Question

In this part of the C# tutorial, we will talk about some of the new features of the C# version 4.0. Microsoft introduced this version of the C# in April 2010 with Visual Studio 2010. Mono C# compiler fully supported the new version by October 2010 with the release of Mono 2.8.

The C# version 4.0 brought the following new features to the language:

• Dynamic programming
• Named parameters
• Optional parameters
• Covariance and contravariance for generics and delegates

Dynamic programming

The new version of the C# language brought a new type dynamic. Once a variable is declared as having type dynamic, operations on these value are not done or verified at compile time, but instead happen entirely at runtime. This is known also as duck typing. The name of the concept refers to the duck test which may be phrased as follows: "When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck". In other words, we are concerned about what the object is doing rather than with the type of the object. The dynamic type is used to simplify access to COM objects, IronPython and IronRuby libraries and to the HTML DOM.

using System;

public class Duck
{
  public void quack()
  {
    Console.WriteLine("Quaaaack");
  }
}

public class Person
{
  public void quack()
  {
    Console.WriteLine("Person imitates a duck");
  }
}

public class DuckTyping
{
  static void Main() 
  {
    Duck donald = new Duck();
    Person josh = new Person();
     
    InTheForest(donald);
    InTheForest(josh);
  }

  public static void InTheForest(dynamic duck)
  { 
    duck.quack();
  }
}

In our example, we have two classes. The Duck class and the Person class. Both have the quack() method.

public static void InTheForest(dynamic duck)
{ 
  duck.quack();
}

The InTheForest() method has a dynamic parameter. The type of the object is not important; we are concerned about the capabilities of the objects. If objects passed to the InTheForest() method can invoke the quack() method, then we are fine.

$ dmcs ducktyping.cs
$ /usr/local/bin/mono ducktyping.exe 
Quaaaack
Person imitates a duck

We compile and run the program. We use the Mono dmcs compiler that is shipped with the Mono 2.8 and supports the C# 4.0 profile.

Named parameters

In earlier versions of the C# language, the arguments were provided in the order in which they appeared in the method's signature. And the position in the parameter list is important when evaluating a method. Named arguments enable us to specify the method parameters by their names. When specifying the named arguments, the position of the parameter is not important anymore.

using System;

public class NamedParameters
{
  static void Main() 
  {
    ShowMessage(name: "Jane", age: 17);
  }

  public static void ShowMessage(int age, string name)
  {
    Console.WriteLine("{0} is {1} years old", name, age);
  }
}

A simple example with named arguments.

ShowMessage(name: "Jane", age: 17);

The parameter name is followed by the colon (:) character and the value. The parameters may not be specified in order of the method signature.

$ ./named.exe 
Jane is 17 years old

This is the output of the named.exe program.

Optional parameters

With C# 4.0 there are required parameters and optional parameters. Any call must provide arguments for all required parameters but it can omit arguments for optional parameters. Optional parameters are defined at the end of the parameter list, after any required parameters. An optional parameter is created when a default value is specified for the parameter.

using System;

public class OptionalParameters
{
  static void Main() 
  {
    Power(4, 4);
    Power(5);
  }

  public static void Power(int x, int y=2)
  {
    int z = 1;

    for (int i=0; i<y; i++)
    {
      z *= x;
    }

    Console.WriteLine(z);
  }
}

We have a Power() method. The method takes two parameters: the base and the exponent. If we do not specify the exponent, then the default 2 is used.

public static void Power(int x, int y=2)

In the method definition, we have a required x parameter and the optional y parameter. The y has a default value.

Power(4, 4);
Power(5);

When we call the Power() method, we can specify one or two parameters.

$ ./optional.exe 
256
25

This is the output of the example.

Covariance & contravariance

Earlier versions of C# support covariance and contravariance in assignments, parameter types and return types. C# version 4.0 brings covariance for generics and delegate types.

Types that are covariant convert from a wider type to narrower type. (For example, from double to float.) Contravariant types convert from a narrower type to a wider type. (For example, from short to int.) Invariant types are not able to convert.

using System;

public class Covariance
{
  static void Main()
  {
    object[] langs = {"C#", "Python", "PHP", "Java"};

    Console.WriteLine(langs[0]);
  }
}

Arrays of reference types are covariant from the beginning.

object[] langs = {"C#", "Python", "PHP", "Java"};

We have an array of string values. The array is declared to be of the object type, from which the string type is derived.

$ ./covariance.exe 
C#

Output of the array covariance example.

In the following example, we have a covariance for generics.

using System;
using System.Collections.Generic;

public class Covariance2
{
  static void Main()
  {
    IEnumerable<string> strings = new List<string>() {"1", "3", "2", "5"};
    PrintAll(strings); 
  }

  static void PrintAll(IEnumerable<object> objects)
  {
    foreach (object o in objects)
    {
      System.Console.WriteLine(o);
    }
  }
}

We have a generic list of strings. We call the PrintAll() method which prints all the elements of the list. Note that the method parameter has a narrower type parameter than our generic list.

$ ./covariance2.exe 
1
3
2
5

This is the output of the covariance2.exe program.

The following is an example for a contravariance in delegates.

using System;
using System.Collections.Generic;

public class Contravariance
{
  static void Main()
  { 
    Action<string> del = ShowMessage; 
    del("Proximity alert");     
  }

  static void ShowMessage(object message) 
  { 
    Console.WriteLine(message);
  }
}

We assign a method with a narrower parameter type to a delegate which has a wider parameter type.

$ ./contravariance.exe 
Proximity alert

This is the output of the contravariance.exe program.

In this part of the C# tutorial we have talked about new features of the C# 4.0.