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Environment: C#
Introduction
If you come from C++ or Java background to C#, no wonder that you will
notice some strange keywords introduced to this new language. The first reaction
will be what are these things for and why were they added Specially that, till
now people were able to develop programs using the existing languages without problems.
Among the set of new keywords added to C# there are new and override
modifiers. Although new may not seem that new to you , however its usage as method
modifier is completely new.
The new Modifier
Similarly to C++ , C# allow redefinition of inherited methods in the derived
classes at the cost of hiding the inherited ones. Although hiding is not
an error, but it does cause the compiler to issue a
warning. To suppress this warning you have to include the new modifier
in the derived method.
The real benefit of this feature is to be sure that you intentionally want
to hide the base method and this did not happen by accident or misspelling.
Conversely, if you declare a method as new and it does not really hide a
base method, the compiler will also issue a warning, that can be suppressed by
removing the new modifier.
Consider the following example :
public class baseHello
{
public void sayHello()
{
System.Console.WriteLine("base says hello");
}
}
class derivedHello:BaseHello
{
public void sayHello()
{
System.Console.WriteLine("derived says hello");
}
}
The preceding code will compile fine but the compiler warns you that method
derivedHello.sayHello() hides the method baseHello.sayHello() :
warning CS0114:
‘derivedHello.sayHello()’ hides inherited member ‘baseHello.sayHello()’. To
make the current method override that implementation, add the override keyword.
Otherwise add the new keyword.
As the warning suggest it is preferable to use new like the following.
class derivedHello:BaseHello
{
new public void sayHello()
{
System.Console.WriteLine("derived says hello");
}
}
The override Modifier
As you certainly know, every object oriented language implements
polymorphism which is summarized by having multiple implementation for the same
interface.
While all methods in Java are virtual by default, C++ and C# require clearly
defining a method as virtual , by using virtual modifier in C++ and C#
or abstract modifier in C# only (abstract is equivalent to pure virtual
in C++ : virtual type methodName(some params) = 0; )
In order to give a different version for the same method it suffice to
derive the base class and redefine the virtual method using same name and
signature, but with different body, plus the override modifier.
This may seem annoying at the beginning specially that C++ and Java developers
did not have to do that. However experience with large projects has shown that
lots of runtime problems occur when some modifications were needed in the
virtual base method signature ,which necessarily require the same modification
in all the derived classes that give their own version.
Look at this Java example :
public class baseHello
{
public void sayHello()
{
System.out.println("baseHello says HELLO");
}
}
public class derivedHello extends baseHello
{
public void sayHello()
{
System.out.println("derivedHello says HELLO");
}
}
In the above example the class derivedHello inherits and redefines the
method sayHello() from baseHello class.
Now suppose that at some time in the project the designer find out that the
sayHello() method should take a String parameter .
The change in the baseHello will be as follows
Public class baseHello
{
public void sayHello(String name )
{
System.out.println("baseHello says HELLO to "+name);
}
}
Now the base class is modified and still the hard work to do is to modify
all the derived classes that could be in dozens . Failing to modify one derived
class will result in a behavior different than expected as the following code
will show:
public class TestHello{
public static void print(baseHello hello)
{
hello.sayHello("TestHello");
}
public static void main(String[] args)
{
baseHello baseH= new baseHello();
derivedHello derivedH= new derivedHello();
print(baseH);
print(derivedH);
}
}
Running the TestHello class will result in two identical strings
baseHello says HELLO to TestHello
baseHello says HELLO to TestHello
The reason is because you failed to change the derivedHello class to be
conform to the baseHello class so the latter is used when
hello.sayHell("TestHello") is called.
Because the java compiler does not have any idea that the sayHello() in
derived class is intended to override the sayHello(String name) in the base
class , it does not complain about it and considers it as a new method defined
only in the derived class.
After exposing the problem, here is what C# offers to solve the problem.
Every method that must override a virtual base method must be declared with override
modifier. The same example applied to C# becomes
public class BaseHello
{
virtual public void sayHello(string name)
{
System.Console.WriteLine("BaseHello says hello to {0}",name);
}
}
class DerivedHello:BaseHello
{
// different signature
override public void sayHello()
{
System.Console.WriteLine("DerivedHello says hello");
}
}
The compilation of the example will result the following error :
error CS0115:DerivedHello.sayHello()’: no suitable method found to override
Telling the method sayHello() in DerivedHello which is intended to override
sayHello(String name) in BaseHello has different signature than its base
method.
In such way no matter how many times a class is derived it is always
possible to detect difference in methods name and signature at compile time.
Conclusion
As a conclusion , you have noticed that new and override modifiers have a
key role in preventing runtime flaws and sparing hours of debugging.
