Reference Types

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ReferenceTypes
(c) Allan C. Milne School of Computing Creative
Technologies University of Abertay dundee
Last updated 22nd October 2004
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Agenda

• Reference Types
• Object Types
• The Object String Classes
• Interface types
• Pointer Types
• Assignment Compatibility

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Reference Types

• are fully inheritable.
• combine a reference identity to a location
  (address) with the value stored in that location.
• values are accessed through strongly-typed
  references.
• are updated if the location is moved by the
  garbage collector.

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A Reference Type Example

• Here is an example of declaring a variable of an
  object reference type and initialising it to a
  reference to an object value.

String str "cat"
str
reference strongly typed to a String
location can only contain String values
variable name
"cat"
value an object of type String
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Reference Type Categories

• Object types
• represent classes
• Interface types
• represent interfaces
• Pointer types
• represent typed addresses

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Object Types

• The CLS specifies two built-in object types with
  support in the CLR
• Object (System.Object)
• String (System.String)
• Other types provided in the .Net Framework are
  considered as user-defined object types.
• All user-defined object types inherit (directly
  or indirectly) from the CLR type Object.
• Note that object type and class are synonymous.

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The Object Class

• The base type for all types in the CLR.
• Enforces a single CLR supported type inheritance
  hierarchy for all types.
• Includes value types since boxing allows these to
  be treated as subtypes of Object.
• Provides a poymorphic type that can be used for
  return types parameters in generalised methods.

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Object Methods

• Object provides anumber of public methods that
  can be used with all types or over-ridden as
  required
• bool Equals (Object o)
• int GetHashCode ()
• Type GetType ()
• static bool ReferenceEquals (Object o1,
  Object o2)
• String ToString ()
• There are also a number of protected methods that
  can be useful.

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The String Class

• is a built-in object type in the CLR.
• is a sealed class (for efficiency).
• defines values (objects) that are immutable.
• uses 16-bit Unicode characters in its string
  representation.
• has many methods providing great functionality.
• has "mutator" methods that return new String
  objects rather than change the current object
  (immutability).

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Interface Types

• support programming with interfaces.
• provide a mechanism for relating classes which do
  not share implementation inheritance but do share
  some common method bahaviour.
• are partial specifications of types with member
  types but no implementations.
• may be supported by many classes and a class may
  support many interface types.

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An Interface Example
interface IPosition int X get int
Y get void Place (int x,int y)
class Eg IPosition private int xpos,
ypos public int X get return xpos
public int Y get return ypos
public void Place (int x,int y) xpos
x ypos y
Eg myEg new Eg() myEg.Place (12, 34) int
a myEg.X int b myEg.Y Iposition ip
myEg ip.Place (67, 89) int i ip.X int j
myEg.Y
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Interface Types Members

• There are no built-in interface types.
• The .Net Framework provides a number of
  interfaces.
• An interface type can contain fields, methods,
  properties and events.
• The methods, properties events must, by
  definition, be
• public available to all users of the interface
• abstract body must be defined in using class
• virtual execution-time binding used

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Pointer Types

• are included here only for completeness, not for
  use.
• represent the locations of code or values (I.e.
  are addresses).
• will usually be abstracted by a programming
  language so as to be invisible to programs.
• are useful in understanding the .Net platform but
  dangerous to use in programs.
• are open to misuse by going outside the semantics
  of the .Net managed code environment (especially
  the gc).

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Pointer Types Supported

• Managed pointers
• known to the garbage collector.
• Unmanaged function pointers
• refer to code
• Unmanaged pointers
• refer to values.
• are not CLS compliant.

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Assignment Compatibility

• A variable of object or interface type T can
  refer to any object
• of type T, or
• of a subtype of T, or
• that supports the interface T.
• The above rules apply when checking if the left
  and right parts of an assignment are compatible.
• Assignment may be explicit ('' operator) or
  implicit (e.g. parameter passing).

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Compatibility Example
int i 42 Object o
null String s "Allan" IComparable ic
null
// compatible o i // uses boxing o
s o ic ic i // uses boxing ic s
// incompatible i o i s i ic s i s
o s ic ic o
// run-time incompatibility o i s (String)
o

• see CompatibilityEg1.cs

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Some Tidying Up

• An object type (class) can inherit from another
  (single) object type and/or multiple interfaces
• The CLR supports a number of member accessibility
  options although in C public, private and
  protected are used.
• Methods may be overloaded over-ridden.
• Virtual methods are supported by run-time
  dispatching.

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An Example of Inheritance
class PlacedBall SnookerBall,
IPosition private int xpos, ypos
public int X get return xpos
public int Y get return ypos public
PlacedBall (BallColour c) base (c)
public void Place (int x, int y) xpos
x ypos y
PlacedBall ball ball new PlacedBall
(BallColour.Red) ball.Place (12,34) int v
ball.Value int x ball.X IPosition ip
ball ip.Place (67, 89) int ipx ip.X int bx
ball.X

• see PlacedBall.cs