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I'm new to c sharp and programming generally. I have a quick question - what is best practise with regards to static/non static variables.
I have a variable,private int x, which belongs to class y. To access this variable, i need to reference y. If x was static however, i can access this variable with no references to y.
Which is the best way to go, in a situation whereby several methods within the class y will be referencing this value ?
Hope this makes sense, and my question isn't too basic !
Many thanks
You need to think about static variables as belonging to the class, not to instances of the class.
If, in all instances of the class this variable should be identical, use a static variable.
If not, use an instance variable.
In general having public static variables is bad practice - it is a shared global resource and if you change it you need to synchronize access to it. Having global state is something you want to avoid as much as possible.
Best practice is to avoid public static. In OOP, class is meant to hide its members. Static is actually not a member of the instance but of the type.
Static comes handy if you are implementing singleton pattern. But then again they need to be made private and accessible through a public property.
You need to read Static Classes and Static Class Members (C# Programming Guide).
Well I can't conclusively say that one is better, because they serve different purposes.
Are you familiar with OOP? In OOP, static objects or members of a class that can be accessed directly from the class, while non-static members can only be accessed from the instance it belongs to.
C# follows a similar principle for the methods. The static methods can by accessed directly from the class, while non-static methods (or instance methods as I like to call them) have to be accessed from an instance. That is why instatiating needs to be done for instance methods, while for static methods it's just not needed, and furthermore impractical (see below).
In OOP, static variables are used for values which cannot be stored by an instance variable. Example: supposed you wanted to keep a count of how many instances of a class exists? How would you store that in a single instance?
The methods use a similar principle. They should be used for procedures for which it is impractical to do within an instance of a class. I tend to use them for broad procedures (not a technical term), meaning those that do not require me to instantiate an object. Example, adding two parameters. (This usage may or may not be correct, but I believe it is)
However, if you wanted to add two properties of an object, the method cannot be static, because as you would soon realize, static methods cannot access instance methods or variables within a class. Of course that makes sense because that static method would not know which instance of the class the get these from unless it were told, since it is not part of an instance itself)
For the sake of no further complicating things, I'll stop here. Let me know if you misunderstood anything.
Your choice depends on your architecture.
Static makes part of a Type, others make part of an instance of that type. If you want have some shared state (say) between different instances of the same type, use static. If you want that every instance have it's own value, independent from others, use instance fields.
In both cases, by the way, avoid to expose like a public fields, but use properties.
I completely agree with Mr Oded:
If, in all instances of the class this variable should be identical, use a static variable.
If not, use an instance variable.
Yes, adding static to a class member basically means you can access it without an instance, and only outside any instance. And yes, it becomes a global resource, or even a global variable if you will.
But I think there's at least another (heavily edited) good point to be made here...
Using static members as global vars go against OOP
This means once you set a static member you can't pass it around as an object. The more you use static as global var, the more difficult it is for unit testing / mocking classes.
There is a solution for that, Singletons. But they should never come without warnings!
At other hand, if you're sure you really need global vars, take a look at the Toolbox pattern. It's a not well known extension of Singleton pattern. It's so unknown in fact, if you google for it you won't find it with those keywords (toolbox pattern).
So plan ahead. Read more. Get to know about every option so you can decide better. Even get a book. Object Oriented Programming is more about applying concepts that will help in the long run than just making things work now.
In general if you want to have a variable public, either static or instance, you must wrap it in a property and expose it like that. This is for sure a principle that you will love to follow.
But despite some of the other answers I cannot say don't use static. Static is not the devil that you should avoid in any case. What you have to do will decide if you are going to use static or not, as long as you keep your program clean and easy to maintain.
Easily speaking, and not in the language of the elders, static stands for something that don't belong to any instance of this class but has an effect on them. An example of a static property in a class that generates instances is for example a factor, which should be global for all instances of the class, to take part in a calculation that is done inside instances. To this case, and to my opinion, it is better to have this factor declared as static rather that have it in every single instance. Especially if this factor changes in the lifetime of your program to affect the next calculation.
