I'm implementing a singleton pattern, and need the initialization to be thread safe.
I've seen several ways to do it, like using the double check lock implementation, or other techniques (i.e.: http://csharpindepth.com/articles/general/singleton.aspx)
I wanted to know if the following approach, which is similar to the fourth version in the article, is thread safe. I'm basically calling a method in the static field initializer, which creates the instance. I don't care about the lazyness. Thanks!
public static class SharedTracerMock
{
private static Mock<ITracer> tracerMock = CreateTracerMock();
private static Mock<ITracer> CreateTracerMock()
{
tracerMock = new Mock<ITracer>();
return tracerMock;
}
public static Mock<ITracer> TracerMock
{
get
{
return tracerMock;
}
}
}
Yes, that's thread-safe - although it's not the normal singleton pattern, as there are no instances of your class itself. It's more of a "single-value factory pattern". The class will be initialized exactly once (assuming nothing calls the type initializer with reflection) and while it's being initialized in one thread, any other thread requesting TracerMock will have to wait.
Your code can also be simplified by removing the method though:
public static class SharedTracerMock
{
private static readonly Mock<ITracer> tracerMock = new Mock<ITracer>();
public static Mock<ITracer> TracerMock { get { return tracerMock; } }
}
Note that I've made the field readonly as well, which helps in terms of clarity. I generally stick trivial getters all on one line like this too, to avoid the bulk of lots of lines with just braces on (7 lines of code for one return statement feels like overkill).
In C# 6, this can be simplified even more using a readonly automatically implemented property:
public static class SharedTracerMock
{
public static Mock<ITracer> TracerMock { get; } = new Mock<ITracer>();
}
Of course, just because this property is thread-safe doesn't mean that the object it returns a reference to will be thread-safe... without knowing about Mock<T>, we can't really tell that.
Related
I am working on some code which is something like this:
class A
{
static SomeClass a = new Someclass("asfae");
}
Someclass contains the required constructor.
The code for this compiles fine without any warning. But I get a code hazard in system:
"The Someclass ctor has been called from static constructor and/or
static initialiser"
This code hazard part of system just to make it better by warning about possible flaws in the system or if system can get into bad state because of this.
I read somewhere on the web that static constructor/initialiser can get into deadlock in c# if they wait for a thread to finish. Does that have something to do with this?
I need to get rid of this warning how can i do this.
I can't make the member unstatic as it's used by a static function.
What should I do in this case , Need help.
You could hide it behind a property and initialize it on first use (not thread-safe);
class A
{
static SomeClass aField;
static SomeClass aProperty
{
get
{
if (aField == null) { aField = new Someclass("asfae"); }
return aField;
}
}
}
or use Lazy (thread-safe):
class A
{
static Lazy<SomeClass> a = new Lazy<SomeClass>(() => new Someclass("asfae"));
}
...or this very verbose thread safe version :)
class A
{
static SomeClass aField;
static object aFieldLock = new object();
static SomeClass aProperty
{
get
{
lock (aFieldLock)
{
if (aField == null) { aField = new Someclass("asfae"); }
return aField;
}
}
}
}
By initialising it as a static field, it behaves as it would in a static constructor, i.e. it probably gets initialised the first time an instance of your class is instantiated, but might happen earlier. If you want more control over exactly when the field is initialised, you could use Lazy<T>, e.g.:
{
static Lazy<SomeClass> a = new Lazy<SomeClass>(() => new Someclass("asfae"));
}
This way, you know that the initialisation of SomeClass will only happen the first time the field is accessed and its Value property called.
I think to understand your problem you need to know the difference between static constructors and type initializers, there is a great article from Jon Skeet about this issue:
http://csharpindepth.com/Articles/General/Beforefieldinit.aspx
The point is that following constructions are not the same, and there are difference in the behavior:
class Test
{
static object o = new object();
}
class Test
{
static object o;
static Test()
{
o = new object();
}
}
In any case, you could try to create a static constructor for your class to be able to have more control on this initialization, and maybe the warning will disappear.
