Today I came up with an interesting problem. I noticed that the following code:
class A
{
public A()
{
Print();
}
public virtual void Print()
{
Console.WriteLine("Print in A");
}
}
class B : A
{
public B()
{
Print();
}
public override void Print()
{
Console.WriteLine("Print in B");
}
}
class Program
{
static void Main(string[] args)
{
A a = new B();
}
}
Prints
Print in B
Print in B
I want to know why does it print the "Print in B" twice.
C programming language is a machine-independent programming language that is mainly used to create many types of applications and operating systems such as Windows, and other complicated programs such as the Oracle database, Git, Python interpreter, and games and is considered a programming foundation in the process of ...
C is a general-purpose language that most programmers learn before moving on to more complex languages. From Unix and Windows to Tic Tac Toe and Photoshop, several of the most commonly used applications today have been built on C. It is easy to learn because: A simple syntax with only 32 keywords.
In the real sense it has no meaning or full form. It was developed by Dennis Ritchie and Ken Thompson at AT&T bell Lab. First, they used to call it as B language then later they made some improvement into it and renamed it as C and its superscript as C++ which was invented by Dr. Stroustroupe.
C is more difficult to learn than JavaScript, but it's a valuable skill to have because most programming languages are actually implemented in C. This is because C is a “machine-level” language. So learning it will teach you how a computer works and will actually make learning new languages in the future easier.
I want to know why does it print the "Print in B" twice.
You're calling a virtual method twice, on the same object. The object is an instance of B
even during A
's constructor, and so the overridden method will be called. (I believe that in C++, the object only "becomes" an instance of the subclass after the base class constructor has executed, as far as polymorphism is concerned.)
Note that this means that overridden methods called from a constructor will be executed before the derived class's constructor body has had a chance to execute. This is dangerous. You should almost never call abstract or virtual methods from a constructor, for precisely this reason.
EDIT: Note that when you don't provide another constructor call to "chain" to using either : this(...)
or : base(...)
in the constructor declaration, it's equivalent to using : base()
. So B
's constructor is equivalent to:
public B() : base()
{
Print();
}
For more on constructor chaining, see my article on the topic.
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