I have a question regarding method calls and stack pointers.
Basically I have a program that reads input from the user. After creating an object of a class "Input", a method call "prompt()" presents a menu with choices, and each choice you make calls a new method that performs some operations. After making a choice, you can always choose to go back to the main menu, and this action calls "prompt()" again.
Now, my question is, will each call of "prompt()" point to a new place in the memory stack or will it enter the same context as when the first call was made? I.e is it possible to create a memory leak by going back to the main menu over and over?
class inOut {
public string[] Prompt(){
...
presentChoices();
...
}
private void PresentChoices(){
...
if(someChoice){
manualInput();
}
...
}
private void ManualInput(){
...
if(goBack){
Prompt();
}
...
}
}
I hope the question was clear and thanks in advance for any answers on this!
For each method you enter there should be a corresponding return. Otherwise it may lead to StackOverlow. It's not a new context, but a values left in stack, which are used for return to return to the point where method was called and for method call itself (to pass parameters).
To have something repeating itself you can use infinite loop:
while(true)
{
... // repeat this action
if(endcondition)
break;
}
In your case repeated action is call to prompt() to show menu. It may have return value to tell whenever repeat or exit, which you use in endcondition.
As Long as you call the method on the same object instance, it's going to be the same pointer to the same adress.
When you create new instances of an object each object has it's own pointer.
If you want to avoid that then you need to define the method as static. In this case you will call the method not from an instance but from the type.
Something you need to be careful about especially when you call the same method over and over again from the same instance context are recursive calls. To many recursive calls (many thousands) will result in a StackOverFlowException (like the Name of this website). You can find out if you have recursive calls in the StackTrace pane in Visual Studio or if you have Resharper installed it will tell you on the left side of the document.
Either way, what you are describing here is not really a "Memory Leak" (ML's are unused objects that do not get collected and stay in memory not doing anything) but rather a stack Overflow Situation.
Objects that are not referenced anymore are garbage collected.Thats what will happen to your Input object.
Unlesss...
... you do it wrong.
In Winform applications doing it wrong usually happens when there are eventhandlers involved that for some reason (the publisher of the event lives longer than the subscriber) prevent the garbage collection.
Suppose your code looks something like this:
void Prompt()
{
// ...
var obj = new Input();
// ...
if (someCondition)
{
Prompt(); // recursive
}
// ...
// Is 'obj' used here?
}
Then when you call Prompt() recursively, yes, a new context is created. The obj variable will point to a new object, etc.
If the recursion becomes very, very deep, you might get a StackOverflowException (no more space on the stack for new "call frames"), or you might get an OutOfMemoryException (no more heap space for Input instances).
However, if you know for some reason that the recursion will not become too deep, the Garbage Collector will clean things for you when it is safe to do so.
But maybe you should consider a while (or do) loop instead of having your method call itself? It really depends on what you want to achieve.
I may have misunderstood your question. Maybe Prompt() is not called from within Prompt itself? You should give simplified structure of your code (like my code sample above) to make it clear what calls what from where.
Related
I have a constructor for a class called ActNode which takes a class parameter called Act like this:
public Act Act; //the act affiliated with this node...
public ActNode(Act moAct, ...some others, not important) {
if (moAct == null)
throw new SomeException(); //this is never entered
Act = moAct;
... some other codes
}
The constructor above is the only constructor the ActNode has and anywhere in the code, the Act passed to the constructor is not null. Then, in one of the ActNode's methods, a bool flag of the Act is checked like this:
public void ActNodeMethod() {
if (Act.AnActFlagToBeChecked) { //FIXME this Act can be surprisingly null!
//do something
}
}
Thus, in a single thread environment, the Act.AnActFlagToBeChecked cannot throw NullException since the Act of an ActNode cannot be null. However, I use the ActNode in a multi-thread environment. It is not always, but sometimes the line above:
if (Act.AnActFlagToBeChecked) //FIXME this Act can be surprisingly null!
Can throw null exception.
Why is this so and how to fix it?
From reading a post in SO, it is said that this may happen if the Act is not initialized (therefore having default value of null) but the method is called. Yet in my implementation, there is no such case, because the Act everywhere in the code is never null when the constructor is called.
The only thing here is that I implement it in multi-thread environment where multiple ActNodes can call ActNodeMethod at the same time. But it shouldn't be a problem (or it could?) since each ActNode will have its own resource called Act associated to it (not a shared resource).
I am pretty confused here and would appreciate if someone can help enlightment me of the possible issues of the above implementation.
To give bigger context, ActNode is a TreeNode which I use to store a value representing the time needed to go to that node. I implement the ActNode in my searching tree algorithm to find the fastest solution to finish up a set of given "Acts". I use multi-threads because it can speed up the searching process.
Introduction
I just thought of a new design pattern. I'm wondering if it exists, and if not, why not (or why I shouldn't use it).
I'm creating a game using an OpenGL. In OpenGL, you often want to "bind" things -- i.e., make them the current context for a little while, and then unbind them. For example, you might call glBegin(GL_TRIANGLES) then you draw some triangles, then call glEnd(). I like to indent all the stuff inbetween so it's clear where it starts and ends, but then my IDE likes to unindent them because there are no braces. Then I thought we could do something clever! It basically works like this:
using(GL.Begin(GL_BeginMode.Triangles)) {
// draw stuff
}
GL.Begin returns a special DrawBind object (with an internal constructor) and implements IDisposable so that it automatically calls GL.End() at the end of the block. This way everything stays nicely aligned, and you can't forget to call end().
