Strict Non-Null Mode

If you compile your code with --enable-experimental-non-null the Vala compiler will run in strict non-null type checking mode and consider every type to be not nullable by default unless it is explicitly declared nullable by marking it with a question mark:

Object o1 = new Object();     // not nullable
Object? o2 = new Object();    // nullable

The compiler will perform a static compile-time analysis to ensure that no nullable reference is assigned to a non-nullable reference, e.g. this won't be possible:

o1 = o2;

o2 could be null and o1 was declared non-nullable, so this assignment is forbidden. However, you can override this behaviour with an explicit non-null cast if you're sure that o2 is not null:

o1 = (!) o2;

The strict non-null mode helps in avoiding unwanted null dereferencing errors. This feature would come to full potential if the nullability of all return types in bindings was marked correctly, which is currently not always the case.