CSharp Bits is a tutorial-based digest that explains the C# programming language. It evolves, covering prerequisite topics and is currently broad in coverage rather than getting deep into a specific topic. That will come after a basis has been established.  I issue the topics daily (:p), and mainly cover one topic in each issue, so readers can fit it in their daily routine.  I email these topics internally at Microsoft and have been asked to blog the digests externally to reach out to the user community.

All code examples are meant to demonstrate the topic only. It is neither shippable nor secure code by any stretch of the imagination.  An archive of the topics I submitted internally within Microsoft before I started blogging these out is not currently available externally.

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Base/Derived Conversions

You can convert a reference to an object of a derived class to an object of its base class, and vice versa, under certain conditions.

Conversion to Base Class Reference

References to objects of one class type can be converted into references to another type if one class inherits from the other, either directly or indirectly.

A reference to an object can always be converted to a reference to a base class object. This conversion can be performed implicitly (by assignment or as part of an expression) or explicitly (by using the cast operator).

The following examples will use two classes: Animal and Bird. Animal is the base class of Bird, or, to put it another way, Bird inherits from Animal.

The following example declares a variable of type Animal and a variable of type Bird:

Animal a;
Bird b = new Bird(...);

Now consider the following assignment, in which the reference in b is copied to a:

a = b;

The Bird class inherits from the Animal class. Therefore, a method that is found in Animal is also found in Bird. (The Bird class might have overridden some of the methods of Animal to create its own version of them, but an implementation of the method will exist nonetheless.) Therefore, it is possible for references to Bird objects to be assigned to variables containing references to objects of type Animal.

In this case, C# performs a type conversion from Bird to Animal. You can explicitly convert Bird to Animal by using a cast operator, as shown:

a = (Animal) b;

The preceding code will produce exactly the same result.

Conversion to Derived Class Reference

You can convert a reference to a derived type, but you must explicitly specify the conversion by using a cast. An explicit conversion is subject to run-time checking to ensure that the types are compatible, as shown in the following example:

Bird b = (Bird) a; // Okay

This code will compile successfully. At run time, the cast operator performs a check to determine whether the object referred to is really of type Bird. If it is not, the run-time InvalidCastException is raised.

If you attempt to assign to a derived type without a conversion operator, as in the following code, the compiler will display an error message stating, "Cannot implicitly convert type 'Animal' to 'Bird.'"

b = a; // Will not compile

You can trap a type conversion error by using try and catch, just like any other exception, as shown in the following code:

try {
   b = (Bird) a;
}
catch (InvalidCastException) {
   Console.WriteLine("Not a bird");
}