Is there a name for this pattern?
Usually when I see using used, you use it like this:
using(var x = new Whatever()) {
// do stuff with `x`
}
But in this case, we don't need to call any methods on our 'used' object, so we don't need to assign it to anything and it serves no purpose other than to call the corresponding end function.
Example
For Anthony Pegram, who wanted a real example of code I'm currently working on:
Before refactoring:
public void Render()
{
_vao.Bind();
_ibo.Bind(BufferTarget.ElementArrayBuffer);
GL.DrawElements(BeginMode.Triangles, _indices.Length, DrawElementsType.UnsignedInt, IntPtr.Zero);
BufferObject.Unbind(BufferTarget.ElementArrayBuffer);
VertexArrayObject.Unbind();
}
After refactoring:
public void Render()
{
using(_vao.Bind())
using(_ibo.Bind(BufferTarget.ElementArrayBuffer))
{
GL.DrawElements(BeginMode.Triangles, _indices.Length, DrawElementsType.UnsignedInt, IntPtr.Zero);
}
}
Notice that there's a 2nd benefit that the object returned by _ibo.Bind also remembers which "BufferTarget" I want to unbind. It also draws your atention to GL.DrawElements, which is really the only significant statement in that function (that does something noticeable), and hides away those lengthy unbind statements.
I guess the one downside is that I can't interlace Buffer Targets with this method. I'm not sure when I would ever want to, but I would have to keep a reference to bind object and call Dispose manually, or call the end function manually.
Naming
If no one objects, I'm dubbing this Disposable Context Object (DCO) Idiom.
Problems
JasonTrue raised a good point, that in this scenario (OpenGL buffers) nested using statements would not work as expected, as only one buffer can be bound at a time. We can remedy this, however, by expanding on "bind object" to use stacks:
public class BufferContext : IDisposable
{
private readonly BufferTarget _target;
private static readonly Dictionary<BufferTarget, Stack<int>> _handles;
static BufferContext()
{
_handles = new Dictionary<BufferTarget, Stack<int>>();
}
internal BufferContext(BufferTarget target, int handle)
{
_target = target;
if (!_handles.ContainsKey(target)) _handles[target] = new Stack<int>();
_handles[target].Push(handle);
GL.BindBuffer(target, handle);
}
public void Dispose()
{
_handles[_target].Pop();
int handle = _handles[_target].Count > 0 ? _handles[_target].Peek() : 0;
GL.BindBuffer(_target, handle);
}
}
Edit: Just noticed a problem with this. Before if you didn't Dispose() of your context object there wasn't really any consequence. The context just wouldn't switch back to whatever it was. Now if you forget to Dispose of it inside some kind of loop, you're wind up with a stackoverflow. Perhaps I should limit the stack size...
A similar tactic is used with Asp.Net MVC with the HtmlHelper. See http://msdn.microsoft.com/en-us/library/system.web.mvc.html.formextensions.beginform.aspx (using (Html.BeginForm()) {....})
So there's at least one precedent for using this pattern for something other than the obvious "need" for IDisposable for unmanaged resources like file handles, database or network connections, fonts, and so on. I don't think there's a special name for it, but in practice, it seems to be the C# idiom that serves as the counterpart to the C++ idiom, Resource Acquisition is Initialization.
When you're opening a file, you're acquiring, and guaranteeing the disposal of, a file context; in your example, the resource you're acquiring is a is a "binding context", in your words. While I've heard "Dispose pattern" or "Using pattern" used to describe the broad category, essentially "deterministic cleanup" is what you're talking about; you're controlling the lifetime the object.
I don't think it's really a "new" pattern, and the only reason it stands out in your use case is that apparently the OpenGL implementation you're depending on didn't make a special effort to match C# idioms, which requires you to build your own proxy object.
The only thing I'd worry about is if there are any non-obvious side effects, if, for example, you had a nested context where there were similar using constructs deeper in your block (or call stack).
ASP.NET/MVC uses this (optional) pattern to render the beginning and ending of a <form> element like this:
#using (Html.BeginForm()) {
<div>...</div>
}
This is similar to your example in that you are not consuming the value of your IDisposable other than for its disposable semantics. I've never heard of a name for this, but I've used this sort of thing before in other similar scenarios, and never considered it as anything other than understanding how to generally leverage the using block with IDisposable similar to how we can tap into the foreach semanatics by implementing IEnumerable.
I would this is more an idiom than a pattern. Patterns usually are more complex involving several moving parts, and idioms are just clever ways to do things in code.
In C++ it is used quite a lot. Whenever you want to aquire something or enter a scope you create an automatic variable (i.e. on the stack) of a class that begins or creates or whatever you need to be done on entry. When you leave the scope where the automatic variable is declared the destructor is called. The destructor should then end or delete or whatever is required to clean up.
class Lock {
private:
CriticalSection* criticalSection;
public:
Lock() {
criticalSection = new CriticalSection();
criticalSection.Enter();
}
~Lock() {
criticalSection.Leave();
delete criticalSection;
}
}
void F() {
Lock lock();
// Everything in here is executed in a critical section and it is exception safe.
}
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.
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.
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.