Possibly-Null Values Tutorial

This tutorial shows in a simple Cω program how to work with possibly-null values.

Tutorial

It is very convenient to be able to express in the type-system that null is an expected value. In the CLR, member access throws a NullReferenceException when the receiver is null. For example, after the assignment Button b = null; subsequent member access b.AllowDrop will fail. In many cases we don't care what happens if the receiver is null (this is the default in Objective C, where null behaves as the zero element of member access), but in order to achieve that we need to write a lot of boiler-plate code:

  if(b!=null){
    ... do something with b ...
   } else {
    ... performs some default action ... } 

When the receiver has static type T? in Cω, member access is lifted for the other stream types, and does nothing except for propagating null. The possibility of failure in recorded in the return type of the lifted member access, which becomes bool? instead of bool.

  Button? b = null;
  bool? d = b.AllowDrop;  // returns null

For value types, T? does not introduce reference identity, that is, values of type T? are still copied on assignment; the important difference between value types T and T? is that instances of T? can be null whereas instances of value type T are never null.

Example

The following is a complete Cω program that demonstrates working with possibly-null values.

using Microsoft.Comega;
using System.Windows.Forms;

public class Test {
    static void Main() {
      Button? b = null;
      bool? d = b.AllowDrop;
      //succeeds
      Button c = (Button)b;
      try {
       bool e = (bool)d;
      } catch(NullReferenceException e) {
          Console.WriteLine("d == null = {0}", d == null);
      }
      Console.ReadLine();
    }
  }

Output

d == null = True