Why is this private member accessible?

Question

I have this class:

class C {     private String msg;      public void F(C obj, String arg)     {         obj.msg = arg; // this is strange, the msg shouldn't be accessible here.     }      public void Output()     {         Console.WriteLine(msg);     } } 

The test code is:

C obj1 = new C(); C obj2 = new C(); obj1.F(obj2, "from obj1"); obj2.Output(); 

The Output is:

from obj1 

So, obj2's private member is accessed from another object obj1. I think this is kind of strange.

ADD

Here is an useful link mentioned by Habib:

Why are private fields private to the type, not the instance?

Answer 1

// this is strange, the msg shouldn't be accessible here.

private members are accessible inside the class, they are not accessible outside the class.

Private - MSDN

The private keyword is a member access modifier. Private access is the least permissive access level. Private members are accessible only within the body of the class or the struct in which they are declared

For your other question:

So, obj2's private member is accessed from another object obj1. I think this is kind of strange.

You are passing address of obj2 to an instance method of obj1, and then accessing obj2's private member msg in the method and changing its value. But since both of them are of same type, you get the impression that you are accessing other class private member.

Try it with two different classes and you will be able to understand it better.

Consider if you have another class defined as:

class B {     public void SomeMethod(C obj, string arg)     {         obj.msg = arg; // that is an error.      } } 

now you can't access private member msg since you are trying to access it outside of the class, in your example, you are accessing the class member inside the class.

There could be an argument that why C# allows instance.PrivateMember inside the class, the language designers could have restricted the usage of private to this.PrivateMember, so that the private member is only accessible with the current instance. If that would have been the case then your code would raise the error on obj.msg = arg;. Apparently the C# designers chosen the private to instance access instead of private to current instance only, so the basic rule is that private members are accessible inside the class, whether with the this (current instance) or with an instance of same type. For more discussion why this was done, you can see this question

Answer 2

If i understand private correctly, it means access only from within the same class, but not only from the same instance of that class.