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// Catch a possible invalid cast.

try

{

Hexagon hex = (Hexagon)frank;

}

catch (InvalidCastException ex)

{

Console.WriteLine(ex.Message);

}

While this is a fine example of defensive programming, C# provides the as keyword to quickly

determine at runtime whether a given type is compatible with another. When you use the as key-

word, you are able to determine compatibility by checking against a null return value. Consider the

following:

// Use 'as' to test compatability.

Hexagon hex2 = frank as Hexagon;

if (hex2 == null)

Console.WriteLine("Sorry, frank is not a Hexagon...");

The C# is Keyword

Given that the FireThisPerson() method has been designed to take any possible type derived from

Employee, one question on your mind may be how this method can determine which derived type

was sent into the method. On a related note, given that the incoming parameter is of type Employee,

how can you gain access to the specialized members of the SalesPerson and Manager types?

In addition to the as keyword, the C# language provides the is keyword to determine whether

two items are compatible. Unlike the as keyword, however, the is keyword returns false, rather

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than a null reference, if the types are incompatible. Consider the following implementation of the

FireThisPerson() method:

static void FireThisPerson(Employee emp)

{

if (emp is SalesPerson)

{

Console.WriteLine("Lost a sales person named {0}", emp.Name);

Console.WriteLine("{0} made {1} sale(s)...", emp.Name,

((SalesPerson)emp).SalesNumber);

Console.WriteLine();

}

if (emp is Manager)

{

Console.WriteLine("Lost a suit named {0}", emp.Name);

Console.WriteLine("{0} had {1} stock options...", emp.Name,

((Manager)emp).StockOptions);

Console.WriteLine();

}

}

Here you are performing a runtime check to determine what the incoming base class reference

is actually pointing to in memory. Once you determine whether you received a SalesPerson or

Manager type, you are able to perform an explicit cast to gain access to the specialized members of

the class. Also notice that you are not required to wrap your casting operations within a try/catch

construct, as you know that the cast is safe if you enter either if scope, given our conditional check.