If the member is only used by a static method, and only by this one, I would recommend you to put it in the scope if this static method and not as class member.
public class MyConfigurationData
{
public double[] Data1 { get; set; }
public double[] Data2 { get; set; }
}
public class MyClass
{
private static object SyncObject = new object();
private static MyConfigurationData = null;
private static MyClass()
{
lock(SyncObject)
{
//Initialize Configuration Data
//This operation is bit slow as it needs to query the DB to retreive configuration data
}
}
public static MyMethodWhichNeedsConfigurationData()
{
lock(SyncObject)
{
//Multilple threads can call this method
//I lock only to an extent where I attempt to read the configuration data
}
}
}
In my application I need to create the configuration data only once and use it several multiple times. In other words, I write once and read many times. And also, I wanted to ensure that read should not happen till write operation is finished. In other words, I don't want to read MyConfigurationData as NULL.
What I know is the static constructor is called only once in an AppDomain. But, while I am preparing the configuration data, if any thread tries to read this data how would I ensure synchronization effectivey? In the end, I wanted to improve the performance of my read operation.
Can I implement my objective in a lock-free manner?
From MSDN:
A static constructor is used to initialize any static data, or to perform a particular action that needs performed once only. It is called automatically before the first instance is created or any static members are referenced.
So you don't need to use lock in your code, it is actually thread-safe. Your static constructor is called before MyMethodWhichNeedsConfigurationData is referenced.
public class MyClass
{
private static MyConfigurationData = null;
private static MyClass()
{
}
public static MyMethodWhichNeedsConfigurationData()
{
}
}
As long as you are only ever reading the data, it should already be thread-safe. Very few data-structures are not thread-safe when just reading (the obvious counter-examples might include lazy loading). Note that the static constructor is automatically synchronized by the runtime, so you don't need to concern yourself with multiple threads running the "Initialize Configuration Data" step.
So: as long as nothing ever mutates the data, you are already safe. You could also make it harder to get wrong by hiding the data behind an immutable interface, i.e.
public class ConfigurationData {
// or some similar immutable API...
public double GetData1(int index) { return data1[index]; }
public double GetData2(int index) { return data2[index]; }
private readonly double[] data1, data2;
public ConfigurationData(double[] data1, double[] data2) {
this.data1 = data1;
this.data2 = data2;
}
}
Then you don't need any locks:
public class MyClass
{
private static MyConfigurationData;
private static MyClass()
{
//Initialize Configuration Data
MyConfigurationData = ...
//This operation is bit slow as it needs to query the DB to retreive configuration data
}
public static MyMethodWhichNeedsConfigurationData()
{ //Multilple threads can call this method
var config = MyConfigurationData;
}
}
Note that removing the locks improves parallelism; it doesn't change raw single-threaded performance.
That said: I should advise against static data generally; it makes it very hard to test, and makes it tricky to do things like multi-tenancy if your needs change. It may be more prudent to have a single configuration instance, but pass it into the system as some form of context. Either approach can be used successfully, though - this is just something to be aware of.
I think you should use Singleton pattern and put your configuration initialization logic in "GetInstance" method which would return the instance of your class.
This way you would not need any locking mechanism for Read.
My question comes after refactoring a class that contained only static methods to be declared as a static class, and experiencing weird issues when starting the application.
I have not performed any thorough investigation but it seems that some call being made from within the static constructor does not complete for some reason.
So, I would like to know where there are any pitfalls when using static constructors in C#? More specifically, are there any things that should be avoided at all cost and not be used from within the static constructor?
There are several pitfalls to static constructors. For example, if a static constructor throws an exception, you would continue getting a TypeInitializationException whenever you access any of its members.
If a static constructor throws an exception, the runtime will not invoke it a second time, and the type will remain uninitialized for the lifetime of the application domain in which your program is running.
In general, static classes should only be used in stateless scenarios where you won’t need any initialization. If your class needs to be initialized, you might be better off using the singleton pattern, which can be lazily initialized on first access:
public class MyClass
{
private static readonly Lazy<MyClass> current =
new Lazy<MyClass>(() => new MyClass());
public static MyClass Current
{
get { return current.Value; }
}
private MyClass()
{
// Initialization goes here.
}
public void Foo()
{
// ...
}
public void Bar()
{
// ...
}
}
static void Main(string[] args)
{
MyClass.Current.Foo(); // Initialization only performed here.
MyClass.Current.Bar();
MyClass.Current.Foo();
}
Edit: I did some further reading up on the matter, and it appears that static constructors do cause deadlocks if you perform blocking operations (e.g. asynchronous callbacks or thread synchronization) within them.
The CLR internally uses locking to prevent type initializers (static constructors) from being executed multiple times concurrently. Thus, if your static constructor attempts to access another member of its declaring type from another thread, it would inevitably deadlock. Since “another member” could be an anonymous function declared as part of a PLINQ or TPL operation, these bugs can be subtle and hard to identify.
