We have a Static field in a abstract class
ABSTRACT CLASS :-
public abstract class BaseController
{
private static string a;
private static string b;
protected abstract SomeArray[] DoSomeThing();
}
And a derived class
public class Controller1:BaseController
{
protected override SomeArray[] DoSomthing()
{
//////
In this method we are setting the static variables with different values
}
}
We also have a class which is starting threads
public class SomeClass
{
public SomeClass()
{
\\ we are setting the controller we want to call, i mean to decide which base class to call
}
public void Run()
{
Thread t = new Thread(StartThread);
t.Start();
}
public void StartThread()
{
_controller.DoSomeThing();
}
}
All the above service is in a WCF service, and the same client tries to call multiple times which means we have multiple threads running at the same time, we have seen issues where the static variable which we are setting and using that for some DB update process is sometimes having wrong values
I have read some blogs which says static fields are not thread safe, Can some body please help me understand what could be going wrrong in our code and why we are having some incorrect values passed to the DB.
By definition, a static field or member is shared among all instances of that class, only one instance of that field or member exisits; thus, it is not thread safe.
You have two options, either to synchronize the access (basically, using Monitor class) to that field or to use the ThreadStaticAttribute on that field.
However, i advice to reorganize your class hierarchy so that each class has its own instance of the field or member.
Please note that if there are multiple threads working on the same instance of the class Controller, then we go back to the same problem and you should synchronize access to that instance field.
Good Luck
Related
Someone once wrote:
The space required for an instance depends only on the fields.
The methods require memory too but only one time per class. Like static fields. That memory is allocated when the class is loaded.
But what happens if a class with say like 5 methods and no fields get multiple instances in fields of other classes(composition).
Do they require more memory? Or would it be the same as static methods?
I do ask this question also because maybe it even gets optimised when compiling?
Is there a differents to static class with static methods? Other than u need to create the class each time or pass it around?
Eg.:
class Test1
{
public void DoThis()
{
...
}
public void DoThat()
{
...
}
}
class Test2
{
public void DoSomething()
{
...
}
private Test1 sample = new Test1();
}
class Test3
{
public void DoSomethingElse()
{
...
}
private Test1 sample = new Test1();
}
And so on...
"Behind the scenes", a class method is just like a static method, with the class instance beeing passes by reference as the first parameter.
That is, unless you use virtual methds, which "behind the scenes" are saved as instance members.
That is, because as long as you don't override a method, there is simply no reason to waste an instance's space.
Therefore, the size of both your class instances won't be affected by any non-virtual method you add to the class.
This concept can change between programming languages tho. For example, in Java and Python class methods are virtual by default.
We all know that we cannot create object of class having private constructor. So the question arises is how many instances of this class can be created .Please find a sample code below.
public class Test
{
public int val ;
private Test(int sent)
{
val=val +sent;
}
public static void Callme(int GetVal)
{
Test obj=new Test(GetVal);
Console.WriteLine(obj.val);
}
}
public class Program
{
public static void Main()
{
Test.Callme(10);
//Console.WriteLine(Test.val);
Test.Callme(20);
//Console.WriteLine(Test.val);
}
}
As per what I know It should create 2 object of the class. Need help understanding this.
We all know that we cannot create object of class having private constructor.
Well, that's not accurate. You can create an object (instance) of a class having only private constructors by using static members of that class, just like in the code in the question.
What you can't do is create an instances of that class from anywhere else in the code.
how many instances of this class can be created
In your code sample there are two instances of class Test.
I think what might be confusing you is you expected the second Console.WriteLine to print 30, but it printed 20. That is because public int val ; is an instance member. If it was a static member, than it would have printed 30
Maybe something like this is what you're looking for:
public static Test Callme(int GetVal)
{
Test obj = new Test(GetVal);
Console.WriteLine(obj.val);
return obj;
}
And then create new instances like:
Test test1 = Test.Callme(10);
Test test2 = Test.Callme(20);
This way you can easily access the members of each instance. E.g. test1.val
Callme method is a static method. Static methods does not require an objects instance to be called upon. They don't have the this (keyword) reference and can be called directly on the class. In your situation Test.CallMe(someValue). Note that there is no object instance involved here.
If CallMe was NOT a static method you would have needed an instance/object to call it. For example
Test ob = new Test();
ob.CallMe(someValue);
What your example illustrates is the use of private fields/methods.
When a method like the constructor or a filed is marked with the private keyword that method/field can only be called/accessed from within the declaring class.
