I have a C# static class accessed from multiple threads. Two questions:
Are my private static fields thread safe when the field is initialized on declaration?
Should I lock when creating private static fields from within static constructor?
Usage of static class from different threads:
class Program
{
static void Main(string[] args)
{
for (int i = 0; i < 100; i++)
{
Task.Run(() =>
{
string name = MyStaticClass.GetValue(9555);
//...
});
}
}
}
Option 1 of static class:
public static class MyStaticClass
{
private static MyClass _myClass = new MyClass();
public static string GetValue(int key)
{
return _myClass.GetValue(key);
}
}
Option 2 of static class:
public static class MyStaticClass
{
private static MyClass _myClass;
private static object _lockObj = new object();
static MyStaticClass()
{
InitMyClass();
}
private static void InitMyClass()
{
if (_myClass == null)
{
lock(_lockObj)
{
if (_myClass == null)
{
_myClass = new MyClass();
}
}
}
}
public static string GetValue(int key)
{
return _myClass.GetValue(key);
}
}
Instance class created from the static class:
public class MyClass
{
private Dictionary<int, Guid> _valuesDict = new Dictionary<int, Guid>();
public MyClass()
{
for (int i = 0; i < 10000; i++)
{
_valuesDict.Add(i, Guid.NewGuid());
}
}
public string GetValue(int key)
{
if (_valuesDict.TryGetValue(key, out Guid value))
{
return value.ToString();
}
return string.Empty;
}
}
Should I lock when initializing private static fields from within static constructor?
Let's not bury the lede here:
Never lock in a static constructor. Static constructors are already locked by the framework so that they run on one thread exactly once.
This is a special case of a more general bit of good advice: never do anything fancy with threads in a static constructor. The fact that static constructors are effectively locked, and that lock can be contested by any code that accesses your type, means that you can very quickly get into deadlocks that you did not expect and are hard to see. I give an example here: https://ericlippert.com/2013/01/31/the-no-lock-deadlock/
If you want lazy initialization, use the Lazy<T> construct; it was written by experts who know how to make it safe.
Are my private static fields thread safe when the field is initialized on declaration?
Thread safety is the preservation of program invariants when program elements are called from multiple threads. You haven't said what your invariants are, so it is impossible to say if your program is "safe".
If the invariant you are worried about is that the static constructor is observed to run before the first static method is executed, or the first instance is created, of a type, C# guarantees that. Of course, if you write crazy code in your static constructor, then crazy things can happen, so again, try to keep your static constructors very simple.
fields of static class are not thread safe by default and should avoid unless it is just for read purpose.
Here down side is "lock" as well, it will create serialized processing in multi threaded environment.
public static class MyStaticClass
{
private static MyClass _myClass;
private static object _lockObj;
static MyStaticClass()
{
_myClass = new MyClass();
_lockObj = new object();
}
public static string GetValue(int key)
{
return _myClass.GetValue(key);
}
public static void SetValue(int key)
{
lock(_lockObj)
{
_myClass.SetValue(key);
}
}
}
Your second version is preferable. You can lock it down a little bit more by making your field readonly:
public static class MyStaticClass
{
private static readonly MyClass _myClass = new MyClass();
public static string GetValue(int key)
{
return _myClass.GetValue(key);
}
}
Your intent appears to be that _myClass is initially set to an instance of MyClass and never set to another. readonly accomplishes that by specifying that it can only be set once, either in a static constructor or by initializing it as above. Not only can another thread not set it, but any attempt to change it will result in a compiler error.
You could omit readonly and just never set _myClass again, but readonly both communicates and enforces your intent.
Here's where it gets trickier: Your reference to an instance of MyClass is thread safe. You don't have to worry about whether various threads will replace it with a different instance (or set it to null), and it will be instantiated before any threads attempt to interact with it.
What this does not do is make MyClass thread safe. Without knowing what it does or how you interact with it, there's no way for me to say what the needs or concerns are.
If that is a concern, one approach is to use a lock to prevent concurrent access that shouldn't occur, exactly as #Mahi1722 demonstrated. I'm including the code from that answer (not to plagiarize, but if anything happens to that answer then this one will refer to an answer that doesn't exist.)
public static class MyStaticClass
{
private static MyClass _myClass = new MyClass();
private static object _lockObj = new object();
public static string GetValue(int key)
{
return _myClass.GetValue(key);
}
public static void SetValue(int key)
{
lock(_lockObj)
{
_myClass.SetValue(key);
}
}
}
Both methods that interact with _myClass lock using _lockObject which means that any execution of either will block while another thread is executing either.
