This won\'t compile:
namespace Constructor0Args
{
class Base
{
public Base(int x)
{
}
}
class Derived : Base
{
}
This is becase when the child class is instantiated, it will also instantiate the base class. By default, it will try to find a arg less constructor. This is work with this code:
class Base
{
public Base(int x) { }
}
class Derived : Base
{
public Derived(int x)
: base(x)
{
}
}
If you don't explicitly define a constructor for a class, a default constructor is automatically defined, which looks like this:
public Derived() : base()
{
}
You need to specify the constructor on the base class as well as which arguments to pass to it:
public Derived() : base(1)
{
}
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.
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:
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)) {}
}