C# for beginners

Introduction to C#
The New Language for .

H.Mössenböck
University of Linz, Austria
moessenboeck@ssw.uni-linz.ac.at

Contents
Introduction to C# Advanced C#
1. Overview 7. Inheritance
2. Types 8. Interfaces
3. Expressions 9. Delegates
4. Declarations 10. Exceptions
5. Statements 11. Namespaces and Assemblies
6. Classes and Structs 12. Attributes
13. Threads
14. XML Comments

References:
• B.Albahari, P.Drayton, B.Merrill: C# Essentials. O'Reilly, 2001
• S.Robinson et al: Professional C#, Wrox Press, 2001
• Online documentation on the .NET SDK CD

2

Features of C#

Very similar to Java
70% Java, 10% C++, 5% Visual Basic, 15% new

As in Java As in C++
• Object-orientation (single inheritance) • (Operator) Overloading
• Interfaces • Pointer arithmetic in unsafe code
• Exceptions • Some syntactic details
• Threads
• Namespaces (like Packages)
• Strong typing
• Garbage Collection
• Reflection
• Dynamic loading of code
• ...

3

New Features in C#

Really new (compared to Java) "Syntactic Sugar"
• Reference and output parameters • Component-based programming
• Objects on the stack (structs) - Properties
• Rectangular arrays - Events
• Enumerations • Delegates
• Unified type system • Indexers
• goto • Operator overloading
• Versioning • foreach statements
• Boxing/unboxing
• Attributes
• ...

4

Hello World

File Hello.cs
using System;
• uses the namespace System
• entry point must be called Main
class Hello {
• output goes to the console
• file name and class name
static void Main() {
need not be identical
Console.WriteLine("Hello World");
}

}

Compilation (in the Console window)
csc Hello.cs
Execution
Hello

5

Structure of C# Programs

Programm

File F1.cs File F2.cs File F3.cs

namespace A {...} namespace B {...} namespace C {...}

class X {...} class Y {...} class Z {...}

• If no namespace is specified => anonymous default namespace
• Namespaces may also contain structs, interfaces, delegates and enums
• Namespace may be "reopened" in other files
• Simplest case: single class, single file, default namespace
6

A Program Consisting of 2 Files
Counter.cs
class Counter { Compilation
int val = 0;
public void Add (int x) { val = val + x; } csc Counter.cs Prog.cs
public int Val () { return val; } => generates Prog.exe
}
Execution
Prog.cs Prog
using System;

class Prog {
Working with DLLs
static void Main() {
Counter c = new Counter(); csc /target:library Counter.cs
c.Add(3); c.Add(5); => generates Counter.dll
Console.WriteLine("val = " + c.Val());
} csc /reference:Counter.dll Prog.cs
} => generates Prog.exe

7

Unified Type System
Types

Value Types Reference Types Pointers

Simple Types Enums Structs Classes Interfaces Arrays Delegates
bool sbyte byte float
char short ushort double
int uint decimal
long ulong
User-defined Types

All types are compatible with object
- can be assigned to variables of type object
- all operations of type object are applicable to them

9

Value Types versus Reference Types
Value Types Reference Types
variable contains value reference
stored on stack heap
initialisation 0, false, '0' null
assignment copies the value copies the reference

example int i = 17; string s = "Hello";
int j = i; string s1 = s;

i 17 s
Hello
j 17 s1

10

Simple Types
Long Form in Java Range
sbyte System.SByte byte -128 .. 127
byte System.Byte --- 0 .. 255
short System.Int16 short -32768 .. 32767
ushort System.UInt16 --- 0 .. 65535
int System.Int32 int -2147483648 .. 2147483647
uint System.UInt32 --- 0 .. 4294967295
long System.Int64 long -263 .. 263-1
ulong System.UInt64 --- 0 .. 264-1
float System.Single float ±1.5E-45 .. ±3.4E38 (32 Bit)
double System.Double double ±5E-324 .. ±1.7E308 (64 Bit)
decimal System.Decimal --- ±1E-28 .. ±7.9E28 (128 Bit)
bool System.Boolean boolean true, false
char System.Char char Unicode character

11

Compatibility Between Simple Types

decimal double float long int short sbyte

only with ulong uint ushort byte
type cast

char

12

Enumerations
List of named constants

Declaration (directly in a namespace)
enum Color {red, blue, green} // values: 0, 1, 2
enum Access {personal=1, group=2, all=4}
enum Access1 : byte {personal=1, group=2, all=4}

Use

Color c = Color.blue; // enumeration constants must be qualified

Access a = Access.personal | Access.group;
if ((Access.personal & a) != 0) Console.WriteLine("access granted");

13

Operations on Enumerations

Compare if (c == Color.red) ...
if (c > Color.red && c <= Color.green) ...
+, - c = c + 2;
++, -- c++;
& if ((c & Color.red) == 0) ...
| c = c | Color.blue;
~ c = ~ Color.red;

The compiler does not check if the result is a valid enumeration value.

