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()
{
}
}
Related
Env.: C#6, Visual Studio 2015 CTP 6
Given the following example:
namespace StaticCTOR
{
struct SavingsAccount
{
// static members
public static double currInterestRate = 0.04;
static SavingsAccount()
{
currInterestRate = 0.06;
Console.WriteLine("static ctor of SavingsAccount");
}
//
public double Balance;
}
class Program
{
static void Main(string[] args)
{
SavingsAccount s1 = new SavingsAccount();
s1.Balance = 10000;
Console.WriteLine("The balance of my account is \{s1.Balance}");
Console.ReadKey();
}
}
}
The static ctor is not being executed for some reason. If I declare SavingsAccount as a class instead of a struct, it works just fine.
The static constructor isn't executed because you are not using any static members of the struct.
If you use the static member currInterestRate, then the static constructor is called first:
Console.WriteLine(SavingsAccount.currInterestRate);
Output:
static ctor of SavingsAccount
0,06
When you are using a class, the static constructor will be called before the instance is created. Calling a constructor for a structure doesn't create an instance, so it doesn't trigger the static constructor.
According to the CLI spec:
If not marked BeforeFieldInit then that type’s initializer method is
executed at (i.e., is triggered by):
first access to any static field of that type, or
first invocation of any static method of that type, or
first invocation of any instance or virtual method of that type if it is a value type or
first invocation of any constructor for that type
For structs which have an implicit default constructor it is not actually invoked, so you can create an instance and access its fields. Everything else (invoking custom constructors, instance property access, method calls, static field access) will trigger static constructor invocation.
Also note that invoking inherited Object methods which are not overridden in the struct (e.g. ToString()) will not trigger static constructor invocation.
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
The MSDN RyuJIT blog entry gives this instruction for setting up CTP3:
Tricky thing necessary until RyuJIT is final: Add a reference to
Microsoft.Numerics.Vectors.Vector to a class constructor that will
be invoked BEFORE your methods that use the new Vector types. I’d
suggest just putting it in your program’s entry class’s constructor.
It must occur in the class constructor, not the instance constructor.
I'm far better versed in class/instance construction in Objective-C than I am in C#. Is he talking about a different concept? What is the difference between a class constructor and an instance constructor in C#? Is the "class constructor" in this case simply the parameterless constructor?
I think this refers to the static constructor
Class constructor = Static constructor
Instance constructor = Normal constructor
For example,
class MyClass
{
// Static/Class constructor.
// Note: Static constructors cannot have visibility modifier (eg. public/private),
// and cannot have any arguments.
static MyClass()
{
... // This will only execute once - when this class is first accessed.
}
// Normal/Instance Constructor.
public MyClass(...)
{
... // This will execute each time an object of this class is created.
}
}
So as an example, consider the following code:
static void Main(string[] args)
{
var a = new MyClass(); // Calls static constructor, then normal constructor.
a.DoSomething();
var b = new MyClass(); // Calls normal constructor only.
b.DoSomething();
}
Also, consider the following code:
static void Main(string[] args)
{
MyClass.SomeStaticMethod(); // Calls static constructor, then SomeStaticMethod().
MyClass.SomeOtherStaticMethod(); // Calls SomeOtherStaticMethod() only.
// Note: None of the above calls the normal constructor.
}
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.