That's a valid approach. Another is to actually make MyClass thread safe, either by using concurrent collections or implementing such locks within that class. That way you don't have to use lock statements in every class that uses an instance of MyClass. You can just use it knowing that it manages that internally.
Both are correct,
but there is no need to lock inside static constructor.
So, i will choose the first option, it is shorter and clearer
Related
I have this class:
class MyFoo
{
private static readonly string _foo = InitFoo();
public static string Foo
{
get
{
return _foo;
}
}
private static string InitFoo()
{
Debug.WriteLine("InitFoo");
// do some job
return "Foo";
}
}
The private static _foo member is initialized only once when there is a reference to MyFoo.Foo.
The data returned from InitFoo() is large and the method might be time consuming (a matter of 1-2 seconds max), My question, is there a chance that while a thread is referencing MyFoo.Foo another thread that reference it will get an uncompleted or un-initialized data back b/c the InitFoo() is not complete yet?
In other words, Is the above thread-safe? if not how to make it thread-safe (if possible avoid a lock object?)
Thanks.
EDIT: following the comments about Lazy<T> is it now better for thread safety?:
public sealed class MyFoo
{
// Explicit static constructor to tell C# compiler not to mark type as beforefieldinit
static MyFoo() { }
private static readonly Lazy<string> _foo = InitFoo();
public static string Foo
{
get
{
return _foo.Value;
}
}
private static Lazy<string> InitFoo()
{
string s = "Foo";
return new Lazy<string>(() => s);
}
}
Is there a chance that while a thread is referencing MyFoo.Foo another thread that reference it will get an uncompleted or un-initialized data back b/c the InitFoo() is not complete yet?
No. Type initialization is thread-safe:
No other threads get to use your type while it's being initialized by another thread
All writes to memory performed by the initialization thread are made visible to other threads when the initialization has been performed
There's one wrinkle which is that if the same thread that's initializing MyFoo ends up reading MyFoo._foo before it's finished initializing, that will cause a problem. That can be particularly awkward to diagnose if there are types that depend on each other for initialization in a cycle.
Here's an example, with two type initializers that each use a value from the other. They both have static constructors to make the behavior deterministic. (The rules for when types are initialized depend on whether or not they have static constructors.)
using System;
public class Program
{
public static void Main(string[] args)
{
// Determine which type to initialize first based on whether there
// are any command line arguemnts.
if (args.Length > 0)
{
Class2.DoNothing();
}
Console.WriteLine($"Class1.Value1: {Class1.Value1}");
Console.WriteLine($"Class2.Value2: {Class2.Value2}");
}
}
public class Class1
{
public static readonly string Value1 =
$"When initializing Class1.Value1, Class2.Value2={Class2.Value2}";
static Class1() {}
}
public class Class2
{
public static readonly string Value2 =
$"When initializing Class2.Value2, Class2.Value2={Class1.Value1}";
static Class2() {}
public static void DoNothing() {}
}
Running this without any command line arguments, Class1 starts initializing first, which in turn initializes Class2:
Class1.Value1: When initializing Class1.Value1, Class2.Value2=When initializing Class2.Value2, Class2.Value2=
Class2.Value2: When initializing Class2.Value2, Class2.Value2=
With any command line argument, we initialize Class2 first, which in turn initializes Class1:
Class1.Value1: When initializing Class1.Value1, Class2.Value2=
Class2.Value2: When initializing Class2.Value2, Class2.Value2=When initializing Class1.Value1, Class2.Value2=
We need to use an unmanaged library in our code that has static methods. I'd like to introduce the library operation as a dependency in my code. And apart from having static methods, the library has an initialization method and a settings method, both are global. So I can't just wrap this in an instance class, because if one instance changes a setting, all other instances will be affected, and if one instance gets initialized, all other instances will be reinitialized.
I thought about introducing it as a singleton class. This way it will be in an instance class, but there will only be one instance thus I won't have to worry about changing the settings or initialization. What do you think about this approach? I'm pretty new to the dependency injection pattern and I'm not sure if the singleton pattern is a good solution? What would your solution be to a similar case?
Edit: The initialization takes a parameter too, so I can't just lock the method calls and re-initialize and change settings every time it is called.
