0.7 Xna0112-type conversion, casting, and assignment compatibility  (Page 2/7)

If not, it may be possible to perform a cast on the value to change its type and assign it to the variable as the new type.

Successful cast depends on class hierarchy

With regard to reference types, whether or not a cast can be successfully performed depends on the relationship of the classes involved in the classhierarchy.

The generic type Object

A reference to any object can be assigned to a reference variable of the type Object , because the Object class is a superclass of every other class.

Calling a method on an object

Whether or not a method can be called on a reference to an object depends on the current type of the reference and the location in the class hierarchy where the method is defined.

In order to use a reference of a class type to call a method, the method must be defined at or above that class in the class hierarchy.

Assignment compatibility and type conversion

As background, for understanding runtime polymorphism, you need to understand assignment compatibility and type conversion .

As I mentioned earlier, a value of a given type is assignment-compatible with another type if a value of the first type can be successfully assigned to avariable of the second type.

Type conversion and the cast operator

In some cases, type conversion happens automatically. In other cases, type conversion must be forced through theuse of a cast operator.

A cast operator is a unary operator, which has a single right operand. The physical representation of the cast operator is the name of a type enclosed by apair of matching parentheses, as in:

Applying a cast operator

Applying a cast operator to the name of a variable doesn't actually change the type of the variable. However, it does cause the contents of the variable tobe treated as a different type for the evaluation of the expression in which the cast operator is contained.

Primitive values and type conversion

Assignment compatibility issues come into play for both primitive types and reference types.

Boolean types

To begin with, values of type bool can only be assigned to variables of type bool (you cannot change the type of a bool ). Thus, a value of type bool is not assignment-compatible with a variable of any other type.

Numeric primitive types

In general, numeric primitive values can be assigned to (are assignment-compatible with) a variable of a type whose numeric range is as wideas or wider than the range of the type of the value. In that case, the type of the value is automatically converted to the type of the variable.

On the other hand, a primitive numeric value of a given type cannot be assigned to (is not assignment-compatible with) a variable of a type with anarrower range than the type of the value.

However, it is possible to use a cast operator to force a type conversion for numeric primitive values.