You need to ask a question to youself: why I need x to be static?
If you make x static it means that x is a part of all objects of class A, but when x is not static it means, than x is a part only of one object.
In geleral using of static fields is painfull for bug tracking, but in some cases this is very helpfull.
I suggest you to look in using of singelton http://en.wikipedia.org/wiki/Singleton
Let me be more precise. In my winforms project im creating classes to manage/create every part of the program. I did it to have more control over my code. E.g. I have a class that manages my DataGridView control. I named it gridManager and in it i set all properties, colors and so on and also i have methods to change those settings (e.g. changeBackgroundColor() etc).
I also have this type of class for each Panel in splitContainer. In those classes i initialize every Control that is a child of panel i add them to that panel set all properties and so on.
I wrote it all to give you better view at the purpose of those classes.
Now my question is: is it good practice to make this classes static? With all controls and methods inside being static?
At first i had them non-static but when i wanted to call methods for (e.g.) changing color from options Form i had to either pass MainForm as a parameter or do it like this:
(Application.OpenForm[0] as MainForm).gridManager.changeColor();
Static version of it makes it a lot easier. But it makes me wonder if its a good thing to do.
Uh a lot of explaining i hope my not perfect English wont make it even more difficult to understand. :)
Global mutable state is usually a bad idea.
Static methods/classes are good for simple sideeffect free functions. Math and Enumerable are good examples.
You on the other hand want controls inside static fields. These are mutable state and thus should be avoided. For example if you tomorrow want to have two instances of your form, you need two instances of your manager class. But it's static and you now need to rewrite all the code using it.
Like anything, static classes have tradeoffs. The two "negative" ones that come to mind are
You can't inherit from static classes
You can't (easily) mock static classes for testing
But it sounds like in your cases you wouldn't be doing any inheritance of these classes anyway, so perhaps in this case it would be okay.
Edit This is assuming you're doing something like a control factory.
For example:
var grid = GridManager.CreateGrid(options);
If you're doing something like
var data = GridManager.GetDataFromGrid(myGrid)
I'd probably reconsider.
Static classes have their place, but this probably is not one of them unless it is a quick and dirty application. If you want to have automated tests around your code, it can be nearly impossible if the code under test uses static classes for preferences.
It might be better to use the singleton pattern. This way you can replace the implementation during the automated test.
You better do it with normal classes that is linked to that grid object. If you need another grid object you may need to instantiate another instance. Controllers are not the best candidate for static classes.
Its neither good nor bad practice, its just a common pattern for certain tasks.
Generally speaking you would use static methods for functionality that is related to a class type but does not rely upon any instance data to work, a classic use would be something like a factory type method that returns an initialized instance of the class its attached to.
public SomeClass = SomeClass.CreateWithSomeInit(parms);
Static classes certainly have their place, but I think that using them whenever you can is a bad advice.
The whole concept of OOP is built around instances and so you should use non-static classes most of the time. The reason? Primarily flexibility. You can have two instances that do the same thing is a slightly different way based on their internal state. You can have more implementations of the same concept and you can easily switch them. You can use things like Inversion of Control containers. And so on.
I need to explain myself why I do not use static methods/propertis. For example,
String s=String.Empty;
is this property (belongs to .Net framework) wrong? is should be like?
String s= new EmptySting();
or
IEmptyStringFactory factory=new EmptyStringFactory();
String s= factory.Create();
Why would you want to create a new object every time you want to use the empty string? Basically the empty string is a singleton object.
As Will says, statics can certainly be problematic when it comes to testing, but that doesn't mean you should use statics everywhere.
(Personally I prefer to use "" instead of string.Empty, but that's a discussion which has been done to death elsewhere.)
I think the worst thing about using statics is that you can end up with tight coupling between classes. See the ASP.NET before System.Web.Abstractions came out. This makes your classes harder to test and, possibly, more prone to bugs causing system-wide issues.
Well, in the case of String.Empty it is more of a constant (kind of like Math.PI or Math.E) and is defined for that type. Creating a sub-class for one specific value is typically bad.