Edit 2: Here are the signatures of some methods:
public static void Initialize(int someParameter)
// Parameter can only be changed by re-initalization which
// will reset all the settings back to their default values.
public static float[] Method1(int someNumber, float[] someArray)
public static void ChangeSetting(string settingName, int settingValue)
If you only need to set the settings once at start up, then I would recommend making a non-static wrapper class which does all the initialization of the static class in its own static constructor. That way you can be assured that it will only happen once:
public class MyWrapper
{
public MyWrapper()
{
// Do any necessary instance initialization here
}
static MyWrapper()
{
UnManagedStaticClass.Initialize();
UnManagedStaticClass.Settings = ...;
}
public void Method1()
{
UnManagedStaticClass.Method1();
}
}
However, if you need to change the settings each time you call it, and you want to make your instances thread-safe, then I would recommend locking on a static object so that you don't accidentally overwrite the static settings while they're still in use by another thread:
public class MyWrapper
{
public MyWrapper()
{
// Do any necessary instance initialization here
}
static MyWrapper()
{
UnManagedStaticClass.Initialize();
}
static object lockRoot = new Object();
public void Method1()
{
lock (lockRoot)
{
UnManagedStaticClass.Settings = ...;
UnManagedStaticClass.Method1();
}
}
}
If you need to pass initialization parameters into your class's instance constructor, then you could do that too by having a static flag field:
public class MyWrapper
{
public MyWrapper(InitParameters p)
{
lock (lockRoot)
{
if (!initialized)
{
UnManagedStaticClass.Initialize(p);
initialized = true;
}
}
}
static bool initialized = false;
static object lockRoot = new Object();
public void Method1()
{
lock (lockRoot)
{
UnManagedStaticClass.Settings = ...;
UnManagedStaticClass.Method1();
}
}
}
If you also need to re-initialize each time, but you are concerned about performance because re-initializing is too slow, then the only other option (outside of the dreaded singleton) is to auto-detect if you need to re-initialize and only do it when necessary. At least then, the only time it will happen is when two threads are using two different instances at the same time. You could do it like this:
public class MyWrapper
{
public MyWrapper(InitParameters initParameters, Settings settings)
{
this.initParameters = initParameters;
this.settings = settings;
}
private InitParameters initParameters;
private Settings settings;
static MyWrapper currentOwnerInstance;
static object lockRoot = new Object();
private void InitializeIfNecessary()
{
if (currentOwnerInstance != this)
{
currentOwnerInstance = this;
UnManagedStaticClass.Initialize(initParameters);
UnManagedStaticClass.Settings = settings;
}
}
public void Method1()
{
lock (lockRoot)
{
InitializeIfNecessary();
UnManagedStaticClass.Method1();
}
}
}
I would use a stateless service class, and pass in state info for the static class with each method call. Without knowing any details of you class, I'll just show another example of this with a c# static class.
public static class LegacyCode
{
public static void Initialize(int p1, string p2)
{
//some static state
}
public static void ChangeSettings(bool p3, double p4)
{
//some static state
}
public static void DoSomething(string someOtherParam)
{
//execute based on some static state
}
}
public class LegacyCodeFacadeService
{
public void PerformLegacyCodeActivity(LegacyCodeState state, LegacyCodeParams legacyParams)
{
lock (_lockObject)
{
LegacyCode.Initialize(state.P1, state.P2);
LegacyCode.ChangeSettings(state.P3, state.P4);
LegacyCode.DoSomething(legacyParams.SomeOtherParam);
//do something to reset state, perhaps
}
}
}
You'll have to fill in the blanks a little bit, but hopefully you get the idea. The point is to set state on the static object for the minimum amount of time needed, and lock access to it that entire time, so no other callers can be affected by your global state change. You must create new instances of this class to use it, so it is fully injectable and testable (except the step of extracting an interface, which I skipped for brevity).
There are a lot of options in implementation here. For example, if you have to change LegacyCodeState a lot, but only to a small number of specific states, you could have overloads that do the work of managing those states.
EDIT
This is preferable to a singleton in a lot of ways, most importantly that you won't be able to accumulate and couple to global state: this turns global state in to non-global state if it is the only entry point to your static class. However, in case you do end up needing a singleton, you can make it easy to switch by encapsulating the constructor here.
public class LegacyCodeFacadeService
{
private LegacyCodeFacadeService() { }
public static LegacyCodeFacadeService GetInstance()
{
//now we can change lifestyle management strategies later, if needed
return new LegacyCodeFacadeService();
}
public void PerformLegacyCodeActivity(LegacyCodeState state, LegacyCodeParams legacyParams)
{
lock (_lockObject)
{
LegacyCode.Initialize(state.P1, state.P2);
LegacyCode.ChangeSettings(state.P3, state.P4);
LegacyCode.DoSomething(legacyParams.SomeOtherParam);
//do something to reset state, perhaps
}
}
}
Does it make sense to make a System.Timers.Timer member of a singleton volatile static?
