what is the default constructor access modifer?

Question

I am confuse on what is the default constructor access modifier and what does this MSDN statemtn says

 If the direct base class does not have an accessible parameterless instance constructor, a compile-time error occurs.

Because when i applied this with a test program it fails. I can make an object or class that is inheriting another class thogh there is no exteranal parameerless constructor defined.

class A 
{
}
class B : A
{
}
class C
{

    public void main()
    {

        B objB = new B();// as per MSDN here should be the compile time error.

    }
}

[Source]

B has an automatically generated public default constructor. — pasztorpisti, Feb 28 '14 at 10:14
So you mean in B if i externally make parameterless constructor private then only there will be a compile time error. — NoviceToDotNet, Feb 28 '14 at 10:16
possible duplicate of [What are the Default Access Modifiers in C#?](http://stackoverflow.com/questions/2521459/what-are-the-default-access-modifiers-in-c) — O. R. Mapper, Feb 28 '14 at 10:19
But the other link answeered by Jon Skeet says The default access for everything in C# is "the most restricted access you could declare for that member". Which is contradictory to this MSDN link. — NoviceToDotNet, Feb 28 '14 at 10:38
If a class is internal then also its default constructor is public.. — NoviceToDotNet, Feb 28 '14 at 12:46
@NoviceToDotNet: I retracted my close vote. I realized just now that the question is actually about the "((default constructor) access modifier)", not about the "(default (constructor access modifier))". — O. R. Mapper, Feb 28 '14 at 12:58

score 4 · Answer 1 · answered Feb 28 '14 at 10:19

4

If the direct base class does not have an accessible parameterless instance constructor, a compile-time error occurs.

If a constructor is not defined for a class, the compiler will automatically generate a public default constructor.

answered Feb 28 '14 at 10:19

F11

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Rafa Paez · Accepted Answer · 2014-02-28T13:36:33.347

3

If a class contains no instance constructor declarations, a default instance constructor is automatically provided. That default constructor simply invokes the parameterless constructor of the direct base class. If the direct base class does not have an accessible parameterless instance constructor, a compile-time error occurs. If the class is abstract then the declared accessibility for the default constructor is protected. Otherwise, the declared accessibility for the default constructor is public.

In your example, all the classes A, B, C has been created with a default internal parameterless constructor.

Since both classes are in the same assembly and are internal with internal constructors by default you are not getting a compilation error. But if you declare a non-accessible constructor (e.g. private or protected) in your class B you will get a compilation error in your example.

edited Feb 28 '14 at 13:36

answered Feb 28 '14 at 10:16

Rafa Paez

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A->B->C if any of base class has private constructor it will be compile time error? – NoviceToDotNet Feb 28 '14 at 12:21
If a class is internal then also its default constructor is public..? – NoviceToDotNet Feb 28 '14 at 12:46
@NoviceToDotNet: I have updated my answer since it was not totally right. – Rafa Paez Feb 28 '14 at 13:37

score 3 · Answer 3 · answered Feb 28 '14 at 10:18

3

From Using Constructors

Unless the class is static, classes without constructors are 
given a public default constructor by the C# compiler in order to 
enable class instantiation.

answered Feb 28 '14 at 10:18

Nagaraj S

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score 3 · Answer 4 · answered Feb 28 '14 at 10:22

The compiler creates parameterless constructor if you don't create one yourself, so as per the MSDN page, your example actually ends up looking like this;

class A
{
    public A(): base() {}
}

class B : A
{
    public B(): base() {}
}

Your call to new B() will ultimately end up calling A's constructor. However, if you created an explicit constructor for A which is private;

class A
{
   private A() {}
}

class B : A
{

}

Then this translates to;

class A
{
   private A() {}
}

class B : A
{
    public B(): base() {}
}

Which will fail to compile with an error about A's constructor being inaccessible.

score 2 · Answer 5 · answered Feb 28 '14 at 10:16

If a constructor is not defined for a class, the compiler will automatically generate a public default constructor.

However, if there is a constructor defined that limits the access, then the compiler will throw.

for example, this should throw an exception:

class A
{
   private A() {}
}
class B : A
{
}

what is the default constructor access modifer?

5 Answers5