This means that CallMe can access the constructor because CallMe is a member of the class and the constructor is a member of the class thus they both can access each other.
When a class has only one constructor and that constructor is private it effectively means that an instance of the class can only be created from within the class.
So in current example CallMe creates an instance of the class each time it's called.
If you call CallMe 2 times you'll create 2 instances of the class.
Because the method Callme is static it is instantiated by the system at some point before it is used and then remains in memory for future calls. There is only one copy of a static memeber of a class ever created regardless of how many instances of the class are created.
My question may sound like many others here but it has a flavor I didn't find.
I am trying to understand the following logic
A generic object
public class GenericClass
{
public static void DoSomething(Object lockObj)
{
lock(lockObj)
{
// do something here
}
}
}
Class A
internal class A
{
private static _aLock = new Object();
public void Do_A_Thing()
{
GenericClass.DoSomething(_aLock);
}
}
Class B
internal class B
{
private static _bLock = new Object();
public void Do_B_Thing()
{
GenericClass.DoSomething(_bLock);
}
}
I just hope to confirm if my explanation is correct:
If multiple threads of class "A" will attempt simultaneously access code in "genericClass" method "DoSomething", this method will be locked to all but one instance of class "A". But a single instance of class "B" will be able to proceed with execution any time. If class "B" will also have multiple instances execute, they will not interfere with class "A" locks.
Is this correct based on what you see above?
Yes, your description sounds correct. It is perhaps a little unusual to pass the lock object in, but it'll work fine. The only change I would suggest is to make the static fields readonly so you can't accidentally change the value to a different object reference.
Your conclusion is correct but it is not a good practice to pass locked object around. I suggest to put the lock inside class A and B respectively.
I suggest to write:
internal class A
{
private static readonly _aLock = new Object();
public void Do_A_Thing()
{
lock (_aLock)
{
GenericClass.DoSomething();
}
}
}
Do you have a specific reason to put the lock in another class? Maybe you can solve your problem in a different way?
Also keep in mind that in some conditions, maybe it is not your case, you can have a deadlock if class A and B call each other (A->B->A).
Yes, that is correct. The locks in A and the locks in B are completely unaware of each other. The code will only be blocked when there is another thread locking it with the same object as identifier.
If you are using generics, then something like
public class MyGadget<T>
{
static readonly SyncRoot = new object() ;
public T SynchronizedMethod()
{
lock ( SyncRoot )
{
SynchronizedMethodGuts() ;
}
}
}
should do what you want because MyGadget<Foo> and MyGadget<Bar> are different classes: they each have their own, different SyncRoot field.
I have done a lot of reading on instance vs. static classes and have not found an answer to my question. Are there any perils to instancing a different class in a static class that was referenced by an instance class?
The current design I am working with is one in which instance classes call a static "Logger" method (passing a series of parameters) to log errors to a text file in the file system. I am refactoring the static "Logger" method to instantiate a parameter class (which is just a series of properties and a few helper methods to return itself as XML or a string) and a DBLogger class to log the error to the database rather than the file system, passing the parameter class as the sole parameter.
This model worked well in my legacy VB6 code, in which the Logger class was instanced, not static.
But now in the .NET code I am not sure if I should make my 2 new classes (parameter and DBLogger) static, or just make the DBLogger static and instance the parameter class. I am concerned about the potential for concurrency/multi-thread data issues with (or without) instances being created from a static class. Am I right to be concerned or am I worrying about nothing?
using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
// all code truncated for illustration purposes
namespace ThisIs.A.Test
{
//INSTANCE
public class ErrorLogParameters
{
private int mThreadId = 0;
private int mErrorNumber = 0;
private string mServerDate = DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff");
public int ThreadId
{
get { return mThreadId; }
set { mThreadId = value; }
}
public int ErrorNumber
{
get { return mErrorNumber; }
set { mErrorNumber = value; }
}
public string ServerDate
{
get { return mServerDate; }
}
}
//INSTANCE
public class ErrorLog
{
public void LogErrorToDatabase(ErrorLogParameters criteria)
{
//Log error to database here
}
}
//STATIC - Instantiates INSTANCE of ErrorLogParameters and ErrorLog
public class Logger
{
public static void WriteLog(string pstrObjectName, string pstrProcedureName, int plngErrNumber, string pstrErrDescription)
{
// create a new parameter object
ErrorLogParameters objParameters = new ErrorLogParameters();
// populate object properties
objParameters.ErrorNumber = mlngErrNumber;
objParameters.ThreadId = System.Threading.Thread.CurrentThread.ManagedThreadId;
ErrorLog objErrorLog = new ErrorLog();
objErrorLog.LogErrorToDatabase(objParameters);
}
}
//INSTANCE - Invokes STATIC method
public class SomeInstance
{
private void ErrorHandler_Log(Exception exception, string procedureName, string additonalDescription, string stackTrace)
{
// call from instance class to static class
Logger.WriteLog(mstrObjectName, procedureName, mlngErrNumber, mstrErrDescription);
}
}
}
No, that's absolutely fine - if you're creating an instance of any class within a method, it doesn't matter whether the class declaring that method is a static class or not.
