In Obj-c there is the static Initialize method which is called the first time that class is used, be it statically or by an instance. Anything like that in C#?
You can write a static constructor with the same syntax as a normal constructor, except with the static modifier (and no access modifiers):
public class Foo {
static Foo() {
// Code here
}
}
Usually you don't need to do this, however - static constructors are there for initialization, which is normally fine to do just in static field initializers:
public class Foo {
private static readonly SomeType SomeField = ...;
}
If you're using a static constructor to do more than initialize static fields, that's usually a design smell - but not always.
Note that the presence of a static constructor subtly affects the timing of type initialization, requiring it to be executed just prior to the first use - either when the first static member access, or before the first instance is created, whichever happens first.
There is a static constructor. As per msdn:
A static constructor is used to initialize any static data, or to perform a particular action that needs to be performed once only. It is called automatically before the first instance is created or any static members are referenced.
public class Foo
{
static Foo() {} //static constructor
}
Yes, it is called a constructor.
By MSDN:
public class Taxi
{
public bool isInitialized;
//This is the normal constructor, which is invoked upon creation.
public Taxi()
{
//All the code in here will be called whenever a new class
//is created.
isInitialized = true;
}
//This is the static constructor, which is invoked before initializing any Taxi class
static Taxi()
{
Console.WriteLine("Invoked static constructor");
}
}
class TestTaxi
{
static void Main()
{
Taxi t = new Taxi(); //Create a new Taxi, therefore call the normal constructor
Console.WriteLine(t.isInitialized);
}
}
//Output:
//Invoked static constructor
//true
Related
I am confused with static constructor in a instance class.
As a static constructor is private by default and we can not use access modifier with them, then do default constructor exists with static constructor in a instance class?
If yes, then Why? because we have already defined a constructor(private static and parameter less) and according to C# concept, if we provide a constructor then the default constructor won't exists. (I might be wrong here)
If No, then Why we are able to create a object of instance class with static constructor.
Below example is complied and executing successfully:
public class OOPS
{
static int i = 0;
static OOPS(){
Console.WriteLine("Static Constructor ");
}
//OOPS() {
// Console.WriteLine("Instance Constructor");
//}
public static void ShowStaticMethod() {
Console.WriteLine("Static Method ");
}
public void ShowInstanceMethod()
{
Console.WriteLine("instance Method");
}
}
class Client
{
public void ClientMethod() {
OOPS o = new OOPS();
o.ShowInstanceMethod();
OOPS.ShowStaticMethod();
Console.WriteLine("Client completed");
Console.ReadLine();
}
}
if we provide a constructor then the default constructor won't exists.
(I might be wrong here)
Well you are wrong about one thing, the above statement is true with respect to instance constructor, not static constructor.
Look at the C# language specification.
10.11.4 Default constructors
If a class contains no instance constructor declarations, a
default instance constructor is automatically provided.
So, when you provided the static constructor, it didn't have any relation with the default instance constructor, and that is the one you are using later in your code.
Static constructor does not affect instance constructors. If you do not want for the class to be instantiated, mark the class as static.
public static class OOPS
{
static OOPS()
{
}
}
I have a static class with a static constructor. I need to pass a parameter somehow to this static class but I'm not sure how the best way is.
What would you recommend?
public static class MyClass {
static MyClass() {
DoStuff("HardCodedParameter")
}
}
Don't use a static constructor, but a static initialization method:
public class A
{
private static string ParamA { get; set; }
public static void Init(string paramA)
{
ParamA = paramA;
}
}
In C#, static constructors are parameterless, and there're few approaches to overcome this limitation. One is what I've suggested you above.
As per MSDN, A static constructor is called automatically to initialize the class before the first instance is created. Therefore you can't send any parameters.
CLR must call a static constructor, how will it know which parameters to pass it?
So don't use a static constructor.
Here's the work around for your requirement.
public class StaticClass
{
private int bar;
private static StaticClass _foo;
private StaticClass() {}
static StaticClass Create(int initialBar)
{
_foo = new StaticClass();
_foo.bar = initialBar;
return _foo;
}
}
Static constructors have the following properties:
A static constructor does not take access modifiers or have parameters. A static constructor is called automatically to
initialize the class before the first instance is created or any
static members are referenced.