Note
- Enumerations cannot be assigned to int (except after a type cast).
- Enumeration types inherit from object (Equals, ToString, ...).
- Class System.Enum provides operations on enumerations
(GetName, Format, GetValues, ...).
14

Arrays
One-dimensional Arrays

int[] a = new int[3];
int[] b = new int[] {3, 4, 5};
int[] c = {3, 4, 5};
SomeClass[] d = new SomeClass[10]; // Array of references
SomeStruct[] e = new SomeStruct[10]; // Array of values (directly in the array)

int len = a.Length; // number of elements in a

15

Multidimensional Arrays
Jagged (like in Java) a
a[0][1]

int[][] a = new int[2][]; a[0]
a[0] = new int[3]; a[1]
a[1] = new int[4];

int x = a[0][1];
int len = a.Length; // 2
len = a[0].Length; // 3

Rectangular (more compact, more efficient access)
int[,] a = new int[2, 3]; a a[0, 1]

int x = a[0, 1];
int len = a.Length; // 6
len = a.GetLength(0); // 2
len = a.GetLength(1); // 3

16

Class System.String
Can be used as standard type string
string s = "Alfonso";

Note
• Strings are immutable (use StringBuilder if you want to modify strings)
• Can be concatenated with +: "Don " + s
• Can be indexed: s[i]
• String length: s.Length
• Strings are reference types => reference semantics in assignments
• but their values can be compared with == and != : if (s == "Alfonso") ...
• Class String defines many useful operations:
CompareTo, IndexOf, StartsWith, Substring, ...

17

Structs
Declaration
struct Point {
public int x, y; // fields
public Point (int x, int y) { this.x = x; this.y = y; } // constructor
public void MoveTo (int a, int b) { x = a; y = b; } // methods
}

Use
Point p = new Point(3, 4); // constructor initializes object on the stack
p.MoveTo(10, 20); // method call

18

Classes
Declaration
class Rectangle {
Point origin;
public int width, height;
public Rectangle() { origin = new Point(0,0); width = height = 0; }
public Rectangle (Point p, int w, int h) { origin = p; width = w; height = h; }
public void MoveTo (Point p) { origin = p; }
}

Use
Rectangle r = new Rectangle(new Point(10, 20), 5, 5);
int area = r.width * r.height;
r.MoveTo(new Point(3, 3));

19

Differences Between Classes and Structs
Classes Structs

Reference Types Value Types
(objects stored on the heap) (objects stored on the stack)

support inheritance no inheritance
(all classes are derived from object) (but compatible with object)

can implement interfaces can implement interfaces

may have a destructor no destructors allowed

20

Boxing and Unboxing
Value types (int, struct, enum) are also compatible with object!

Boxing
The assignment
object obj = 3;
wraps up the value 3 into a heap object
obj

3

Unboxing
The assignment
int x = (int) obj;
unwraps the value again

21

Boxing/Unboxing
Allows the implementation of generic container types
class Queue {
...
public void Enqueue(object x) {...}
public object Dequeue() {...}
...
}

This Queue can then be used for reference types and value types
Queue q = new Queue();

q.Enqueue(new Rectangle());
q.Enqueue(3);

Rectangle r = (Rectangle) q.Dequeue();
int x = (int) q.Dequeue();

22

Operators and their Priority
Primary (x) x.y f(x) a[x] x++ x-- new typeof sizeof checked unchecked
Unary + - ~ ! ++x --x (T)x
Multiplicative * / %
Additive + -
Shift << >>
Relational < > <= >= is as
Equality == !=
Logical AND &
Logical XOR ^
Logical OR |
Conditional AND &&
Conditional OR ||
Conditional c?x:y
Assignment = += -= *= /= %= <<= >>= &= ^= |=

Operators on the same level are evaluated from left to right

24

Overflow Check
Overflow is not checked by default

int x = 1000000;
x = x * x; // -727379968, no error

Overflow check can be turned on

x = checked(x * x); // System.OverflowException

checked {
...
x = x * x; // System.OverflowException
...
}

Overflow check can also be turned on with a compiler switch

csc /checked Test.cs

25

typeof and sizeof
typeof
• Returns the Type descriptor for a given type
(the Type descriptor of an object o can be retrieved with o.GetType()).

