How can a class have no constructor?

Question

Hi,

A while back I asked about instantiating a HttpContext object. Now that I have learnt what I didn't know, what confuses me is that you cannot say HttpContext ctx = new HttpContext(); because the object does not have a constructor.

But doesn't every class need a constructor? In C#, if you don't provide one, the compiler automatically provides a default cstr for you.

Also, if I have a string (example: "Hello There!") and I say Convert.ToBoolean("Hello"), or any string, how does this work? What happens behind the scenes? I guess a book like CLR Via C# would be handy in this case.

What am I missing?

Answer 1

Constructor can be private or protected.
Also you can't create instance of abstract class, even if that class has public constructor.

Answer 2

I believe the HttpContext constructor has been marked private. That means that you cannot instantiate it yourself. The .net framework creates one for you behind the scenes...

Answer 3

In one word : static.

Otherwise, a class might be instantiated internally or privately (Factory or Singleton)

Signleton :

Class A{
public static readonly A Instance = new A();

private A()
{
}

}

Answer 4

This was actually meant to be a C# question, I pressed C# by accident and can't edit the tags now. :(

Answer 5

If you make the constructor private, you cannot extarnaly instatiate a class. But within the class it is possible. So you can provide a static method that returns a instance of the class. The singleton pattern is based on this.

Answer 6

Singletons, for example, do not have constructors, or at least, no public constructors. So if your class is a singleton, instead of writing

MyClass c = new MyClass();

You would write instead

MyClass c = MyClass.getInstance();

Answer 7

Thanks for the heads up guys.

Answer 8

MyClass c = MyClass.getInstance();

What would the getInstance method look like?

Answer 9

Take a look at the Singleton Design Pattern.

Answer 10

HttpContext has a public constructor with two overloads but it's not the default (no params) one.

As an example, you need to pass in a SimpleWorkerRequest instance in order to instatiate an HttpContext instance and assign it to HttpContext.Current:

//Initialize this stuff with some crap
string appVirtualDir = "/"; 
string appPhysicalDir = @"C:\Documents and Settings\"; 
string page = @"localhost"; 
string query = string.Empty; 
TextWriter output = null;    
//Create a SimpleWorkerRequest object passing down the crap
SimpleWorkerRequest workerRequest = new SimpleWorkerRequest(appVirtualDir, appPhysicalDir, page, query, output);
//Create your fake HttpContext instance 
HttpContext.Current = new HttpContext(workerRequest);

See this link for details.

Anyway some classes don't have public constructors - think of a singleton class for example, constructor is private (and you can call the static getInstance method to get current instance or create it if it is null).

Cheers

Answer 11

You have 3 questions there...

HttpContext; it actually has two public constructors - but in reality you aren't expected to use them. In more general terms, you can use non-default constructors like so: MyType foo = new MyType("abc");.

Missing constructor Fairly well covered already, but no: abstract / static are the simplest, but it also isn't necessary to have a public constructor.

ToBoolean Behing the scenes, this will do the moral equivalent of bool.Parse("Hello"), which simply checks for known strings - in particular "True" and "False" (using OrdinalIgnoreCase, having dealt with null/trimming/etc).