New C# 4.0 features

1) dynamic

"most anything that you could do with some variable, you can do with a dynamic variable."

static double SumDouble(double a, double b) { return a + b; }
static decimal SumDecimal(decimal a, decimal b) {return a + b;}

this can now be writen as :

static dynamic Sum1(dynamic a, dynamic b) { return a + b; }

or more generic :

static dynamic Sum2(T1 a, T2 b) {
dynamic ad = a;
dynamic bd = b;
return ad + bd;
}

more:
Dynamically Typed Objects with C# 4.0 (xml example)



2) Named arguments, optional arguments, and default values

By giving method parameters default values.

public class ContactList
{
  List<Contact> SearchForContacts(
      string name = "any",
      int age = -1,
      string address = "any") { ... }

  static void Main()
  {
      ContactList list = new ContactList();
      var x = list.SearchForContacts(age:26);
  }
}

U can then call the method like this:

list.SearchForContacts(age:26);

or

list.SearchForContacts(address:"home", name:"sam", age:30);