Why is it necessary for a base class to have a constructor that takes 0 args?

Question

This won't compile:

namespace Constructor0Args {     class Base     {         public Base(int x)         {         }     }      class Derived : Base     {     }      class Program     {         static void Main(string[] args)         {         }     } } 

Instead, I get the following error:

'Constructor0Args.Base' does not contain a constructor that takes 0 arguments

Why is that? Is it necessary for a base class to have a constructor that takes 0 arguments?

Answer 1

It isn't - the problem is that it needs to call some base constructor, in order to initialise the base type, and the default is to call base(). You can tweak that by specifying the specific constructor (and arguments) yourself in the derived-types constructor:

class Derived : Base {     public Derived() : base(123) {} } 

For parameters to base (or alternatively, this) constructors, you can use:

• parameters to the current constructor
• literals / constants
• static method calls (also involving the above)

For example, the following is also valid, using all three bullets above:

class Derived : Base {     public Derived(string s) : base(int.Parse(s, NumberStyles.Any)) {} } 

Answer 2

When you derive a class from another, the base class will be called before the derived classes constructor. When you don't explicitly call a constructor you are essentially writing

class Derived : Base {     public Derived() : base()     {     } } 

Since the Base class doesn't have a 0 argument constructor, this is invalid.