Igor Ostrovsky (MSFT) explains this in his Static constructor deadlocks article, providing the following example of a deadlock:
using System.Threading;
class MyClass
{
static void Main() { /* Won’t run... the static constructor deadlocks */ }
static MyClass()
{
Thread thread = new Thread(arg => { });
thread.Start();
thread.Join();
}
}
In the above example, the new thread needs to access the empty anonymous function, { }, defined as its callback. However, since the anonymous function is compiled as another private method of MyClass behind the scenes, the new thread cannot access it before the MyClass type initializes. And, since the MyClass static constructor needs to wait for the new thread to complete first (because of thread.Join()), a deadlock ensues.
Yes, there are some pitfalls, mostly related to when the class is initialized. Basically, a class with a static constructor will not be marked with the beforefieldinit flag, which allows the runtime to initialize it at a later time.
Have a look at this article for more details.
This is not an answer to the question, but it's too long for a comment, so I offer it here...
Since I didn't know for static class construct, I have used following scheme (simplified) to provide me with singletons:
public class SomeSingleton {
static _instance;
static public SomeSingleton Instance {
get {
if (_instance==null) {
_instance=new SomeSingleton();
}
return _instance;
}
}
}
Later, you use
SomeSingleton.Instance.MyProp = 3;
And first usage of the Instance member will construct your singleton.
I guess that it is OK since instantiation of the singletons if there are many of such classes is done in proper order.
Okay, newbie multi-threading question:
I have a Singleton class. The class has a Static List and essentially works like this:
class MyClass {
private static MyClass _instance;
private static List<string> _list;
private static bool IsRecording;
public static void StartRecording() {
_list = new List<string>();
IsRecording = true;
}
public static IEnumerable<string> StopRecording() {
IsRecording = false;
return new List<string>(_list).AsReadOnly();
}
public MyClass GetInstance(){
}
public void DoSomething(){
if(IsRecording) _list.Add("Something");
}
}
Basically a user can call StartRecording() to initialize a List and then all calls to an instance-method may add stuff to the list. However, multiple threads may hold an instance to MyClass, so multiple threads may add entries to the list.
However, both list creation and reading are single operations, so the usual Reader-Writer Problem in multi-threading situations does not apply. The only problem I could see is the insertion order being weird, but that is not a problem.
Can I leave the code as-is, or do I need to take any precautions for multi-threading? I should add that in the real application this is not a List of strings but a List of Custom Objects (so the code is _list.Add(new Object(somedata))), but these objects only hold data, no code besides a call to DateTime.Now.
Edit: Clarifications following some answers: DoSomething cannot be static (the class here is abbreviated, there is a lot of stuff going on that is using instance-variables, but these created by the constructor and then only read).
Is it good enough to do
lock(_list){
_list.Add(something);
}
and
lock(_list){
return new List<string>(_list).AsReadOnly();
}
or do I need some deeper magic?
You certainly must lock the _list. And since you are creating multiple instances for _list you can not lock on _list itself but you should use something like:
private static object _listLock = new object();
As an aside, to follow a few best practices:
DoSomething(), as shown, can be static and so it should be.
for Library classes the recommended pattern is to make static members thread-safe, that would apply to StartRecording(), StopRecording() and DoSomething().
I would also make StopRecording() set _list = null and check it for null in DoSomething().
And before you ask, all this takes so little time that there really are no performance reasons not to do it.
You need to lock the list if multiple threads are adding to it.
A few observations...
Maybe there's a reason not to, but I would suggest making the class static and hence all of its members static. There's no real reason, at least from what you've shown, to require clients of MyClass to call the GetInstance() method just so they can call an instance method, DoSomething() in this case.
I don't see what prevents someone from calling the StartRecording() method multiple times. You might consider putting a check in there so that if it is already recording you don't create a new list, pulling the rug out from everyone's feet.
