Constructor rules - please verify

  • Thread starter Thread starter Manco
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Manco

1. If you do not declare an explicit constructor, C-sharp will generate a
default paramaterless constructor that calls Object:ctor()

2. All derived class constructors implicity call base(), this can be
verified by looking at the MSIL.

3. If you declare a constructor in the base class with a parameter, but not
the paramaterless one, and you derive from this class, you will get a
compile error, because the C-sharp compiler wants to call base() and it
isn't there in the base class.
 
Manco said:
1. If you do not declare an explicit constructor, C-sharp will generate a
default paramaterless constructor that calls Object:ctor()
Click to expand...
Pretty sure the default constructor calls base:ctor(), not Object:ctor(),
but I haven't checked the MSIL...
2. All derived class constructors implicity call base(), this can be
verified by looking at the MSIL.
Click to expand...
yes, unless you provide a different call.
3. If you declare a constructor in the base class with a parameter, but not
the paramaterless one, and you derive from this class, you will get a
compile error, because the C-sharp compiler wants to call base() and it
isn't there in the base class.
Click to expand...
You can avoid this by simply specifying the parameter to the constructor for
the base class:

Class MyClass : MyBaseClass
{
public MyClass() : base("parameter")
{
return;
}
}
 
1. If you do not declare an explicit constructor, C-sharp will generate a
default paramaterless constructor that calls Object:ctor()
Click to expand...

Yep... To be more precise, it will call the parent class's parameterless
constructor, which could be explicitly written or implicitly created.
Eventually it will propagate to Object.ctor(). If the parent class has a
constructor with parameters and no parameterless constructor, a compiler
error will be generated.
2. All derived class constructors implicity call base(), this can be
verified by looking at the MSIL.
Yep.

3. If you declare a constructor in the base class with a parameter, but
not the paramaterless one, and you derive from this class, you will get a
compile error, because the C-sharp compiler wants to call base() and it
isn't there in the base class.
Click to expand...

Yep. If you declare a constructor and remove the parameterless one, a
default constructor will not be created implicitly, hence the compiler
error.

I recommend taking a peek at the C# spec...very informative:
http://blogs.msdn.com/csharpfaq/archive/2004/03/11/87814.aspx

ShaneB
 
This looks correct but applies to classes only. Structs don't allow explicit
parameterless constructors whereas classes do. Of course you can't derive
from structs anyway. You won't get a compiler error if you invoke an
overloaded constructor in the base class explicitly, e.g., base(<params>). A
derived class constructor will only implicitly call the default base class
constructor if you don't have an explicit call.

The following compiles without error:

class A //has no default parameterless constructor
{
public A(int i) {}
}
class B: A
{
public B(): base(0) {}
}

Thomas P. Skinner [MVP]
 
Manco said:
1. If you do not declare an explicit constructor, C-sharp will generate a
default paramaterless constructor that calls Object:ctor()
Click to expand...

Well, it calls the base class parameterless constructor.
2. All derived class constructors implicity call base(), this can be
verified by looking at the MSIL.
Click to expand...

Unless you declare it to call a different constructor, either this(...)
or base(...).
3. If you declare a constructor in the base class with a parameter, but not
the paramaterless one, and you derive from this class, you will get a
compile error, because the C-sharp compiler wants to call base() and it
isn't there in the base class.
Click to expand...

Only if you don't explicitly call that constructor from the derived
class too.

See http://www.pobox.com/~skeet/csharp/constructors.html
 
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