Type t = typeof(int);
Console.WriteLine(t.Name); // Int32

sizeof
• Returns the size of a type in bytes.
• Can only be applied to value types.
• Can only be used in an unsafe block (the size of structs may be system dependent).
Must be compiled with csc /unsafe xxx.cs

unsafe {
Console.WriteLine(sizeof(int));
Console.WriteLine(sizeof(MyEnumType));
Console.WriteLine(sizeof(MyStructType));
}

26

Declaration Space
The program area to which a declaration belongs

Entities can be declared in a ...
- namespace: Declaration of classes, interfaces, structs, enums, delegates
- class, interface, struct: Declaration of fields, methods, properties, events, indexers, ...
- enum: Declaration of enumeration constants
- block: Declaration of local variables

Scoping rules
- A name must not be declared twice in the same declaration space.
- Declarations may occur in arbitrary order.
Exception: local variables must be declared before they are used

Visibility rules
- A name is only visible within its declaration space
(local variables are only visible after their point of declaration).
- The visibility can be restricted by modifiers (private, protected, ...)
28

Namespaces
File: X.cs
namespace A {
... Classes ...
... Interfaces ...
... Structs ...
... Enums ...
... Delegates ...
namespace B { // full name: A.B
...
}
}

File: Y.cs
namespace A {
...
namespace B {...}
}

namespace C {...}

Equally named namespaces in different files constitute a single declaration space.
Nested namespaces constitute a declaration space on their own.
29

Using Other Namespaces
Color.cs Figures.cs Triangle.cs
namespace Util { namespace Util.Figures { namespace Util.Figures {
public enum Color {...} public class Rect {...} public class Triangle {...}
} public class Circle {...} }
}

using Util.Figures;

class Test {
Rect r; // without qualification (because of using Util.Figures)
Triangle t;
Util.Color c; // with qualification
}

Foreign namespaces
• must either be imported (e.g. using Util;)
• or specified in a qualified name (e.g. Util.Color)

Most programs need the namespace System => using System;
30

Blocks
Various kinds of blocks
void foo (int x) { // method block
... local variables ...

{ // nested block
}

for (int i = 0; ...) { // structured statement block
}
}

Note
• The declaration space of a block includes the declaration spaces of nested blocks.
• Formal parameters belong to the declaration space of the method block.
• The loop variable in a for statement belongs to the block of the for statement.
• The declaration of a local variable must precede its use.

31

Declaration of Local Variables
void foo(int a) {
int b;
if (...) {
int b; // error: b already declared in outer block
int c; // ok so far, but wait ...
int d;
...
} else {
int a; // error: a already declared in outer block
int d; // ok: no conflict with d from previous block
}
for (int i = 0; ...) {...}
for (int i = 0; ...) {...} // ok: no conflict with i from previous loop
int c; // error: c already declared in this declaration space
}

32

Simple Statements
Empty statement
; // ; is a terminator, not a separator

Assigment
x = 3 * y + 1;

Method call
string s = "a,b,c";
string[] parts = s.Split(','); // invocation of an object method (non-static)

s = String.Join(" + ", parts); // invocation of a class method (static)

34

if Statement
if ('0' <= ch && ch <= '9')
val = ch - '0';
else if ('A' <= ch && ch <= 'Z')
val = 10 + ch - 'A';
else {
val = 0;
Console.WriteLine("invalid character {0}", ch);
}

35

switch Statement
switch (country) {
case "Germany": case "Austria": case "Switzerland":
language = "German";
break;
case "England": case "USA":
language = "English";
break;
case null:
Console.WriteLine("no country specified");
break;
default:
Console.WriteLine("don't know language of {0}", country);
break;
}

