C

1
C Data Types
2
some common C types
3
type conversion

• there are various ways to accomplish conversions

int i 5 double d 3.2 string s
"496" d i i (int) d i
System.Convert.ToInt32(s)
implicit conversion
cast required
conversion required
4
value types, vs. reference types

• value types are C fields that correspond to .NET
  primitives, such as
• bool, int, float, enum, struct
• when sent as parameters in a method call, value
  types are allocated on the stack
• they have a lifetime limited by their scope (as
  in C)
• they are not allocated on the managed heap
• reference types
• fields that act as shortcuts to objects in the
  managed heap

5
value vs. reference types

• .NET separates data types into two categories
• value types
• variable represents a value ("bits")
• reference types
• variable represents a reference to a heap-based
  object
• actual data resides in the object

int i i 10
10
string s s "calico"
6
how do you know which types are which?

• primitive types (except strings) are value types
• all other types are references, including arrays
• why do we care?
• efficiency
• comparison

int x, y string s1, s2 Customer c1,
c2 . . . if (x y) ... if (x lt y) ... if
(s1.Equals(s2)) ... if (s1.CompareTo(s2) lt 0)
... if (c1.Equals(c2)) ...
7
boxing and unboxing

• when necessary, C will auto-convert between
  value and object
• value gt object is called "boxing"
• object gt value is called "unboxing"

int i, j object obj string s i
32 obj i // boxed (implicit) i
19 j (int) obj // unboxed via cast
8
The Common Type System (CTS)

• CTS is based on a hierarchy of classes defined in
  FCL
• All types inherit from Object (all except
  interface types)

9
user-defined reference types
10
classes

• classes yield user-defined reference types
• example
• Customer class

public class Customer public string Name
// fields public int ID public
Customer(string name, int id) // constructor
this.Name name this.ID id
public override string ToString() // method
return "Customer " this.Name
11
creating objects

• objects are created using the new operator
• strings are a special case and don't require the
  use of new

Customer c1, c2 string s1, s2 c1 new
Customer("jim bag", 36259) c2 new
Customer("jane doe", 55298) s1 "an apple a
day" s2 "keeps the doctor away"
12
defining equality

• all classes inherit from System.Object
• and it has a method .Equals
• classes should override Equals
• example
• Customers are equal if their IDs are the same

public class Customer . . . public
override bool Equals(object obj) Customer
other if ( obj null
!(obj.GetType().Equals(this.GetType())) )
return false // definitely not equal other
(Customer) obj // type-cast to access id
return this.ID other.ID // equal if same
id...
13
GetHashCode

• If you override Equals, you must also override
  GetHashCode
• here's the rule
• if obj1.Equals(obj2) true, then obj1.GHC()
  obj2.GHC()
• otherwise, it doesn't matter what GHC returns

public class Customer . . . public
override int GetHashCode() // delegate
hash code computation to underlying integer
class return this.ID.GetHashCode()
14
all objects derive from System.Object

• user-defined classes can (and sometimes should)
  override some of the System.Object methods,
  including
• Equals - comparison between objects
• Finalize - performs cleanup operations before an
  object is automatically reclaimed.
• GetHashCode - generates a number corresponding to
  the value of the object to support the use of a
  hash table.
• ToString - manufactures a human-readable text
  string that describes an instance of the class

15
C arrays
16
Arrays

• Arrays are reference types
• based on Array class in FxCL
• must be created using new
• 0-based indexing
• assigned default values (0 for numeric, null for
  references, etc.)

int a a new int5 a0 17 a1 32
a4 2 int x a0 a1 a4 int len
a.Length
create
element access
number of elements
17
types of methods

• classes may contain 2 types of methods
• subroutines with no return value (void)
• functions with a return value (int, string, etc.)
• methods may be
• static
• instance (default if static keyword not present)
• static methods are global and thus require only
  class name
• instance methods require a reference to an object
  of the class in order to be called

18
Array class in the framework library

• fully-qualified name is System.Array

namespace System public class Array
public int GetUpperBound(int dimension)
... public static void Sort(Array a)
... . . .
instance method(absence of static)
static method(presence of static)
19
an example of calling into the Array class

• .Sort is a static method
• the argument to .Sort is an instance of an Array
• or an Array object

/ main.cs / using System public class App
public static void Main() int data
11, 7, 38, 55, 3 Array.Sort(data)
for (int i0 iltdata.GetUpperBound(0)
i) Console.WriteLine(i " "
datai)
20
multi-dimensional arrays

• C supports arrays as a single object, or array
  of arrays (potentially jagged arrays)

Customer, twoD int jaggedD // 2D
array as single object twoD new Customer10,
100 twoD0, 0 new Customer() twoD9, 99
new Customer() // 2D array as array of
arrays jaggedD new int10 jaggedD0 new
int10 jaggedD1 new int20 jaggedD9
new int100 jaggedD00 1 jaggedD999
100
21
foreach

• specialized foreach loop provided for collections
  like Array
• reduces risk of indexing error
• provides read-only access to collection (but
  objects within collection can be modified)

int myArray 1, 2, 3, 4, 5 int sum
0 foreach (int x in myArray) sum x
foreach
type
value
collection
22
the end of this PowerPoint file
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