为什么MessageBox类在C＃中没有默认构造函数？

Question

Case 1:

I am trying this

MessageBox m = new MessageBox();

And got compilation error

'System.Windows.Forms.MessageBox' has no constructors defined

Case 2:
Then, I have made a class without constructor

class myClass
{

}

and tried myClass my = new myClass(); This time I found no error.

Now, my question:

• Why I am getting error in 1st case?

Since, both are classes and every class have default constructor, then

• Where is default constructor in 1st case?

Answer 1

The constructor may be private or protected in order to forbid direct instantiation. Use the static factory method instead. There is a static method Show in the MessageBox class.

Archil is right, too. If theres a explicit constructor defined, the implicit default constructor is not created anymore.

And regarding x0ns comments: Yes, it's also impossible to instantiate static classes. Don't use static classes, thats poor design (there are exceptions).

Answer 2

In c#, evey class automatically has default constructor if NONE is defined. MessageBox defines other constructors, so it does not automatically have default constructor

Answer 3

MessageBox is designed to be used as a static class - see http://msdn.microsoft.com/en-us/library/79b3xss3(VS.80).aspx

You can make your class static using:

static class myclass {}

Answer 4

System.Windows.Forms.MessageBox has no default (empty) constructor.

A constructor can be hidden by setting its accessibility to something other than public.

The class' design declares that you can't use it as an object.
It only has static methods that can be used without instantiating an object of that class.

Answer 5

In case 1, MessageBox is a static class, it has no constructors (update - it has a private constructor says reflector but the OP gave a misleading/incorrect compiler error message.) Static classes are defined like this:

public static class MessageBox { }

A static class can only have static methods, and as such is not meant to be instantiated.

In case 2, MyClass is not a static class, and the compiler generates a default constructor for you if you don't define any constructors.

UPDATE: to all downvoters: go compile up a project with a static class and examine it in reflector - it decompiles without the static keyword becuase there is no MSIL or metadata for a static class; the compiler (in .net 2.0 or later) generates an abstract sealed class with no constructors. The keyword "static" is just syntactic sugar. Additionally, in 1.0/1.1 of .NET (when MessageBox was created,) the static keyword did not exist for classes and the sealed/private ctor was the accepted pattern.