A static constructor cannot be called directly.
The user has no control on when the static constructor is executed in the program.
A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrary
method.
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.
If by "HardCodedParameter" you really mean hard coded, you can use constants.
public static class YoursClass
{
public const string AnotherHardCodedParam = "Foo";
}
public static class MyClass
{
private const string HardCodedParam = "FooBar";
static MyClass()
{
DoStuff(MyClass.HardCodedParam);
DoStuff(YoursClass.AnotherHardCodedParam);
}
}
Also, you can use static readonly properties.
Constructors on non-static class have have the benefit to ensure they're properly initialized before they're actually being used.
Since static classes don't have this benefit, you have to make ensure that yourself.
Use a static constructor with an obvious name, then in the relevant portion of your static procedures check to make sure the initialization has been performed.
The example below assumes your want to "initialize" your static class with a Form object.
public static class MyClass
{
private static Form FormMain { get; set; }
public static void Init(Form initForm)
{
FormMain = initForm;
}
private static bool InitCheck()
{
return FormMain != null ? true: false;
}
public static void DoStuff()
{
if (InitCheck())
{
// Do your things
}
else
{
throw new Exception("Object reference not set to an instance of an object");
}
}
}
How are static constructors and private constructors different?
public class WorkstationDevicePresenter
{
private WorkstationDevicePresenter()
{}
}
What's the point in leaving them blank?
Whats the point in leaving them blank?
There are a number of reasons to make "blank" constructors.
You might make a blank constructor because you want a place to set a breakpoint during debugging.
You might make a blank static constructor because doing so changes the semantics of static field initializers. Read Jon's article on the subject for details.
Let's leave static constructors and consider blank instance constructors.
The key rule that motivates blank constructors is: By default if there are no constructors in a type then you get a "blank" parameterless public constructor for free. If there are any constructors in a type then you do not get a blank parameterless public constructor for free.
So the first obvious reason why you'd want a blank constructor is: I want a blank parameterless constructor, but I've already made another ctor, so I no longer get one for free.
The second reason is that you don't have any ctors and you do not want a blank parameterless public constructor. You might want a blank parameterless private, internal or protected constructor. If that's what you want then you'll have to make one yourself.
In particular, making an empty private ctor as the only ctor means that the class cannot be instantiated via a constructor from outside the class. This is very useful if you want to use the factory pattern. It also prevents code outside the class from making derived classes, because derived classes must be able to call a constructor. If all the constructors are private then they can't derive.
I frequently use this variation on the factory pattern:
public abstract class Thing
{
private Thing() {}
private class RedThing : Thing { ... }
public static Thing GetRedThing() { return new RedThing(); }
}
See, by making a private constructor I can make a public abstract class that can only be instantiated by my code and only extended by my code, and therefore I have a nice invariant: every time I see an object of type Thing, I know where it came from.
Static constructors happen once when the class is loaded, private constructors happen when they are called by some public static method typically used to create singletons, or with the Builder pattern. There is no reason to have a blank private constructor (that I know of).
Static constructors initialize the static parts of a class and private constructors can only be used by the class itself, like for creating a singleton-object of the class.
public class MyClass {
private static int staticitem;
private int instanceitem;
static MyClass(){
staticitem = 0; //define value for staticitem
}
private MyClass() { //can only be called from within the class
instanceitem = 0; //define value for instanceitem
}
public static MyClass GetMyClass() {
MyClass m = new MyClass();
return m;
}
}
Blank private constructor will make the class uninstantiable by anything other than itself. If you don't have this piece of code, by default the compiler creates a blank public parameterless contstructor.
Static constructor is called when creating the static instance.
You can use both to create a Singleton pattern, for instance.
