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This chapter is from the book

This chapter is from the book

Constructors

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 problem, you need to provide a means of specifying the required data when the object is created. You do this using a constructor as demonstrated in Listing 5.25.

LISTING 5.25: Defining a Constructor

class Employee
{
  // Employee constructor                                                   
  public Employee(string firstName, string lastName)                        
  {                                                                         
    FirstName = firstName;                                                  
    LastName = lastName;                                                    
  }                                                                         
 
  public string FirstName{ get; set; }
  public string LastName{ get; set; }
  public string Salary{ get; set; } = "Not Enough";
 
  // ...
}

As shown here, 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 runtime calls to initialize 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 5.26 is an example of how to call a constructor.

LISTING 5.26: Calling a Constructor

class Program
{
  static void Main()
  {
      Employee employee;
      employee = new Employee("Inigo", "Montoya");                            
      employee.Salary = "Too Little";
 
      System.Console.WriteLine(
          "{0} {1}: {2}",
          employee.FirstName,
          employee.LastName,
          employee.Salary);
  }
  // ...
}

Notice that the new operator 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 initialization takes place 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 override any value assigned at declaration time. This subtlety can lead to a misinterpretation of the code by a casual reader who assumes the value after instantiation is the one assigned in the field declaration. Therefore, it is worth considering a coding style that does not mix both declaration assignment and constructor assignment within the same class.

Default Constructors

When you add 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 5.27, therefore, will not compile.

LISTING 5.27: Default Constructor No Longer Available

class Program
{
  static void Main()
  {
      Employee employee;
      // ERROR: No overload because method 'Employee'                        
      // takes '0' arguments.                                                
      employee = new Employee();                                             
 
      // ...
  }
}

If a class has no explicitly defined constructor, the C# compiler adds one during compilation. This constructor takes no parameters and, therefore, is 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 manually 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.

Object Initializers

Begin 3.0

Starting with C# 3.0, the C# language team added functionality to initialize an object’s accessible fields and properties using an object initializer. The object initializer consists of a set of member initializers enclosed in curly braces following the constructor call to create the object. Each member initializer is the assignment of an accessible field or property name with a value (see Listing 5.28).

LISTING 5.28: Calling an Object Initializer

class Program
{
  static void Main()
  {
        Employee employee1 = new Employee("Inigo", "Montoya")
            { Title = "Computer Nerd", Salary = "Not enough"};
      // ...
  }
}

Notice that the same constructor rules apply even when using an object initializer. In fact, the resultant CIL is exactly the same as it would be if the fields or properties were assigned within separate statements immediately following the constructor call. The order of member initializers in C# provides the sequence for property and field assignment in the statements following the constructor call within CIL.

In general, all properties should be initialized to reasonable default values by the time the constructor exits. Moreover, by using validation logic on the setter, it is possible to restrict the assignment of invalid data to a property. On occasion, the values on one or more properties may cause other properties on the same object to contain invalid values. When this occurs, exceptions from the invalid state should be postponed until the invalid interrelated property values become relevant.

End 3.0

Overloading Constructors

Constructors can be overloaded—you can have more than one constructor as long as the number or types of the parameters vary. For example, as Listing 5.30 shows, you could provide a constructor that has an employee ID with first and last names, or even just the employee ID.

LISTING 5.30: Overloading a Constructor

class Employee
{
  public Employee(string firstName, string lastName)
  {
      FirstName = firstName;
      LastName = lastName;
  }
 
  public Employee(                                                                 
      int id, string firstName, string lastName )                                  
  {                                                                                
      Id = id;                                                                     
      FirstName = firstName;                                                       
      LastName = lastName;                                                         
  }                                                                                
                                                                                   
  public Employee(int id)                                                          
  {                                                                                
      Id = id;                                                                     
                                                                                   
      // Look up employee name...                                                  
      // ...                                                                       
  }                                                                                
                                                                                   
  public int Id { get; set; }                                                      
  public string FirstName { get; set; }
  public string LastName { get; set; }
  public string Salary { get; set; } = "Not Enough";
 
  // ...
}

This approach 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.

As is the case with method overloading, multiple constructors are used to support simple scenarios using a small number of parameters and complex scenarios with additional parameters. Consider using optional parameters in favor of overloading so that the default values for “defaulted” properties are visible in the API. For example, a constructor signature of Person(string firstName, string lastName, int? age = null) provides signature documentation that if the Age of a Person is not specified, it will default to null.

Constructor Chaining: Calling Another Constructor Using this

Notice in Listing 5.30 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—constructor chaining—using constructor initializers. Constructor initializers determine which constructor to call before executing the implementation of the current constructor (see Listing 5.31).

LISTING 5.31: Calling One Constructor from Another

class Employee
{
  public Employee(string firstName, string lastName)
  {
      FirstName = firstName;
      LastName = lastName;
  }
 
  public Employee(                                                    
      int id, string firstName, string lastName )                     
      : this(firstName, lastName)                                     
  {
      Id = id;
  }
 
  public Employee(int id)
  {
      Id = id;
 
      // Look up employee name...
      // ...
 
      // NOTE:  Member constructors cannot be                         
      // called explicitly inline                                     
      // this(id, firstName, lastName);                               
  }
 
  public int Id { get; private set; }
  public string FirstName { get; set; }
  public string LastName { get; set; }
  public string Salary { get; set; } = "Not Enough";
 
  // ...
}

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 parameters. Often, this calling pattern is reversed—that is, the constructor with the fewest parameters calls the constructor with the most parameters, passing defaults for the parameters that are not known.

Begin 3.0
End 3.0
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