On to your other (main) question as to how they are "inconvenient:"
I've only found static properties and methods to be inconvenient when they are abused to create a more functional solution instead of the object-oriented approach that is meant with C#.
Most of my static members are either constants like above or factory-like methods (like Int.TryParse).
If the class has a lot of static properties or methods that are used to define the "object" that is represented by the class, I would say that is typically bad design.
One major thing that does bother me with the static methods/properties is that you sometimes they are too tied to one way of doing something without providing an easy way to create an instance the provides with easy overrides to the behavior. For example, imagine that you want to do your mathematical computations in degrees instead of radians. Since Math is all static, you can't do that and instead have to convert each time. If Math were instance-based, you could create a new Math object that defaulted to radians or degrees as you wished and could still have a static property for the typical behaviors.
For example, I wish I could say this:
Math mD = new Math(AngleMode.Degrees); // ooooh, use one with degrees instead
double x = mD.Sin(angleInDegrees);
but instead I have to write this:
double x = Math.Sin(angleInDegrees * Math.PI / 180);
(of course, you can write extension methods and constants for the conversions, but you get my point).
This may not be the best example, but I hope it conveys the problem of not being able to use the methods with variations on the default. It creates a functional construct and breaks with the usual object-oriented approach.
(As a side note, in this example, I would have a static property for each mode. That in my eyes would be a decent use of the static properties).
The semantics of your three different examples are very different. I'll try to break it down as I do it in practice.
String s=String.Empty;
This is a singleton. You would use this when you want to ensure that there's only ever one of something. In this case, since a string is immutable, there only ever needs to be one "empty" string. Don't overuse singletons, because they're hard to test. When they make sense, though, they're very powerful.
String s= new EmptySting();
This is your standard constructor. You should use this whenever possible. Refactor to the singleton pattern only when the case for a singleton is overwhelming. In the case of string.Empty, it very much makes sense to use singleton because the string's state cannot be changed by referring classes.
IEmptyStringFactory factory=new EmptyStringFactory();
String s= factory.Create();
Instance factories and static factories, like singletons, should be used sparingly. Mostly, they should be used when the construction of a class is complex and relies on multiple steps, and possibly state.
If the construction of an object relies on state that might not be known by the caller, then you should use instance factories (like in your example). When the construction is complex, but the caller knows the conditions that would affect construction, then you should use a static factory (such as StringFactory.CreateEmpty() or StringFactory.Create("foo"). In the case of a string, however, the construction is simple enough that using a factory would smell of a solution looking for a problem.
Generally, it is a bad idea to create a new empty string - this creates extra objects on the heap, so extra work for the garbage collector. You should always use String.Empty or "" when you want the empty string as those are references to existing objects.
In general, the purpose of a static is to make sure that there is ever only one instance of the static "thing" in your program.
Static fields maintain the same value throughout all instances of a type
Static methods and properties do not need an instance in order to be invoked
Static types may only contain static methods/properties/fields
Statics are useful when you know that the "thing" you are creating will never change through the lifetime of the program. In your example, System.String defines a private static field to store the empty string, which is allocated only once, and exposed through a static property.
As mentioned, there are testability issues with statics. For example, it is hard to mock static types since they can't be instantiated or derived from. It is also hard to introduce mocks into some static methods since the fields they use must also be static. (You can use a static setter property to get around this issue, but I personally try to avoid this as it usually breaks encapsulation).
For the most part, use of statics is o.k. You need to decide when to make the trade-off of using static and instance entities based on the complexity of your program.
In a purist OO approach, static methods break the OO paradigm because you're attaching actual data to the definition of data. A class is a definition of a set of objects that conform to semantics. Just like there are mathematical sets that contain one or zero elements, there can be classes that contain only one or zero possible states.
The way of sharing a common object and allowing multiple actors on its state is to pass a reference.
The main problem with static methods comes from, what if in the future you want two of them? We're writing computer programs, one would assume that if we can make one of something, we should be able to make two very simply, with statics this isn't the case. To change something from a static state to a normal instance state is a complete rewrite of the class in question.