Check the following code:
public class Singleton
{
public static Singleton Instance;
static Singleton
{
Instance = new Singleton();
}
private Singleton()
{
}
}
public class SomeOtherClass
{
public static Singleton CompileError = new Singleton();
public static Singleton CompileOK = Singleton.Instance;
}
Consider this example code:
public class A<T>
{
public static T TheT { get; set; }
}
public class B : A<string>
{
static B() {
TheT = "Test";
}
}
public class Program {
public static void Main(String[] args) {
Console.WriteLine(B.TheT);
}
}
Here B.TheT is null. However, changing the Main method like this:
public static void Main() {
new B();
Console.WriteLine(B.TheT);
}
B.TheT is "Test", as expected. I can understand that this forces the static constructor to run, but why does this not happen for the first case?
I tried reading the spec, and this caught my attention (§10.12):
[...] The execution of a static constructor is triggered by the first of the
following events to occur within an application domain:
• [...]
• Any of the static members of the class type are referenced.
My interpretation of this is that since TheT is not a member of B, the static constructor of B is not forced to be run. Is this correct?
If that is correct, how would I best let B specify how to initialize TheT?
A.TheT is "Test", as expected. I can understand that this forces the static constructor to run, but why does this not happen for the first case?
Basically, you haven't really referenced B. If you look in the IL, I think you'll find that your code was actually equivalent to:
public static void Main(String[] args) {
Console.WriteLine(A<string>.TheT);
}
The compiler worked out that that's really the member you meant, even though you wrote B.TheT.
If that is correct, how would I best let A specify how to initialize TheT?
I would try to avoid doing this to start with, to be honest... but you could always just add a static method to B:
public static void Initialize() {
// Type initializer will be executed now.
}
Static constructor is called before the first access or at the latest when the first access is made. I.e you know it has been called when first access is made but not how long before. However, if no access is made it won't be called. So you cannot control when only if it s called.
As per MSDN reference
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.
In second case called static contructor and it will call the actual types' static contructor, and not that one of derived one. So in your case it calls the static ctor of the A<T> =>A<string> and not of A.
To prove this behaviour just do the following:
public class Base {
static Base() {
"Base static".Dump();
}
}
public class Derived : Base {
static Derived() {
"Derived static".Dump();
}
public static string temp = "Hello";
}
and call
Derived.temp.Dump();
You will get:
Derived static
Hello
This is what you actually do in your code, you acess derived type A<string> and it's default static ctor is called.
So here is what I'm thinking...
public class MyClass
{
public const string MyConstant = "MyConstantValue";
private static MyClass DefaultInstance;
static MyClass()
{
DefaultInstance = new MyClass();
}
}
...
NotificationService.RegisterForNotification(MyClass.MyConstant, Callback);
Will this work or do I need to use something like a static readonly property field to trigger the static constructor?
Use of a constant doesn't necessarily result in a member access which would cause the static constructor to be called. The compiler is allowed to (encouraged, even) substitute the value of the constant at compile time.
Your suggested workaround of static readonly should be ok, although readonly suggests a field, not a property. Properties are read-only when they have no setter, the readonly keyword isn't involved.
A simple example:
class HasSConstructor
{
internal const int Answer = 42;
static HasSConstructor()
{
System.Console.WriteLine("static constructor running");
}
}
public class Program
{
public static void Main()
{
System.Console.WriteLine("The answer is " + HasSConstructor.Answer.ToString());
}
}
Output under .NET 4.0:
The answer is 42
The static constructor never runs!
Static constructor is called automatically before the first instance is created or any static members are referenced. See here: MSDN: Static Constructors
By the way, constant fields are inherently static, but as pointed they may (and probably will) be substituted by the value itself.
The static constructor will not be called if you're just accessing public constants. For example, consider this class:
class Testo
{
public const string MyValue = "Hello, world";
public static readonly int xyzzy;
static Testo()
{
Console.WriteLine("In static constructor");
xyzzy = 27;
}
}
Now, in another class, execute:
Console.WriteLine(Testo.MyValue);
Console.WriteLine(Testo.xyzzy);
Your output will be:
Hello, world
In static constructor
27
Constants are hoisted from the class at compile time. So the value of Testo.MyValue does not reference the Testo class at runtime. The static constructor is not called until you call something that requires initialization.
So, yes, you need to access something like a static readonly if you want to ensure that the constructor is called.
No, you don't need such. Static constructor is called when the class is loaded.