为什么我因“保护级别”错误而“无法访问”？

Question

I am getting this error:

'CTest.AA()' is inaccessible due to its protection level.

when compiling this code:

public class A
{
    private A()
    {
    }
}

public class B : A
{
    public void SayHello()
    {
        Console.WriteLine("Hello");
    }
}

Can anyone explain why?

Answer 1

Because the default constructor for A is private, try protected A() {} as the constructor.

Class B automatically calls the default constructor of A , if that is inaccessible to B or there is no default constructor (if you have constructor protected A(string s) {} ) B can not be instantiated correctly.

The compiler automatically generates the following default constructor in B

public B() : base()
{
}

Where base() is the actual call to the default constructor of A .

Answer 2

The constructor on class B (which is added by the compiler) needs to call the default (no-args) constructor on A , however the default constructor is marked as private , which means it can only be called inside A , hence the error.

Change the constructor on A to protected or public , or internal if B is in the same assembly.

Answer 3

The constructor for A is private, it cannot be accessed from outside. If you want to create an instance of A from outside, make the constructor public or protected.

Answer 4

将private A()更改为public A() ，你很高兴。

Answer 5

It's because A's constructor is private, but B's constructor is public. When you construct B (which constructs A as well) there is no way to access A's private constructor.