I might assume I want to only ever use one SqlConnection pool, but now what if I want a high priority pool and a low priority pool. If the connection pool was instanced instead of static the solution would be simple, instead I have to couple pooling with connection instantiation. I better hope the library writer had forsight or else I have to reimplement the pooling.
Edit:
Static methods in single inheritance languages are a hack to provide reuse of code. Normally if there are methods one wanted to share common code between classes you could pull it in through multiple inheritance or a mixin. Single inheritance languages force you to call static methods; there's no way to use multiple abstract classes with state.
There are draw backs to using statics such as:
Statics dont allow extension methods.
Static constructor is called automatically to initialize the class before the first instance is created (depending on the static class being called of course)
Static class data lives throughout the lifespan of the execution scope, this wastes memory.
Reasons to use static methods
Statics are good for helper methods, as you dont want to create a local copy of a non-static class, just to calla single helper method.
Eeerm, static classes make the singleton pattern possible.
From a scenario-driven design, the criteria for choosing statics vs. instance methods should be: if a method can be called without an instance of a class to be created, make it static. Else, make it an instance method. First option makes the call a once line process, and avoid .ctor calls.
Another useful criteria here is whether responsabilities are in the right place. For ex. you got an Account class. Say you need functionality for currency conversion e.g. from dollars to euros. Do you make that a member of the Account class? account.ConvertTo(Currency.Euro)? Or do you create a different class that encapsulates that responsibility? CurrencyConverter.Convert(account, Currency.Euro)? To me, the latter is better in the sense that encapsulates responsibilities on a different class, while in the former I would be spreading currency conversion knowledge across different accounts.
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Should C# methods that can be static be static?
We were discussing this today and I'm kind of on the fence. Imagine you have a long method that you refactor a few lines out of. The new method probably takes a few local variables from the parent method and returns a value. This means it could be static.
The question is: should it be static? It's not static by design or choice, simply by its nature in that it doesn't reference any instance values.
It depends.
There are really 2 types of static methods:
Methods that are static because they CAN be
Methods that are static because they HAVE to be
In a small to medium size code base you can really treat the two methods interchangeably.
If you have a method that is in the first category (can-be-static), and you need to change it to access class state, it's relatively straight forward to figure out if it's possible to turn the static method into a instance method.
In a large code base, however, the sheer number of call sites might make searching to see if it's possible to convert a static method to a non static one too costly. Many times people will see the number of calls, and say "ok... I better not change this method, but instead create a new one that does what I need".
That can result in either:
A lot of code duplication
An explosion in the number of method arguments
Both of those things are bad.
So, my advice would be that if you have a code base over 200K LOC, that I would only make methods static if they are must-be-static methods.
The refactoring from non-static to static is relatively easy (just add a keyword), so if you want to make a can-be-static into an actual static later (when you need it's functionality outside of an instance) then you can. However, the inverse refactoring, turning a can-be-static into a instance method is MUCH more expensive.
With large code bases it's better to error on the side of ease of extension, rather than on the side of idealogical purity.
So, for big projects don't make things static unless you need them to be. For small projects, just do what ever you like best.
I would not make it a public static member of that class. The reason is that making it public static is saying something about the class' type: not only that "this type knows how to do this behavior", but also "it is the responsibility of this type to perform this behavior." And odds are the behavior no longer has any real relationship with the larger type.
That doesn't mean I wouldn't make it static at all, though. Ask yourself this: could the new method logically belong elsewhere? If you can answer "yes" to that, you probably do want to make it static (and move it as well). Even if that's not true, you could still make it static. Just don't mark it public.
As a matter of convenience, you could at least mark it internal. This typically avoids needing to move the method if you don't have easy access to a more appropriate type, but still leaves it accessible where needed in a way that it won't show up as part of the public interface to users of your class.
Not necessarily.
Moving public methods from static to non-static is a breaking change, and would require changes to all of your callers or consumers. If a method seems like an instance method, but happens to not use any instance members, I would suggest making it an instance method as a measure of future-proofing.
