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.
Related
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
This question already has answers here:
C# static class constructor
(7 answers)
What is the use of static constructors?
(8 answers)
Closed 5 years ago.
In my class there are several methods with the following signature:
public static void SomeMethod()
{
...
}
Most of these methods depend on the value of the private static field.
It is necessary that the caller had any way to assign a value to this field before calling any of these static methods.
I want to have a single Random object for use by all classes in the application. How do I pass a reference to this object to use it in the static methods of another class?
I have a private static field in the class with static methods and a static initializer:
public static void Init(Random random)
{
_random = random;
}
But how to make sure that the initializer was called? To do the check and throw exceptions in every method, it seems to me redundant. There may be another option.
Finally I found an acceptable solution for me:
Define public static property in the class that contains application entry point:public static Random Rnd { get; } = new Random();
Define private static Random _random in other classes;
Implement a static constructor for other classes that includes this code:
Type type = Assembly.GetExecutingAssembly().EntryPoint.DeclaringType;
var randomTypeProperties =
type.GetProperties().Where(p => p.PropertyType == typeof(Random));
foreach (var propertyInfo in randomTypeProperties)
{
_random = (Random)propertyInfo.GetValue(null);
}
I can mark a message from #Andy as the answer, because it is after all a static constructor.
Thank you to everyone who took part in the discussion.
It's never too late to learn. Yesterday, as I read about the design patterns, I realized that the Singlton is exactly what I needed:
public sealed class RandomAsSingleton : Random
{
private static volatile RandomAsSingleton _instance;
private static readonly object _syncRoot = new object();
private RandomAsSingleton() { }
public static RandomAsSingleton Instance
{
get
{
lock (_syncRoot)
{
if (ReferenceEquals(_instance, null)) { _instance = new RandomAsSingleton(); }
return _instance;
}
}
}
}
My previous solution seems to me a little bit ugly right now.
You might want to use a static constructor
// Static constructor is called at most one time, before any
// instance constructor is invoked or member is accessed.
static SimpleClass()
{
// set your private static field
}
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/static-constructors
Do I need to mark public method as static if I want to initialize private variable only once or it is enough for making "singleton property" in the following code?
public IEqualityComparer<T> GetComparer<T>()
{
if (typeof (IUserShift).IsAssignableFrom(typeof (T)))
return UserShiftComparer.Value as IEqualityComparer<T>;
throw new ArgumentOutOfRangeException("There is no avaliable comparer for the type!", nameof(T));
}
private static readonly Lazy<UserShiftTrackingComparer> UserShiftComparer = new Lazy<UserShiftTrackingComparer>();
If you make your field static then only one copy will exist and in this case since you have it within Lazy, it will only be created when it is accessed. If it is never accessed, it will never be created.
Making your method static means it is not tied to an instance of the class but the class itself. All instance methods can access static methods and static fields and instance fields and instance methods. On the other hand, static methods can only access static fields and other static methods.
To answer your question, you DO NOT need to make the method static to initialize the UserShiftComparer only once.
I was making this to implement a singleton pattern
private static ProcessDao _dao;
public static ProcessDao Dao
{
get { return _dao ?? (_dao = new ProcessDao()); }
}
but in C# 6 auto properties has default values.
Is this a correct implementation of singleton?
public static ProcessDao Dao { get; } = new ProcessDao();
Is this a correct implementation of singleton?
Your first example is wrong in the sense that it isn't a thread-safe implementation of a singleton. If multiple threads called ProcessDao.Instance, you're likely to see different instances of the private field being created.
In contrary to that, your second example is thread-safe, as the compiler actually translates your auto-implemented getter only property to:
public class ProcessDao
{
[CompilerGenerated]
private static readonly ProcessDao <Dao>k__BackingField;
public static ProcessDao Dao
{
[CompilerGenerated]
get
{
return ProcessDao.<Dao>k__BackingField;
}
}
static ProcessDao()
{
ProcessDao.<Dao>k__BackingField = new ProcessDao();
}
}
The static constructor invocation is guaranteed by the runtime to at most once, so you're also getting the guarantee that this will not create multiple instances of the backing field.
