still a problem suggests that either your coworkers need a helpful reminder, or they're rather sub-par, or the interface wasn't very clear (perhaps its perfectly good, but the author didn't realise it wouldn't be clear if you only used it in passing rather than getting to know it), or you yourself have misunderstood the situation. A technical solution would be good, but you should probably think about why the problem occurred and how to communicate more clearly, probably asking someone senior's input.

After great deliberation and trial and error, it turns out that throwing an exception from the Finalize() method was not going to work for me. Apparently, you simply can't do that; the exception gets eaten up, because garbage collection operates non-deterministically. I was unable to get the software to call Dispose() automatically from the destructor either. Jack V.'s comment explains this well; here was the link he posted, for redundancy/emphasis:
The difference between a destructor and a finalizer?
Changing the syntax to use a callback was a clever way to make the behavior foolproof, but the agreed-upon syntax was fixed, and I had to work with it. Our company is all about fluent method chains. I was also a fan of the "out parameter" solution to be honest, but again, the bottom line is the method signatures simply could not change.
Helpful information about my particular problem includes the fact that my software is only ever to be run as part of a suite of unit tests - so efficiency is not a problem.
What I ended up doing was use Mono.Cecil to Reflect upon the Calling Assembly (the code calling into my software). Note that System.Reflection was insufficient for my purposes, because it cannot pinpoint method references, but I still needed(?) to use it to get the "calling assembly" itself (Mono.Cecil remains underdocumented, so it's possible I just need to get more familiar with it in order to do away with System.Reflection altogether; that remains to be seen....)
I placed the Mono.Cecil code in the Init() method, and the structure now looks something like:
public IntermediateClass<T> Init<T>()
{
ValidateUsage(Assembly.GetCallingAssembly());
return new IntermediateClass<T>();
}
void ValidateUsage(Assembly assembly)
{
// 1) Use Mono.Cecil to inspect the codebase inside the assembly
var assemblyLocation = assembly.CodeBase.Replace("file:///", "");
var monoCecilAssembly = AssemblyFactory.GetAssembly(assemblyLocation);
// 2) Retrieve the list of Instructions in the calling method
var methods = monoCecilAssembly.Modules...Types...Methods...Instructions
// (It's a little more complicated than that...
// if anybody would like more specific information on how I got this,
// let me know... I just didn't want to clutter up this post)
// 3) Those instructions refer to OpCodes and Operands....
// Defining "invalid method" as a method that calls "Init" but not "Save"
var methodCallingInit = method.Body.Instructions.Any
(instruction => instruction.OpCode.Name.Equals("callvirt")
&& instruction.Operand is IMethodReference
&& instruction.Operand.ToString.Equals(INITMETHODSIGNATURE);
var methodNotCallingSave = !method.Body.Instructions.Any
(instruction => instruction.OpCode.Name.Equals("callvirt")
&& instruction.Operand is IMethodReference
&& instruction.Operand.ToString.Equals(SAVEMETHODSIGNATURE);
var methodInvalid = methodCallingInit && methodNotCallingSave;
// Note: this is partially pseudocode;
// It doesn't 100% faithfully represent either Mono.Cecil's syntax or my own
// There are actually a lot of annoying casts involved, omitted for sanity
// 4) Obviously, if the method is invalid, throw
if (methodInvalid)
{
throw new Exception(String.Format("Bad developer! BAD! {0}", method.Name));
}
}
Trust me, the actual code is even uglier looking than my pseudocode.... :-)
But Mono.Cecil just might be my new favorite toy.
I now have a method that refuses to be run its main body unless the calling code "promises" to also call a second method afterwards. It's like a strange kind of code contract. I'm actually thinking about making this generic and reusable. Would any of you have a use for such a thing? Say, if it were an attribute?

What if you made it so Init and With don't return objects of type FluentClass? Have them return, e.g., UninitializedFluentClass which wraps a FluentClass object. Then calling .Save(0 on the UnitializedFluentClass object calls it on the wrapped FluentClass object and returns it. If they don't call Save they don't get a FluentClass object.

In Debug mode beside implementing IDisposable you can setup a timer that will throw a exception after 1 second if the resultmethod has not been called.

Use an out parameter! All the outs must be used.
Edit: I am not sure of it will help, tho...
It would break the fluent syntax.