Finally, when you lock the list, don't do it like this:
static object _sync = new object();
lock(_sync){
_list.Add(new object(somedata));
}
Minimize the amount of time spent inside the lock by moving the new object creation outside of the lock.
static object _sync = new object();
object data = new object(somedata);
lock(_sync){
_list.Add(data);
}
EDIT
You said that DoSomething() cannot be static, but I bet it can. You can still use an object of MyClass inside DoSomething() for any instance-related stuff you have to do. But from a programming usability perspective, don't require the users to MyClass to call GetInstance() first. Consider this:
class MyClass {
private static MyClass _instance;
private static List<string> _list;
private static bool IsRecording;
public static void StartRecording()
{
_list = new List<string>();
IsRecording = true;
}
public static IEnumerable<string> StopRecording()
{
IsRecording = false;
return new List<string>(_list).AsReadOnly();
}
private static MyClass GetInstance() // make this private, not public
{ return _instance; }
public static void DoSomething()
{
// use inst internally to the function to get access to instance variables
MyClass inst = GetInstance();
}
}
Doing this, the users of MyClass can go from
MyClass.GetInstance().DoSomething();
to
MyClass.DoSomething();
.NET collections are not fully thread-safe. From MSDN: "Multiple readers can read the collection with confidence; however, any modification to the collection produces undefined results for all threads that access the collection, including the reader threads." You can follow the suggestions on that MSDN page to make your accesses thread-safe.
One problem that you would probably run into with your current code is if StopRecording is called while some other thread is inside DoSomething. Since creating a new list from an existing one requires enumerating over it, you are likely to run into the old "Collection was modified; enumeration operation may not execute" problem.
The bottom line: practice safe threading!
It's possible, albeit tricky, to write a linked list that allows simultaneous insertions from multiple threads without a lock, but this isn't it. It's just not safe to call _list.Add in parallel and hope for the best. Depending how it's written, you could lose one or both values, or corrupt the entire structure. Just lock it.
In a C# app, suppose I have a single global class that contains some configuration items, like so :
public class Options
{
int myConfigInt;
string myConfigString;
..etc.
}
static Options GlobalOptions;
the members of this class will be uses across different threads :
Thread1: GlobalOptions.myConfigString = blah;
while
Thread2: string thingie = GlobalOptions.myConfigString;
Using a lock for access to the GlobalOptions object would also unnecessary block when 2 threads are accessing different members, but on the other hand creating a sync-object for every member seems a bit over the top too.
Also, using a lock on the global options would make my code less nice I think;
if I have to write
string stringiwanttouse;
lock(GlobalOptions)
{
stringiwanttouse = GlobalOptions.myConfigString;
}
everywhere (and is this thread-safe or is stringiwanttouse now just a pointer to myConfigString ? Yeah, I'm new to C#....) instead of
string stringiwanttouse = GlobalOptions.myConfigString;
it makes the code look horrible.
So...
What is the best (and simplest!) way to ensure thread-safety ?
You could wrap the field in question (myConfigString in this case) in a Property, and have code in the Get/Set that uses either a Monitor.Lock or a Mutex. Then, accessing the property only locks that single field, and doesn't lock the whole class.
Edit: adding code
private static object obj = new object(); // only used for locking
public static string MyConfigString {
get {
lock(obj)
{
return myConfigstring;
}
}
set {
lock(obj)
{
myConfigstring = value;
}
}
}
The following was written before the OP's edit:
public static class Options
{
private static int _myConfigInt;
private static string _myConfigString;
private static bool _initialized = false;
private static object _locker = new object();
private static void InitializeIfNeeded()
{
if (!_initialized) {
lock (_locker) {
if (!_initialized) {
ReadConfiguration();
_initalized = true;
}
}
}
}
private static void ReadConfiguration() { // ... }
public static int MyConfigInt {
get {
InitializeIfNeeded();
return _myConfigInt;
}
}
public static string MyConfigString {
get {
InitializeIfNeeded();
return _myConfigstring;
}
}
//..etc.
}
After that edit, I can say that you should do something like the above, and only set configuration in one place - the configuration class. That way, it will be the only class modifying the configuration at runtime, and only when a configuration option is to be retrieved.
Your configurations may be 'global', but they should not be exposed as a global variable. If configurations don't change, they should be used to construct the objects that need the information - either manually or through a factory object. If they can change, then an object that watches the configuration file/database/whatever and implements the Observer pattern should be used.
Global variables (even those that happen to be a class instance) are a Bad Thing™
What do you mean by thread safety here? It's not the global object that needs to be thread safe, it is the accessing code. If two threads write to a member variable near the same instant, one of them will "win", but is that a problem? If your client code depends on the global value staying constant until it is done with some unit of processing, then you will need to create a synchronization object for each property that needs to be locked. There isn't any great way around that. You could just cache a local copy of the value to avoid problems, but the applicability of that fix will depend on your circumstances. Also, I wouldn't create a synch object for each property by default, but instead as you realize you will need it.