Programming C#, Third Edition

Creating Objects

In Chapter 3, a distinction is drawn between value types and reference types. The primitive C# types (int, char, etc.) are value types, and are created on the stack. Objects, however, are reference types, and are created on the heap, using the keyword new, as in the following:

Time t = new Time( );

t does not actually contain the value for the Time object; it contains the address of that (unnamed) object that is created on the heap. t itself is just a reference to that object.

Tip

VB6 programmers take note: While there is a performance penalty in using the VB6 keywords Dim and New on the same line, in C# this penalty has been removed. Thus in C# there is no drawback to using the new keyword when declaring an object variable.

Constructors

In Example 4-1, notice that the statement that creates the Time object looks as though it is invoking a method:

Time t = new Time( );

In fact, a method is invoked whenever you instantiate an object. This method is called a constructor, and you must either define one as part of your class definition or let the Common Language Runtime (CLR) provide one on your behalf. The job of a constructor is to create the object specified by a class and to put it into a valid state. Before the constructor runs, the object is undifferentiated memory; after the constructor completes, the memory holds a valid instance of the class type.

The Time class of Example 4-1 does not define a constructor. If a constructor is not declared, the compiler provides ...