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Objectives

.NET is designed around the CTS, or Common Type System. TheCTS is what allows assemblies, written in different languages, towork together. To ensure interoperability across languages,Microsoft has also defined the CLS, or Common LanguageSpecification, a subset of the CTS that all languages support.Otherwise, the types in C# are what you would expect from a

modern OOPL

The Common Type System

Value vs. reference types Arrays

Namespaces

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Part 1

The Common Type System

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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)

String Array ValueType Exception Delegate Class1

MulticastDelegate

Class2

Class3

Object

Enum1

Structure1EnumPrimitive types

Boolean

Byte

Int16

Int32

Int64

Char

Single

Double

Decimal

DateTime

System-defined types

User-defined types

Delegate1

TimeSpan

Guid

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The Common Language Specification (CLS)

Not all languages support all CTS types and features C# supports unsigned integer types, VB.NET does not

C# is case sensitive, VB.NET is not

C# supports pointer types (in unsafe mode), VB.NET does not

C# supports operator overloading, VB.NET does not

CLS was drafted to promote language interoperability

vast majority of classes within FCL are CLS-compliant

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Mapping C# to CTS

Language keywords map to common CTS classes:

Keyword Description Special format for literals

bool Boolean true false

char 16 bit Unicode character 'A' '\x0041' '\u0041'

sbyte 8 bit signed integer none

byte 8 bit unsigned integer none

short 16 bit signed integer none

ushort 16 bit unsigned integer none

int 32 bit signed integer none

uint 32 bit unsigned integer U suffix

long 64 bit signed integer L or l suffixulong 64 bit unsigned integer U/u and L/l suffix

float 32 bit floating point F or f suffix

double 64 bit floating point no suffix

decimal 128 bit high precision Mormsuffix

string character sequence "hello", @"C:\dir\file.txt"

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Example

An example of using types in C#

declare before you use (compiler enforced)

initialize before you use (compiler enforced)

public class App{public static void Main(){int width, height;width = 2;height = 4;

int area = width * height;

int x;int y = x * 2;...

}

}

declarations

decl + initializer

error, x not set

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Part 2

Value vs. reference types

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Value vs. reference types

C# 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";

"calico"

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How do you know which types are which?

Memorization!

Though it's pretty obvious based on past experience

primitive types like bool, int and double are values

remainder are reference types

int i;string s;Customer c1, c2;

i = 23;s = "a message";

c1 = null;c2 = new Customer();

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Boxing and Unboxing

When necessary, C# will auto-convert value object

value ==> object is called "boxing"

object ==> value is called "unboxing"

int i, j;

object obj;string s;

i = 32;obj = i; // boxed copy!i = 19;

j = (int) obj; // unboxed!

s = j.ToString(); // boxed!s = 99.ToString(); // boxed!

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User-defined reference types

Classes!

for example, Customer class we worked with earlier

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; }

}

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Working with reference types

Creating, assigning, and comparing:

Customer c1, c2, c3;string s1, s2;

c1 = new Customer("joe hummel", 36259);

c2 = new Customer("marybeth lore", 55298);c3 = null; // c3 references no object

c3 = c1; // c3 now references same obj as c1

if (c1 == null) ... // do I ref an object?

if (c1 == c2) ... // compares referencesif (c1.Equals(c2)) ... // compares objects

if (s1 == s2) ... // exception: == overloaded to// compare string data

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Defining equality

Classes should override Equals

public class Customer

{...

public override bool Equals(object obj)

{

Customer other;

if ((obj == null) || (!(obj is Customer)))

return false; // definitely not equal

other = (Customer) obj; // typecast to access

return this.ID == other.ID; // equal if same id...

}

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Part 3

Arrays

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Arrays

Arrays are reference types

based on Array class in FCL

must be created using new

0-based indexing

assigned default values (0 for numeric, null for references, etc.)

int[] a;a = new int[5];

a[0] = 17;

a[1] = 32;int x = a[0] + a[1] + a[4];

int l = a.Length;

element access

create

number of elements

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Multi-dimensional arrays

C# supports arrays as a single object OR array of arrays

latter allows you to implement jagged arrays

Customer[,] twoD;int[][] jagged2D;

// 2D array as single objecttwoD = new Customer[10, 100];twoD[0, 0] = new Customer();twoD[9, 99] = new Customer();

// 2D array as array of arrays

jagged2D = new int[10][];jagged2D[0] = new int[10];jagged2D[1] = new int[20];jagged2D[9] = new int[100];

jagged2D[0][0] = 1;jagged2D[9][99] = 100;

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Part 4

Namespaces

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Namespaces

Namespaces are a means for organizing types

a namespace N is a set of names scoped by N

namespaces are often nested

namespace Workshop

{

public class Customer{...

}

public class Product{...

}

}//namespace

Workshop.Customer

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Example

Framework Class Library (FCL)contains 1000's of classes

how to organize?

how to avoid name collisions?

with FCL

within FCL

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FCL namespaces

FCL's outermost namespace is "System"

FCL technologies nested within System

Namespace Purpose Assembly

System Core classes, types mscorlib.dll

System.Collections Data structures mscorlib.dll

System.Data Database access System.Data.dll

System.Windows.Forms GUI System.Windows.Forms.dll

System.XML XML processing System.Xml.dll

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Namespace != Assembly

Orthogonal concepts:

namespace for organization

assembly for packaging

One namespace could be spread across multiple assemblies

One assembly may contain multiple namesspaces

e.g. mscorlib.dll

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Summary

CTS is the common type system

same type system for all languages

types implemented by classes in FCL

fundamental difference between value & reference types

CLS is the common language specification

types that are guaranteed to work across languages

Try not to confuse namespaces with assemblies

namespaces help with organization assemblies denote implementation / packaging

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References

Books:

I. Pohl, "C# by Dissection"

S. Lippman, "C# Primer"

J. Mayo, "C# Unleashed"