Type of switch expression
numeric, char, enum or string (null ok as a case label).
No fall-through!
Every statement sequence in a case must be terminated with break (or return, goto, throw).
If no case label matches default
If no default specified continuation after the switch statement
36

switch with Gotos
E.g. for the implementation of automata
b
a c
0 1 2
c

int state = 0;
int ch = Console.Read();
switch (state) {
case 0: if (ch == 'a') { ch = Console.Read(); goto case 1; }
else if (ch == 'c') goto case 2;
else goto default;
case 1: if (ch == 'b') { ch = Console.Read(); goto case 1; }
else if (ch == 'c') goto case 2;
else goto default;
case 2: Console.WriteLine("input valid");
break;
default: Console.WriteLine("illegal character {0}", ch);
break;
}

37

Loops
while
while (i < n) {
sum += i;
i++;
}

do while
do {
sum += a[i];
i--;
} while (i > 0);

for Short form for
for (int i = 0; i < n; i++) int i = 0;
sum += i; while (i < n) {
sum += i;
i++;
}

38

foreach Statement
For iterating over collections and arrays

int[] a = {3, 17, 4, 8, 2, 29};
foreach (int x in a) sum += x;

string s = "Hello";
foreach (char ch in s) Console.WriteLine(ch);

Queue q = new Queue();
q.Enqueue("John"); q.Enqueue("Alice"); ...
foreach (string s in q) Console.WriteLine(s);

39

Jumps
break; For exiting a loop or a switch statement.
There is no break with a label like in Java (use goto instead).

continue; Continues with the next loop iteration.

goto case 3: Can be used in a switch statement to jump to a case label.

myLab:
...
goto myLab; Jumps to the label myLab.
Restrictions:
- no jumps into a block
- no jumps out of a finally block of a try statement

40

return Statement
Returning from a void method

void f(int x) {
if (x == 0) return;
...
}

Returning a value from a function method

int max(int a, int b) {
if (a > b) return a; else return b;
}

class C {
static int Main() {
...
return errorCode; // The Main method can be declared as a function;
} // the returned error code can be checked with the
// DOS variable errorlevel
}

41

Contents of Classes or Structs
class C {
... fields, constants ... // for object-oriented programming
... methods ...
... constructors, destructors ...

... properties ... // for component-based programming
... events ...

... indexers ... // for amenity
... overloaded operators ...

... nested types (classes, interfaces, structs, enums, delegates) ...
}

43

Classes
class Stack {
int[] values;
int top = 0;
public Stack(int size) { ... }
public void Push(int x) {...}
public int Pop() {...}
}

• Objects are allocated on the heap (classes are reference types)
• Objects must be created with new
Stack s = new Stack(100);

• Classes can inherit from one other class (single code inheritance)
• Classes can implement multiple interfaces (multiple type inheritance)

44

Structs
struct Point {
int x, y;
public Point(int x, int y) { this.x = x; this.y = y; }
public MoveTo(int x, int y) {...}
}

• Objects are allocated on the stack not on the heap (structs are value types)
+ efficient, low memory consumption, no burden for the garbage collector.
- live only as long as their container (not suitable for dynamic data structures)
• Can be allocated with new
Point p; // fields of p are not yet initialized
Point q = new Point();

• Fields must not be initialized at their declaration
struct Point {
int x = 0; // compilation error
}

• Parameterless construcors cannot be declared
• Can neither inherit nor be inherited, but can implement interfaces
45

Visibility Modifiers (excerpt)
public visible where the declaring namespace is known
- Members of interfaces and enumerations are public by default.
- Types in a namespace (classes, structs, interfaces, enums, delegates)
have default visibility internal (visible in the declaring assembly)
private only visible in declaring class or struct
- Members of classes and structs are private by default
(fields, methods, properties, ..., nested types)

Example

public class Stack {
private int[] val; // private is also default
private int top; // private is also default
public Stack() {...}
public void Push(int x) {...}
public int Pop() {...}
}

46

Fields and Constants
class C {

int value = 0; Field
- Initialization is optional
- Initialization must not access other fields or methods
of the same type
- Fields of a struct must not be initialized

const long size = ((long)int.MaxValue + 1) / 4;

Constant
- Value must be computable at compile time

readonly DateTime date;

Read Only Field
- Must be initialized in their declaration or in a constructor
- Value needs not be computable at compile time
- Consumes a memory location (like a field)
}

Access within C Access from other classes
... value ... size ... date ... C c = new C();
... c.value ... c.size ... c.date ...
47

Static Fields and Constants
Belong to a class, not to an object
class Rectangle {
static Color defaultColor; // once per class
static readonly int scale; // -- " –
// static constants are not allowed
int x, y, width,height; // once per object
...
}

Access within the class Access from other classes
... defaultColor ... scale ... ... Rectangle.defaultColor ... Rectangle.scale ...

