Parameters are optional when a default value is specified as part of a declaration.
Example: public List<Product> GetProductByCategory(string category, int pageIndex=0){}
Here pageIndex is an optional parameter.
Named arguments allow us to explicitly name an argument we are passing to a method – instead of just identifying it by argument position. 
Example: var products= GetProductByCategory(“beverages”, pageIndex=2){} here pageIndex is a named argument.

Upcasting: An upcast operation creates a base class reference from a subclass reference.
Example:
Stock msft = new Stock();
       Asset a = msft;
Downcasting: A downcast operation creates a subclass reference from a base class reference.
Example:
Stock msft = new Stock();
       Asset a = msft;
       Stock s = (Stock)a;

When an instance method declaration includes a sealed modifier, that method is said to be a sealed method. If an instance method declaration includes the sealed modifier, it must also include the override modifier. Use of the sealed modifier prevents a derived class from further overriding the method.

Dynamic binding :Dynamic  binding defers  binding—the  process  of  resolving  types,  members,  and
operations—from  compile  time  to  run-time.Static binding : It is defined as, when we compile our program and an object type is determined then it is known as static binding or early binding.Custom binding : Defines a binding from a list of binding elements.Language binding : Language  binding  occurs  when  a  dynamic  object  does  not  implement
IDynamicMetaObjectProvider

Boxing:Boxing is the process of converting a value type to the object type or to any interface type implemented by this value type. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap. Unboxing: Unboxing extracts the value type from the object. Boxing is implicit; unboxing is explicit. The concept of boxing and unboxing underlies the C# unified view of the type system in which a value of any type can be treated as an object.