Does C# have a way to mimic Software Transactional Memory, on a small scale?

Does C# have a way to temporarily change the value of a variable in a specific scope and revert it back automatically at the end of the scope/block?
For instance (not real code):
bool UpdateOnInput = true;
using (UpdateOnInput = false)
{
//Doing my changes without notifying anyone
Console.WriteLine (UpdateOnInput) // prints false
}
//UpdateOnInput is true again.
EDIT:
The reason I want the above is because I don't want to do this:
UpdateOnInput = false
//Doing my changes without notifying anyone
Console.WriteLine (UpdateOnInput) // prints false
UpdateOnInput = true

No, there's no way to do this directly. There are a few different schools of thought on how to do this sort of thing. Compare and contrast these two:
originalState = GetState();
SetState(newState);
DoSomething();
SetState(originalState);
vs
originalState = GetState();
SetState(newState);
try
{
DoSomething();
}
finally
{
SetState(originalState);
}
Many people will tell you that the latter is "safer".
It ain't necessarily so.
The difference between the two is of course the the latter restores the state even if DoSomething() throws an exception. Is that better than keeping the state mutated in an exception scenario? What makes it better? You have an unexpected, unhandled exception reporting that something awful and unexpected has happened. Your internal state could be completely inconsistent and arbitrarily messed up; no one knows what might have been happening at the point of the exception. All we know is that DoSomething probably was trying to do something to the mutated state.
Is it really the right thing to do in the scenario where something terrible and unknown has happened to keep on stirring that particular pot and trying to mutate the state that just caused an exception again?
Sometimes that is going to be the right thing to do, and sometimes its going to make matters worse. Which scenario you're actually in depends on what exactly the code is doing, so think carefully about what the right thing to do is before just blindly choosing one or the other.
Frankly, I would rather solve the problem by not getting into the situation in the first place. Our existing compiler design uses this design pattern, and frankly, it is freakin' irritating. In the existing C# compiler the error reporting mechanism is "side effecting". That is, when part of the compiler gets an error, it calls the error reporting mechanism which then displays the error to the user.
This is a major problem for lambda binding. If you have:
void M(Func<int, int> f) {}
void M(Func<string, int> f) {}
...
M(x=>x.Length);
then the way this works is we try to bind
M((int x)=>{return x.Length;});
and
M((string x)=>{return x.Length;});
and we see which one, if any, gives us an error. In this case, the former gives an error, the latter compiles without error, so this is a legal lambda conversion and overload resolution succeeds. What do we do with the error? We cannot report it to the user because this program is error free!
Therefore what we do when we bind the body of a lambda is exactly what you say: we tell the error reporter "don't report your errors to the user; save them in this buffer over here instead". Then we bind the lambda, restore the error reporter to its earlier state, and look at the contents of the error buffer.
We could avoid this problem entirely by changing the expression analyzer so that it returned the errors along with the result, rather than making errors a state-related side effect. Then the need for mutation of the error reporting state goes away entirely and we don't even have to worry about it.
So I would encourage you to revisit your design. Is there a way you can make the operation you are performing not dependent upon the state you are mutating? If so, then do that, and then you don't need to worry about how to restore the mutated state.
(And of course in our case we do want to restore the state upon an exception. If something inside the compiler throws during lambda binding, we want to be able to report that to the user! We don't want the error reporter to stay in the "suppress reporting errors" state.)

No, but it is pretty simple to just do this:
bool UpdateOnTrue = true;
// ....
bool temp = UpdateOnTrue;
try
{
UpdateOnTrue = false;
// do stuff
}
finally
{
UpdateOnTrue = temp;
}

Try:
public void WithAssignment<T>(ref T var, T val, Action action)
{
T original = var;
var = val;
try
{
action();
}
finally
{
var = original;
}
}
Now you can say:
bool flag = false;
WithAssignment(ref flag, true, () =>
{
// flag is true during this block
});
// flag is false again

No, you have to do it manually with a try/finally block. I dare say you could write an IDisposable implementation which would do something hacky in conjunction with lambda expressions, but I suspect a try/finally block is simpler (and doesn't abuse the using statement).

