Initialization of Variables, Revisited

We said back in Chapter 1 that trying to access variables without explicitly initializing them will result in a compilation error. For example, this next bit of code

public class Example
{
  static void Main() {
      // Declare several local variables within the Main() method.
      int i; // not automatically initialized
      int j; // ditto
      j = i; // compilation error!
    }
}

was shown to produce the following compilation error on the line that is highlighted in the snippet:

  error CS0165: Use of unassigned local variable 'i'

We also stated in Chapter 3 that variables are implicitly assigned their zero-equivalent value in some situations, if we haven't explicitly assigned them a value.

Both of these statements regarding initialization of variables were a bit over-simplified, however, and we'd like to correct the oversimplification now.

To properly understand the notion of initialization in C#, we must differentiate between local variables—that is, variables declared within a method, and whose scope is therefore limited to that method (recall our discussion of the scope of a variable in Chapter 1)—and fields of a class (whether instance or static variables), which are declared at the class scope level. As it turns out:

• All local variables, of any type, are considered by the compiler to be uninitialized until they have been explicitly initialized within a program.

• All fields, on the other hand, of any type, are automatically initialized to their zero-equivalent values—that is, bools are initialized to false, numerics to either 0 or 0.0, reference types to null, and so forth.

Here is an example illustrating all of these points:

public class Student
{
  // Fields ARE automatically initialized.
  private int age;                   // initialized to 0
  private double gpa;                // initialized to 0.0
  private bool isHonorsStudent;      // initialized to false
  private Professor myAdvisor;       // initialized to null
  // This includes STATIC variables.
  private static int studentCount;   // initialized to 0
  // etc.

  // Methods.
  public void UpdateGPA() {
    // Local variables are NOT automatically initialized.
    double val;    // NOT initialized -- value is undefined.
    Course c;      // NOT initialized -- value is undefined.
    // etc.
  }
}