48

Methods
Examples
class C {
int sum = 0, n = 0;

public void Add (int x) { // procedure
sum = sum + x; n++;
}

public float Mean() { // function (must return a value)
return (float)sum / n;
}
}

this.Add(3); C c = new C();
float x = Mean(); c.Add(3);
float x = c.Mean();

49

Static Methods
Operations on class data (static fields)

class Rectangle {
static Color defaultColor;

public static void ResetColor() {
defaultColor = Color.white;
}
}

ResetColor(); Rectangle.ResetColor();

50

Parameters
Value Parameters (input values) - "call by value"
void Inc(int x) {x = x + 1;} - formal parameter is a copy of the
void f() { actual parameter
int val = 3; - actual parameter is an expression
Inc(val); // val == 3
}

ref Parameters (transition values) - "call by reference"
void Inc(ref int x) { x = x + 1; } - formal parameter is an alias for the
void f() { actual parameter
int val = 3; (address of actual parameter is passed)
Inc(ref val); // val == 4 - actual parameter must be a variable
}

out Parameters (output values)
- similar to ref parameters
void Read (out int first, out int next) {
first = Console.Read(); next = Console.Read(); but no value is passed by the caller.
} - must not be used in the method before
void f() { it got a value.
int first, next;
Read(out first, out next);
}
51

Variable Number of Parameters
Last n parameters may be a sequence of values of a certain type.

keyword
array type
params

void Add (out int sum, params int[] val) {
sum = 0;
foreach (int i in val) sum = sum + i;
}

params cannot be used for ref and out parameters

Use
Add(out sum, 3, 5, 2, 9); // sum == 19

52

Method Overloading
Methods of a class may have the same name
- if they have different numbers of parameters, or
- if they have different parameter types, or
- if they have different parameter kinds (value, ref/out)

Examples
void F (int x) {...}
void F (char x) {...}
void F (int x, long y) {...}
void F (long x, int y) {...}
void F (ref int x) {...}

Calls
int i; long n; short s;
F(i); // F(int x)
F('a'); // F(char x)
F(i, n); // F(int x, long y)
F(n, s); // F(long x, int y);
F(i, s); // cannot distinguish F(int x, long y) and F(long x, int y); => compilation error
F(i, i); // cannot distinguish F(int x, long y) and F(long x, int y); => compilation error

Overloaded methods must not differ only in their function types, in the presence of params
or in ref versus out!
53

Constructors for Classes
Example

class Rectangle {
int x, y, width, height;
public Rectangle (int x, int y, int w, int h) {this.x = x; this.y = y; width = x; height = h; }
public Rectangle (int w, int h) : this(0, 0, w, h) {}
public Rectangle () : this(0, 0, 0, 0) {}
...
}

Rectangle r1 = new Rectangle();
Rectangle r2 = new Rectangle(2, 5);
Rectangle r3 = new Rectangle(2, 2, 10, 5);

• Constructors can be overloaded.

• A construcor may call another constructor with this
(specified in the constructor head, not in its body as in Java!).

• Before a construcor is called, fields are possibly initialized.

54

Default Constructor
If no constructor was declared in a class, the compiler generates a
parameterless default constructor:
class C { int x; }
C c = new C(); // ok

The default constructor initializes all fields as follows:
numeric 0
enum 0
bool false
char '0'
reference null

If a constructor was declared, no default constructor is generated:
class C {
int x;
public C(int y) { x = y; }
}

C c1 = new C(); // compilation error
C c2 = new C(3); // ok

55

Constructors for Structs
Example
struct Complex {
double re, im;
public Complex(double re, double im) { this.re = re; this.im = im; }
public Complex(double re) : this (re, 0) {}
...
}

Complex c0; // c0.re and c0.im are still uninitialized
Complex c1 = new Complex(); // c1.re == 0, c1.im == 0
Complex c2 = new Complex(5); // c2.re == 5, c2.im == 0
Complex c3 = new Complex(10, 3); // c3.re == 10, c3.im == 3

• For every struct the compiler generates a parameterless default constructor
(even if there are other constructors).
The default constructor zeroes all fields.

• Programmers must not declare a parameterless constructor for structs
(for implementation reasons of the CLR).