Yes. The reason "it can be static" is that it does not operate on the state of the object upon which it is called. Therefore it is not an instance method, but a class method. If it can do what it needs to do without ever accessing the data for the instance, then it should be static.
Yes, it should. There are various metrics of coupling that measure how your class depends on other things, like other classes, methods, etc. Making methods static is a way to keep the degree of coupling down, since you can be sure a static method does not reference any members.
I think it would make it a bit more readable if you marked it as static...Then someone who comes along would know that it doesn't reference any instance variables without having to read the entire function...
Personally, I'm a great fan of statelessness. Does your method need access to the state of the class? If the answer is no (and it is probably no, otherwise you wouldn't consider making it a static method), then yeah, go for it.
No access to state is less headache. Just as it is a good idea to hide private members that are not needed by other classes, it is a good idea to hide the state from members that don't need it. Reduced access can mean less bugs. Also, it makes threading easier as it is much easier to keep static members thread-safe. There is also a performance consideration as the runtime does not need to pass a reference to this as a parameter for static methods.
Of course the downside is that if you ever find that your previously static method will have to access the state for some reason, then you have to change it. Now I understand that this can be a problem for public APIs so if this is a public method in a public class, then perhaps you should think about the implications of this a bit. Still, I've never faced a situtation in the real world where this actually caused a problem, but maybe I'm just lucky.
So yeah, go for it, by all means.
Static methods are faster than the non-static ones so yes, they should be static if they can and there is no special reason for leaving them nonstatic.
I am surprised that so few are mentioning encapsulation here in fact. An instance method will automatically have access to all private (instance) fields, properties and methods. In addition to all protected ones inherited from base classes.
When you write code you should write it so that you expose as little as possible and also so that you have access to as little as possible.
So yes, it might be important to make your code fast which would happen if you're making your methods static, but usually more important then that is to make your code as incapable of creating bugs as possible too. One way to achieve that is to have your code have access to as little as possible of "private stuff".
This might seem irrelevant at first glance since the OP is obviously talking about refactoring which can not go wrong in this scenario and create any new bugs, however this refactored code must be maintained in the future and modified which makes your code have a bigger "attack surface" in regards to new bugs if it has access to private instance members. So in general I think the conclusion here is that "yes mostly your methods should be static" unless there are any other reasons for not having them static. And this simply because it's "better use of encapsulation and data hiding and creates 'safer' code"...
Making something static just because you can is not a good idea. Static methods should be static due to their design, not due to happenstance.
Like Michael said, changing this later will break code that's using it.
With that said, it sounds like you are creating a private utility function for the class that is, in fact, static by design.
If you were able to refactor a few lines out and the resulting method could be static, it is probably an indication that the lines you pulled out of that method don't belong in the containing class at all, and you should consider moving them into their own class.
It depends but generally I do not make those methods static. Code is always changing and perhaps someday I will want to make that function virtual and override it in a subclass. Or perhaps some day it will need to reference instance variables. It will be harder to make those changes if every call site has to be changed.
Personally I would have no choice but to make it static. Resharper issues a warning in this case and our PM has a rule "No warnings from the Resharper".
Inherently static methods that are for some reason made non-static are simply annoying. To wit:
I call my bank and ask for my balance.
They ask for my account number. Fair enough. Instance method.
I call my bank and ask for their mailing address.
They ask for my account number. WTF? Fail—should have been static method.
I suggest that the best way to think about it is this: If you need a class method that needs to be called when no instances of the class are instantioated, or maintains some kind of global state, then static is a good idea. But in general, I suggest you should prefer making members non-static.
You should think about your methods and classes:
How are you going to use them?
Do you need a lot of acces to them from different levels of your code?
Is this a method/class I can use in almost every thinkable project.
If the last two are 'yes', then your method/class should probably be static.
The most used example is probably the Math class. Every major OO language has it and all the methods are static. Because you need to be able to use them anywhere, anytime, without making an instance.
Another good example is the Reverse() method in C#.This is a static method in the Array class. It reverses the order of your array.
