Now that you have added fields to a class and can store data, you need to consider the validity of that data. As you saw in Listing 5.3, it is possible to instantiate an object using the new operator. The result, however, is the ability to create an employee with invalid data. Immediately following the assignment of employee, you have an Employee object whose name and salary are not initialized. In this particular listing, you assigned the uninitialized fields immediately following the instantiation of an employee, but if you failed to do the initialization, you would not receive a warning from the compiler. As a result, you could end up with an Employee object with an invalid name.
Declaring a Constructor
To correct this, you need to provide a means of specifying the required data when the object is created. You do this using a constructor, demonstrated in Listing 5.16.
Listing 5.16. Defining a Constructor
To define a constructor you create a method with no return type, whose method name is identical to the class name.
The constructor is the method that the code calls to create an instance of the object. In this case, the constructor takes the first name and the last name as parameters, allowing the programmer to specify these names when instantiating the Employee object. Listing 5.17 is an example of how to call a constructor.
Listing 5.17. Calling a Constructor
Notice that the constructor returns the type of the object being instantiated (even though no return type or return statement was specified explicitly in the constructor's declaration or implementation). In addition, you have removed the initialization code for the first and last names because that occurs within the constructor. In this example, you don't initialize Salary within the constructor, so the code assigning the salary still appears.
Developers should take care when using both assignment at declaration time and assignment within constructors. Assignments within the constructor will occur after any assignments are made when a field is declared (such as string Salary = "Not enough" in Listing 5.5). Therefore, assignment within a constructor will take precedence and will override any value assigned at declaration time. This subtlety can lead to a misinterpretation of the code by a casual reader that assumes the value after instantiation is assigned at declaration time. Therefore, it is worth considering a coding style that does not mix both declaration assignment and constructor assignment within the same class.
It is important to note that by adding a constructor explicitly, you can no longer instantiate an Employee from within Main() without specifying the first and last names. The code shown in Listing 5.18, therefore, will not compile.
Listing 5.18. Default Constructor No Longer Available
If a class has no explicitly defined constructor, then the C# compiler adds one during compilation. This constructor takes no parameters and is, therefore, the default constructor by definition. As soon as you add an explicit constructor to a class, the C# compiler no longer provides a default constructor. Therefore, with Employee(string firstName, string lastName) defined, the default constructor, Employee(), is not added by the compiler. You could explicitly add such a constructor, but then you would again be allowing construction of an Employee without specifying the employee name.
It is not necessary to rely on the default constructor defined by the compiler. It is also possible for programmers to define a default constructor explicitly, perhaps one that initializes some fields to particular values. Defining the default constructor simply involves declaring a constructor that takes no parameters.
Constructors can be overloaded. For example, as Listing 5.19 shows, you could provide a constructor that has an employee ID with first and last names, or even just the employee ID.
Listing 5.19. Overloading a Constructor
This enables Program.Main() to instantiate an employee from the first and last names either by passing in the employee ID only, or by passing both the names and the IDs. You would use the constructor with both the names and the IDs when creating a new employee in the system. You would use the constructor with only the ID to load up the employee from a file or a database.
Calling Another Constructor Using this
Notice in Listing 5.19 that the initialization code for the Employee object is now duplicated in multiple places and, therefore, has to be maintained in multiple places. The amount of code is small, but there are ways to eliminate the duplication by calling one constructor from another, using constructor initializers. Constructor initializers determine which constructor to call before executing the implementation of the current constructor (see Listing 5.20).
Listing 5.20. Calling One Constructor from Another
To call one constructor from another within the same class (for the same object instance) C# uses a colon followed by the this keyword followed by the parameter list on the callee constructor's declaration. In this case, the constructor that takes all three parameters calls the constructor that takes two. Often, the calling pattern is reversed; the constructor with the fewest parameters calls the constructor with the most parameters, passing defaults for the parameters that are not known.