Would it make any difference if I make the _fooTimer static and or volatile in singleton instance context?
Would it make any difference if I dont make _instance static?
EDIT2: I corrected the codesample and made it now ba better Singleton without unecessary static or volatile fields and changed to Interlock.Increment
public sealed class Foo
{
private static readonly object _syncRoot;
private int _counter;
private Timer _fooTimer;
private static Foo _instance;
private Foo()
{
_counter = 0;
_syncRoot = new object();
_fooTimer = new new Timer();
_fooTimer.Intervall = 3600000;
_fooTimer.Elapsed += new ElapsedEventHandler(LogFoo);
}
public static Foo Instance
{
get
{
lock(_syncRoot)
{
if (_instance == null)
{
_instance = new Foo();
}
}
return _instance;
}
}
private void LogFoo()
{
// write a logfile with _counter - then restart timer and set _counter to 0
}
public void Increment()
{
Interlocked.Increment(_counter);
}
}
public class UseTheFoo
{
// Foo.Instance.Increment()
...
}
Typically the only static variable in a singleton class is a reference to the single instance. You'd then use instance variables for the remaining state of the type. If you make it static then you don't even need to create a single instance of the class to use the timer - but I would expect that you'd want to do so anyway.
I'd be nervous of using volatile, too... it almost certainly doesn't mean exactly what you think it means. I'd probably use Interlocked instead to achieve atomic updates to the variable.
(Note that there are plenty of better ways of implementing it, as per my article on the topic.)
EDIT: Now that the code has changed to show more members, it's a bit confusing. There's a static method which would use _counter (an instance variable) - presumably via the singleton instance. Basically, the class doesn't seem to have made up its mind about whether it wants to be a bunch of static methods, or a singleton instance. I suggest you decide and make everything accessible one way or the other, but not a mixture.
By making the whole class static, you can push any thread synchronization worries during the creation of the instance to the CLR:
public static class Foo
{
private static Timer _fooTimer;
static Foo()
{
_fooTimer = new Timer();
}
}
public class MyClass<T>
{
public static readonly String MyStringValue;
static MyClass()
{
MyStringValue = GenerateString();
}
private static String GenerateString()
{
//Dynamically generated ONCE per type (hence, not const)
}
public void Foo()
{
Console.WriteLine(MyStringValue);
}
}
It's my understanding that the static readonly String won't get generated until the static constructor is called on the class. But, the static constructor won't be called until one of the static methods or variables is accessed.
In a multi-threaded environment is it possible to run into issues because of this? Basically, is the static constructor by default singleton locked or do I have to do this myself? That is... do I have to do the following:
private static Object MyLock;
static MyClass()
{
lock(MyLock)
{
if (MyStringValue == null)
MyStringValue = GenerateString();
}
}
The static constructor is guaranteed to run only once per instantiated type. So you don't need your locking.
Note that it will run once for each generic parameter. And the static fields on the generic class aren't shared between different generic parameters either.
To avoid this why not make the value a static property with only a get accessor which returned the cached value, which you could then make private? Accessing the property get would assure the static constructor ran first.
I know there are lot of ways to implement a thread safe singleton pattern like (Double Check Locking , static readonly method, lock method) but i just tried below code
static void Main(string[] args)
{
for (int i = 0; i <= 100; i++)
{
Thread t = new Thread(new ParameterizedThreadStart(doSome));
t.Start(null);
}
Console.ReadLine();
}
private static void doSome(object obj)
{
MyReadOnly obj1 = MyReadOnly.getInstance;
Console.WriteLine(obj1.GetHashCode().ToString());
}
class MyReadOnly
{
private static MyReadOnly instance = new MyReadOnly();
int counter = 0;
// static MyReadOnly()
// {
// } treat is as commented code.
public static MyReadOnly getInstance { get { return instance; } }
private MyReadOnly()
{
Console.WriteLine((++counter).ToString());
}
}
when i see the output of this program , i see just single object created (because of same hashcode)
how to prove that this code is not thread safe?
EDIT
removing static constructor which causes some confusion
That code is thread-safe due to the nature of type initializers in .NET. The type initializer is guaranteed to run exactly once, and if two threads try to run it at the same time, one will do so and the other will block.
See my article on singleton implementation for more details.
This is actually thread-safe code because you are (indirectly) using static constructor to create the instance (And CLR guarantees invocation of static constructor is thread-safe manner on/before access to any other type member).