Code:
public static void Reverse(Array array)
It doesn't even return anything, your array is reversed, because all arrays are instances of the Array class.
As long as you make the new method private static it is not a breaking change. In fact, FxCop includes this guidance as one of its rules (http://msdn.microsoft.com/en-us/library/ms245046(VS.80).aspx), with the following information:
After you mark the methods as static, the compiler will emit non-virtual call sites to these members. Emitting non-virtual call sites will prevent a check at runtime for each call that ensures that the current object pointer is non-null. This can result in a measurable performance gain for performance-sensitive code. In some cases, the failure to access the current object instance represents a correctness issue.
That being said, the first comment from David Kean more succinctly summarizes the concerns by saying this is actually more about being correct than about the performance gain:
Although this rule is classified as a performance issue, the performance improvement of making a method static is only around 1%.
Rather, it is more a correctness issue that could indicate an either an incomplete or a bug in the member by its failure to use other instance members. Marking a method static (Shared in Visual Basic) makes it clear on its intention not to touch instance state.
I would definitely turn anything I can into static for a different reason:
Static functions, when JIT'd, are called without a "this" parameter.
That means, for example, that a 3 parameter non-static function (member method)
gets pushed with 4 params on the stack.
The same function compiled as a static function would get called with 3 parameters.
This can free up registers for the JIT and conserve stack space...
I'm in the "only make private methods static" camp. Making a public method can introduce coupling that you don't want and may decrease testability: You can't stub a public static method.
If you want to unit test a method that uses a public static method, you end up testing the static method as well, which might not be what you want.
I look at it generally from a functional perspective of pure functions. Does it need to be an instance method? If not, you may benefit from forcing the user to pass in the variables and not mangling the current instance's state. (Well, you could still mangle state, but the point is to, by design, not do so.) I generally design instance methods as public members and do my best to make private members static. If necessary (you can then more easily extract them into other classes later.
In those cases, i tend to move the method to a static or utils library, so i don't be mixing the concept of the "object" with the concept of "class"
I often have methods which are called regularly and have some "state" which has to be preserved between calls, as in:
float lastTime = 0.0f;
void Draw( float currentTime )
{
if( currentTime - lastTime > 0.5f )
{
// not enough FPS
}
lastTime = currentTime;
}
And it drives me nuts that the global "state" field (here "lastTime") is visible throughout the whole class while it is only needed in this one method.
Is there any way to limit this global field's visibility scope to only the method or to make it local, but keep it's value between calls?
The same question applies to fields used by Properties (which have some logic and can't be automatic).
Thanks in advance for any suggestion...
The only way to make it private in just a portion of the class would be to make it a separate object that managed this property, internal in the class.
However, I would recommend rethinking this a bit. The idea of privates it to keep a member private to the class itself - if you're worried about your class seeing its own data, it's time to refactor into smaller classes, each with their own function.
This has a "smell" of a class getting too large.
In C#, no. This is what static variables are in VB.NET (and I believe C, though I'm no expert). I am fairly certain, though, that static variables in VB.NET are just compiled to IL with autogenerated names as instance fields.
Just stick to instance fields that are appropriately named (like lastDrawTime for your example).
Nope, sorry. C# doesn't allow function-level statics like C does. I'd recommend giving it an "icky" name, like Draw_Data_lastTime or something. That at least conveys information about where it should be used.
No, the most restrictive scope is "private" which allows access from same type.
I'm afraid what your asking is not possible.
I think your only other option is to pass the current state as a parameter in the method but that means that the calling object would be responsible for managing the state.
I also think that you could refactor your code because if the rest of your class don't need to know about this variable maybe this part could be somewhere else.
No, there is no way; but your question raises some interesting issues. If you are having concerns about your member variable for tracking this is visible throughout the whole class, that raises flags that perhaps the scope of the class has grown too large, and that the class needs to be refactored into several smaller classes.
In general, if you start worrying about segregation of members within your classes, you might have a situation that requires refactoring into smaller classes. Then again, you might not (situations vary), but at the very least, it's probably worth considering.