Regarding laziness, that depends on the implementation of your ProcessDao class. The static constructor is guaranteed to run before the first reference of a static field in the class. If you have multiple static fields exposed, then the first will cause the invocation and the allocation of these objects. If you know you'll be using other static members and want maximum laziness, look into the Lazy<T> type introduced in .NET 4.0.
What is the need of private constructor in C#?
I got it as a question for a C# test.
For example if you have a class that should only be created through factory methods. Or if you have overloads of the constructor, and some of them should only be used by the other constructors. Probably other reasons as well =)
If you know some design pattern, it's obvious: a class could create a new instance of itself internally, and not let others do it.
An example in Java (I don't know C# well enough, sorry) with a singleton-class:
class Meh
{
private Meh() { }
private static Meh theMeh = new Meh();
public static Meh getInstance() { return theMeh; }
}
Whenever you want to prevent direct instantiation of a class from outside of it, you'll use a private constructor. For example, prior to C# 2.0 which introduced static classes, you used a private constructor to accomplish roughly the same thing:
sealed class StaticClass {
private StaticClass() {
}
public static void DoSomething() {
}
}
When you want to prevent the users of your class from instantiating the class directly. Some common cases are:
Classes containing only static methods
Singletons
I can can recall few usages for it:
You could use it from a static factory method inside the same class
You could do some common work inside it and then call it from other contructure
You could use it to prevent the runtime from adding an empty contructure automatically
It could be used (although private) from some mocking and ORM tools (like nhibernate)
For example when you provide factory methods to control instantiation...
public class Test(){
private Test(){
}
void DoSomething(){
// instance method
}
public static Test CreateCoolTest(){
return new Test();
}
}
Private constructors are used to prevent the creation of instances of a class when there are no instance fields or methods, such as the Math class, or when a method is called to obtain an instance of a class. If all the methods in the class are static, consider making the entire class static. For more information see Static Classes and Static Class Members.
class NLog
{
// Private Constructor:
private NLog() { }
public static double e = System.Math.E; //2.71828...
}
The following is an example of a class using a private constructor.
public class Counter
{
private Counter() { }
public static int currentCount;
public static int IncrementCount()
{
return ++currentCount;
}
}
class TestCounter
{
static void Main()
{
// If you uncomment the following statement, it will generate
// an error because the constructor is inaccessible:
// Counter aCounter = new Counter(); // Error
Counter.currentCount = 100;
Counter.IncrementCount();
System.Console.WriteLine("New count: {0}", Counter.currentCount);
}
}
While this link is related to java, I think it should help you understand the reason why as the idea is pretty much the same.
Private constructors prevent a class from being explicitly instantiated by callers. There are some common cases where a private constructor can be useful:
classes containing only static utility methods
classes containing only constants
type safe enumerations
singletons
You can use it with inheritance in a case where the arguments to the constructor for the base class are of different types to those of the child classes constructor but you still need the functionality of the base class in the child class eg. protected methods.
Generally though this should be avoided wherever possible as this is a bad form of inheritance to be using.
I'm late to the game, but reading through all the other answers, I don't see this usage mentioned:
I use private constructors in scenarios where I have multiple (public) constructors, and they all have some code in common. With constructor chaining, the code becomes really neat and DRY.
Remember, the private readonly variables can only be set in constructors, so I can't use a regular method.
Example:
public class MyClass
{
private readonly int _a;
private readonly int _b;
private readonly string _x;
public MyClass(int a, int b, string x)
: this(x)
{
_a = a;
_b = b;
}
public MyClass()
: this("(not set)")
{
// Nothing set here...
}
private MyClass(string x)
{
_x = x;
}
}
Basically you use private constructors when you are following a singleton design pattern. In this case, you have a static method defined inside the class that internally calls the private constructor.
So to create the instance of the class for the first time, the user calls the classname.static_method_name. In this method, since the class's object doesn't yet exist, the static method internally calls the private constructor and returns the class's instance.
If the class's instance already exists, then the static method simply returns the instance to the calling method.
And of course you can use private constructor to prevent subclassing.