Sounds like you really want
Stack<bool>

No , there is not standard way, you should implement it manually. Generic implementation of IEditableObject via TypeDescriptor and Reflection can be helpfull

Canned... I doubt it. Given your example is a simple use of a temporary bool value I'm assuming you've got something wacky in mind :-) You can implement some kind of Stack stucture:
1) Push old value onto stack
2) Load new value
3) Do Stuff
4) Pop from stack and replace used value.
Rough (AKA Untested) example (Can't look up stack syntax right now)
bool CurrentValue = true;
Stack<bool> Storage= new Stack<bool>
Storage.Push(CurrentValue);
CurrentValue=false;
DoStuff();
CurrentValue = Storage.Pop();
//Continue

You should refactor your code to use a separate function, like so:
bool b = GetSomeValue();
DoSomething(ModifyValue(b));
//b still has the original value.
For this to work for a reference type, you need to copy it before messing with it:
ICloneable obj = GetSomeValue();
DoSomething(ModifyValue(obj.Clone()));
//obj still has the original value.
It's hard to write correct code when the values of your variables change around a lot. Strive to have as few reassignments in your code as possible.

How do I automatically reset a boolean when any method other is called in C#?

Using C#, I need to do some extra work if function A() was called right before function C(). If any other function was called in between A() and C() then I don't want to do that extra work. Any ideas that would require the least amount of code duplication?
I'm trying to avoid adding lines like flag = false; into every function B1..BN.
Here is a very basic example:
bool flag = false;
void A()
{
flag = true;
}
void B1()
{
...
}
void B2()
{
...
}
void C()
{
if (flag)
{
//do something
}
}
The above example was just using a simple case but I'm open to using something other than booleans. The important thing is that I want to be able to set and reset a flag of sorts so that C() knows how to behave accordingly.
Thank you for your help. If you require clarification I will edit my post.

Why not just factor your "Extra work" into a memoised function (i.e. one that caches its results)? Whenever you need that work you just call this function, which will short circuit if the cache is fresh. Whenever that work becomes stale, invalidate the cache. In your rather odd examples above, I presume you'll need a function call in each of the Bs, and one in C. Calls to A will invalidate the cache.
If you're looking for away around that (i.e. some clever way to catch all function calls and insert this call), I really wouldn't bother. I can conceive of some insane runtime reflection proxy class generation, but you should make your code flow clear and obvious; if each function depends on the work being already done, just call "doWork" in each one.

Sounds like your design is way too tightly coupled if calling one method changes the behavior of another such that you have to make sure to call them in the right order. That's a major red flag.
Sounds like some refactoring is in order. It's a little tricky to give advice without seeing more of the real code, but here is a point in the right direction.
Consider adding a parameter to C like so:
void C(bool DoExtraWork) {
if (DoExtraWork)...
}
Of course "DoExtraWork" should be named something meaningful in the context of the caller.

I solved a problem with a similar situation (i.e., the need to know whether A was called directly before C) by having a simply state machine in place. Essentially, I built a state object using an enum and a property to manage/query the state.
When my equivalent of A() was called, it would have the business logic piece store off the state indicating that A was called. If other methods (your B's ) were called, it would toggle the state to one of a few other states (my situation was a bit more complicated) and then when C() was called, the business logic piece was queried to determine if we were going to call some method D() that held the "only if A was just called" functionality.
I suspect there are multiple ways to solve this problem, but I liked the state machine approach I took because it allowed me to expand what was initially a binary situation to handle a more complicated multi-state situation.
I was fortunate that multi-threading was not an issue in my case because that tends to make things more entertaining, but the state machine would likely work in that scenario as well.
Just my two cents.

I don't recommend this, but what the hell: If you're willing to replace all your simple method calls:
A();
... with syntax like this:
// _lastAction is a class-level Action member
(_lastAction = new Action(A)).Invoke();
... then inside of C() you can just do a check like this:
void C()
{
if (_lastAction.Method.Name == "A")
{
}
}
This probably isn't thread-safe (and it wouldn't work in code run through an obfuscator without a bit of tinkering), so I wouldn't use something like this without heavy testing. I also wouldn't use something like this period.
Note: my ancient version of C# only has Action<T> (and not Action or Action<T, T> etc.), so if you're stuck there, too, you'd have to add a dummy parameter to each method to use this approach.

Can I detect whether I've been given a new object as a parameter?

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.