In this post, I am going to discuss about the casting of object form one type to another type and what the things required to be kept in mind when casting object.

Casting basically takes place when we are relating between classes i.e. INHERITANCE. Classes have a parent child relationship, i.e. is-a relationship between them. To understand it, take an example of the below diagram:

As show in the above image, Employee and Manager are having is-a relationship with person, i.e., inherited from person class.
Now I have one method in my main class which takes Employee as argument and does processing on the property of the class.

public void ProcessData(Person p)
{
....process data 
Console.WriteLine(p.ToString());
}

Implicit Conversion

Implicit conversion takes place when there is an is-a relation between two classes, i.e., inherited from other. As per the diagram, Employee is Inherited from Person class and so that as per OOD rule, there is no need to convert Employee class to Person class.

Example:

public void testdata()
{
 Employee emp = new Employee();
 emp.Age = 25;
 emp.BirthDate = DateTime.Now;
 emp.Height = "5 ' 3";
 emp.salary = 2000;
 emp.type = 1;
 ProcessData(emp);
}

When you compile and run the code, there is no error because it is Implicit conversion from child to parent.

Explicit Conversion

Where there is no relation between classes and if you want to pass the object of another type to method, then you need to convert it explicitly.

Example

Consultant cons = new Consultant();
 cons.Age = 25;
 ....

object obj = cons;
 ProcessData((Employee) obj);

Note

The above code does not give any kind of compiler error, but it throws runtime InvalidCastException if it is not able to cast to the type.

Following is solution to avoid InvalidCastException at runtime because of Explicit conversion.

Solution 1

Catch the exception and handle it.

Consultant cons = new Consultant();
 cons.Age = 25;
 ....
 try
 {
   ProcessData((Employee) cons);
 }
 catch(InvalidCastException ex)
 {
    ... process exception
 }

Solution 2

Make use of 'as' keyword of C#. It does conversion from one object to another object and returns Null if it is not able to convert. Use it when you want to convert object from one type to another object.

Syntax

type t =expression as type;

Example

Employee emp = new Employee();
  Person p = emp as Person; 
  if(p!=null)
  {
   .. process data
  } 
  else
  {
   .. display error message or do another code
  }

Solution 3

Make use of 'is' keyword of C#. It does conversion from one object to another object and returns false if it is not able to convert. Use it when you want to check whether it is convertible or not.

Syntax

type t = expression is type

Example

Employee emp = new Employee();
  if(emp is Person)
  {
    Person p = (Person)emp;
    .. process data
  } 
  else
  {
   .. display error message or do another code
  }

Difference between as and is

as operator does conversion form one type to another type and returns Null if conversion fails. There is no need for conversion again if it's convertible as shown in the example code.
is operator checks whether one object is convertible to another type or not and returns false if not. So you need to convert object to base type if it is convertible as shown in the example code.

Summary

'is' and 'as' keywords play an important role when we do the casting of the related objects explicitly. But you need to use it according to the situation as we discuss in the Difference section of this post.

CodeProject

Explicit and Implicit Casting of object and Role of 'is' and 'as' keyword in Explicit casting