56

Static Constructors
Both for classes and for structs

class Rectangle {
...
static Rectangle() {
Console.WriteLine("Rectangle initialized");
}
}

struct Point {
...
static Point() {
Console.WriteLine("Point initialized");
}
}

• Must be parameterless (also for structs) and have no public or private modifier.
• There must be just one static constructor per class/struct.
• Is invoked once before this type is used for the first time.

57

Destructors
class Test {

~Test() {
... finalization work ...
// automatically calls the destructor of the base class
}

}

• Correspond to finalizers in Java.
• Called for an object before it is removed by the garbage collector.
• No public or private.
• Is dangerous (object resurrection) and should be avoided.

58

Properties
Syntactic sugar for get/set methods
class Data { property type
property name
FileStream s;

public string FileName {
set {
s = new FileStream(value, FileMode.Create);
}
get {
"input parameter"
return s.Name;
of the set method
}
}
}

Used as "smart fields"
Data d = new Data();

d.FileName = "myFile.txt"; // invokes set("myFile.txt")
string s = d.FileName; // invokes get()

JIT compilers often inline get/set methods no efficiency penalty
59

Properties (continued)
get or set can be omitted
class Account {
long balance;

public long Balance {
get { return balance; }
}
}

x = account.Balance; // ok
account.Balance = ...; // compilation error

Why are properties a good idea?
• Interface and implementation of data may differ.
• Allows read-only and write-only fields.
• Can validate a field when it is assigned.
• Substitute for fields in interfaces.

60

Indexers
Programmable operator for indexing a collection
class File { type of the name type and name
FileStream s; indexed expression (always this) of the index value

public int this [int index] {
get { s.Seek(index, SeekOrigin.Begin);
return s.ReadByte();
}
set { s.Seek(index, SeekOrigin.Begin);
s.WriteByte((byte)value);
}
}
}

Use
File f = ...;
int x = f[10]; // calls f.get(10)
f[10] = 'A'; // calls f.set(10, 'A')

• get or set method can be omitted (write-only / read-only)
• Indexers can be overloaded with different index types

61

Indexers (other example)
class MonthlySales {
int[] product1 = new int[12];
int[] product2 = new int[12];
...
public int this[int i] { // set method omitted => read-only
get { return product1[i-1] + product2[i-1]; }
}

public int this[string month] { // overloaded read-only indexer
get {
switch (month) {
case "Jan": return product1[0] + product2[0];
case "Feb": return product1[1] + product2[1];
...
}
}
}
}

MonthlySales sales = new MonthlySales();
...
Console.WriteLine(sales[1] + sales["Feb"]);

62

Overloaded Operators
Static method for implementing a certain operator
struct Fraction {
int x, y;
public Fraction (int x, int y) {this.x = x; this.y = y; }

public static Fraction operator + (Fraction a, Fraction b) {
return new Fraction(a.x * b.y + b.x * a.y, a.y * b.y);
}
}

Use
Fraction a = new Fraction(1, 2);
Fraction b = new Fraction(3, 4);
Fraction c = a + b; // c.x == 10, c.y == 8

• The following operators can be overloaded:
– arithmetic: +, - (unary and binary), *, /, %, ++, --
– relational: ==, !=, <, >, <=, >=
– bit operators: &, |, ^
– others: !, ~, >>, <<, true, false
• Must return a value

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Conversion Operators
Implicit conversion
- If the conversion is always possible without loss of precision
- e.g. long = int;

Explicit conversion
- If a run time check is necessary or truncation is possible
- e.g. int = (int) long;

Conversion operators for custom types
class Fraction {
int x, y;
...
public static implicit operator Fraction (int x) { return new Fraction(x, 1); }
public static explicit operator int (Fraction f) { return f.x / f.y; }
}

Use
Fraction f = 3; // implicit conversion, f.x == 3, f.y == 1
int i = (int) f; // explicit conversion, i == 3

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Nested Types
class A {
int x;
B b = new B(this);
public void f() { b.f(); }
public class B {
A a;
public B(A a) { this.a = a; }
public void f() { a.x = ...; ... a.f(); }
}
}

class C {
A a = new A();
A.B b = new A.B(a);
}

For auxiliary classes that should be hidden
- Inner class can access all members of the outer class (even private members).
- Outer class can access only public members of the inner class.
- Other classes can access an inner class only if it is public.
Nested types can also be structs, enums, interfaces and delegates.
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C# for beginners

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