Furthermore, unless you've got something "special" (e.g. a static variable counting the number of instances created) you're less likely to run into concurrency issues when creating new objects than when using existing objects. Basically, the tricky part of almost all concurrency is working out where mutable data is shared - it doesn't sound like you've got any here (although sample code would help to clarify that).
I would use a combination of the provider and singleton pattern for this.
Create an abstract class called Logger.
The Logger class contains abstract methods for writing to log. For example:
abstract void LogInfo(LogInfo info);
abstract void LogError(Exception exception);
etc
The Logger class contains a private instance of Logger object.
The Logger class contains a static property that returns the private instance.
The Logger class contains a static constructor that instantiate the private instance of Logger object. You would probably use Reflection and instantiate the object based on the configuration.
Implement a FileLogger that inherits from the Logger object. This logger writes to a file.
Implement a SQLLogger that inherits from the Logger object. This logger writes to a database.
Call the logger like so:
Logger.Instance.WriteInfo(info);
Logger.Instance.WriteError(exception);
There are a few advantages of using this design:
Your logging functionality is fully abstracted. This completely decouple the logging callers from the code that writes the logs. This allows you to write the log to any data stores.
You can change which logger to use without compiling the code. Just update the config file.
Singleton guarantees thread-safety
Testability. You can write Mock tests against abstract classes.
Hope this helps.
There are no concurrency issues with static methods.
Static variables are a different matter.
A colleague of mine told me that I should never use static variables because if you change them in one place, they are changed everywhere. He told me that instead of using static variables I should use Singleton.
I know that Singleton is for limitation of the number of instances of one class to one.
How can Singleton help me with static variables?
Let's address your statements one at a time:
A colleague of mine told me that I should never use static variables because if you change them in one place, they are changed everywhere.
It seems fairly clear that your colleague means the basic feature of static variables: there is only one instance of a static variable. No matter how many instances of any class you create, any access to a static variable is to the same variable. There is not a separate variable for each instance.
He told me that instead of using static variables I should use Singleton.
This is not good global advice. Static variables and singletons aren't in competition with each other and aren't really substitutes for each other. A singleton is an instance of a class, managed in such a way that only one instance is possible to create. A static variable is similarly tied to exactly one (static) instance of a class, but could be assigned with not only a class instance but any data type such as a scalar. In actuality, to effectively use the singleton pattern, you must store it in a static variable. There is no way to "use a singleton instead of a static variable".
On the other hand, perhaps he meant something slightly different: perhaps he was trying to say that instead of your static class having many different static variables, methods, properties, and fields (altogether, members) that function as if they were a class, you should make those fields non-static, and then expose the wrapping class as a Singleton instance. You would still need a private static field with a method or property (or perhaps just use a get-only property) to expose the singleton.
I know that Singleton is for limitation of the number of instances of one class to one. How can Singleton help me with static variables?
A static class's variables and a singleton are alike in that they both are instantiated once (the former enforced by the compiler and the latter enforced by your implementation). The reason you'd want to use a singleton instead of a static variable inside of a class is when your singleton needs to be a true instance of a class, and not consist simply of the collected static members of a static class. This singleton then gets assigned to a static variable so that all callers can acquire a copy of that same instance. As I said above, you can convert all the different static members of your static class to be instance members of your new non-static class which you will expose as a singleton.
I would also like to mention that the other answers given so far all have issues around thread safety. Below are some correct patterns for managing Singletons.
Below, you can see that an instance of the Singleton class, which has instance (or non-static) members, is created either by static initialization or within the static constructor, and is assigned to the variable _singleton.. We use this pattern to ensure that it is instantiated only once. Then, the static method Instance provides read-only access to the backing field variable, which contains our one, and only one, instance of Singleton.
public class Singleton {
// static members
private static readonly Singleton _singleton = new Singleton();
public static Singleton Instance => _singleton
// instance members
private Singleton() { } // private so no one else can accidentally create an instance
public string Gorp { get; set; }
}
or, the exact same thing but with an explicit static constructor:
public class Singleton {
// static members
private static readonly Singleton _singleton; // instead of here, you can...
static Singleton() {
_singleton = new Singleton(); // do it here
}
public static Singleton Instance => _singleton;
// instance members
private Singleton() { } // private so no one else can accidentally create an instance
public string Gorp { get; set; }
}
You could also use a property default without an explicit backing field (below) or in a static constructor can assign the get-only property (not shown).
public class Singleton {
// static members
public static Singleton Instance { get; } = new Singleton();
// instance members
private Singleton() { } // private so no one else can accidentally create an instance
public string Gorp { get; set; }
}
Since static constructors are guaranteed to run exactly once, whether implicit or explicit, then there are no thread safety issues. Note that any access to the Singleton class can trigger static initialization, even reflection-type access.
You can think of static members of a class as almost like a separate, though conjoined, class:
Instance (non-static) members function like a normal class. They don't live until you perform new Class() on them. Each time you do new, you get a new instance. Instance members have access to all static members, including private members (in the same class).
Static members are like members of a separate, special instance of the class that you cannot explicitly create using new. Inside this class, only static members can be accessed or set. There is an implicit or explicit static constructor which .Net runs at the time of first access (just like the class instance, only you don't explicitly create it, it's created when needed). Static members of a class can be accessed by any other class at any time, in or out of an instance, though respecting access modifiers such as internal or private.
EDIT #ErikE's response is the correct approach.
For thread safety, the field should be initialized thusly:
private static readonly Singleton instance = new Singleton();
One way to use a singleton (lifted from http://msdn.microsoft.com/en-us/library/ff650316.aspx)
using System;
public class Singleton
{
private static Singleton instance;
private Singleton() {}
public static Singleton Instance
{
get
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
/// non-static members
public string Foo { get; set; }
}
Then,
var foo = Singleton.Instance.Foo;
Singleton.Instance.Foo = "Potential thread collision here.";
Note that the instance member is a static field. You can't implement a singleton without using a static variable, and (I seem to recall - it's been awhile) this instance will be shared across all requests. Because of that, it's inherently not thread safe.
Instead, consider putting these values in a database or other persistent store that's more thread-friendly, and creating a class that interfaces with that portion of your database to provide transparent access.
public static class Foo
{
public static string Bar
{
get { /// retrieve Bar from the db }
set { /// update Bar in the db }
}
}
The whole point of the static modifier is to ensure that the object thus modified is the same wherever it is used as it requires no instantiation. The name originally came about as a static variable has a fixed location in memory and all items referring to it will reference the same memory location.
Now you may wish to use a static field within a class, in which case it exists before the class is instantiated (constructed). There may be instances where you would want this.
A singleton is a different beast. It is a class that is limited to a single instantiation by use of a private constructor and a static property. So in that regard you still can't avoid statics by using a singleton.
To answer the stated question:
It is incredibly stupid (but possible) to create a singleton without a static field.
To do it, you need to use someone else's static field, such as AppDomain.GetData or (in ASP.Net) HttpContext.Application.
Just to answer your question (I hope): Instead of using a static class containing static members like Class1 you can implement the Singleton pattern like in Class2 (please don't begin a discussion about lazy initialization at this point):
public static class Class1
{
public static void DoSomething ()
{
}
}
public static class Class2
{
private Class2() {
}
private Class2 instance;
public Class2 GetInstance(){
if (instance == null)
instance = new Class2();
return instance;
}
public void DoSomething ()
{
}
}
Instead of calling Class1.DoSomething() you can use Class2.GetInstance().DoSomething().
Edit: As you can see there's still a (private) static field inside Class2 holding it's instance.
Edit2 in answer to user966638's comment:
Do I understand you correct that you have code like this:
public class Foo {
private static Bar bar;
}
And your collegue suggests to replace it by this?
public class Foo {
private BarSingleton bar;
}
This could be the case if you want to have different Foo instances where each instance's bar attribute could be set to null for example. But I'm not sure if he meant this what exactly is the use case he is talking about.
Both singleton and static variables give you one instance of a class. Why you should prefer singleton over static is
With Singleton you can manage the lifetime of the instance yourself, they way you want
With Singleton, you have greater control over the initialization of the instance. This useful when initializing an instance of a class is complicated affair.
It's challenging to make static variables thread-safe, with singleton, that task becomes very easy
Hope this helps