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Overview of C# - Programming Language Concepts | CS A331, Study notes of Programming Languages

Material Type: Notes; Class: Programming Language Concepts; Subject: Computer Science ; University: University of Alaska - Anchorage; Term: Unknown 1989;

Typology: Study notes

2009/2010

Uploaded on 03/28/2010

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Overview of C#
CS331
As described in the previous lecture, C# is one of the languages designed for the .NET
platform. Here we will present an overview of the language, focusing on the unique
aspects that differ from Java or C++. As you will see, it is quite similar to Java in many
respects.
The structure of a C# program looks like the following:
// Specify namespaces we use classes from here
using System;
using System.Threading; // Specify
namespace AppNamespace
{
// Comments that start with /// used for
// creating online documentation, like javadoc
/// <summary>
/// Summary description for Class1.
/// </summary>
class Class1
{
static void Main(string[] args)
{
// Your code would go here, e.g.
Console.WriteLine("hi");
}
/* We can define other methods and vars for the class */
// Constructor
Class1()
{
// Code
}
// Some method, use public, private, protected
// Use static as well just like Java
public void foo()
{
// Code
}
// Variables
private int m_number;
public static double m_stuff;
}
}
C# code normally uses the file extension of “.cs”. Everything above is quite similar to
Java; note the use of “Main” instead of “main”. If a namespace is left out, your code is
placed into the default, global, namespace.
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Overview of C#

CS

As described in the previous lecture, C# is one of the languages designed for the .NET

platform. Here we will present an overview of the language, focusing on the unique

aspects that differ from Java or C++. As you will see, it is quite similar to Java in many

respects.

The structure of a C# program looks like the following:

// Specify namespaces we use classes from here using System; using System.Threading; // Specify

namespace AppNamespace { // Comments that start with /// used for // creating online documentation, like javadoc ///

/// Summary description for Class1. /// class Class { static void Main(string[] args) { // Your code would go here, e.g. Console.WriteLine("hi"); } /* We can define other methods and vars for the class */

// Constructor Class1() { // Code }

// Some method, use public, private, protected // Use static as well just like Java public void foo() { // Code } // Variables private int m_number; public static double m_stuff; } }

C# code normally uses the file extension of “.cs”. Everything above is quite similar to

Java; note the use of “Main” instead of “main”. If a namespace is left out, your code is

placed into the default, global, namespace.

The “using” directive tells C# what methods you would like to use from that namespace.

If we left out the “using System” statement, then we would have had to write

“System.Console.WriteLine” instead of just “Console.WriteLine”.

It is normal for each class to be defined in a separate file, but you could put all the classes

in one file if you wish. Using Visual Studio .NET’s “P)roject, Add C)lass” menu option

will create separate files for your classes by default.

Based on what little we have covered, together with your knowledge of Java, you should

already be able to write quite sophisticated programs!

Getting Help

If you have installed MSDN on the system, you have a great online help resource built

into Visual Studio .NET. You can look up the C# programming language reference from

the Help menu and also get Dynamic Help, which will show related help as you are

typing.

If MSDN is not installed, you can go online to access the references. It is accessible

from:

http://msdn.microsoft.com/library/default.asp

You will have to drill down to VS.NET, Documentation, VB and C#, and then to the C#

reference. There are also numerous tutorials here. Or you could just enter search terms

into the search engine for the class or keyword you are interested in.

Output : Using WriteLine

Usually we cover how to do output first, so here are the basics.

System.Console.WriteLine() will output a string to the console. You can use this just

like Java’s System.out.println():

System.Console.WriteLine(“hello world “ + 10/2);

will output:

hello world 5

We can also use {0}, {1}, {2}, … etc. to indicate arguments in the WriteLine statement

to print. For example:

Console.WriteLine(“hi {0} you are {0} and your age is {1}”, “Kenrick”, 23);

will output:

hi Kenrick you are Kenrick and your age is 23

Public Object Methods

Equals Overloaded. Determines whether two

Object instances are equal.

GetHashCode Serves as a hash function for a particular

type, suitable for use in hashing algorithms

and data structures like a hash table.

GetType Gets the Type of the current instance.

ToString Returns a String that represents the current

Object.

We can then write code such as:

int i;

Console.WriteLine(i.ToString());

int hash = i.GetHashCode();

This is equivalent to performing:

z = new Object(i);

Console.WriteLine(z.ToString());

But is much more efficient since the value type is converted to reference on demand and

at the virtual code level instead of at the programmer’s level.

The struct is another value type we can create. A struct can contain constructors,

constants, fields, methods, properties, indexers, operators, and nested types. The

declaration of a struct looks just like a declaration of a class, except we use the keyword

struct instead of class. For example:

public struct Point { public int x, y; public Point(int p1, int p2) { x = p1; y = p2; } }

So what is the difference with a struct? Unlike classes, structs can be created on the stack

without using the keyword new, e.g.:

Point p1, p2;

p1.x = 3; p1.y = 5;

etc. We also cannot use inheritance with structs.

Finally, C# provides an enumeration type that is also a value type. This is most easily

shown through an example:

// Enum goes outside in the class definition enum Days {Sat, Sun, Mon, Tue, Wed, Thu, Fri};

// Inside some method Days day1, day2; int day3;

day1 = Days.Sat; day2 = Days.Tue; day3 = (int) Days.Fri; Console.WriteLine(day1); Console.WriteLine(day2); Console.WriteLine(day3);

This program outputs:

Sat

Tue

As you can see, the enumeration really maps to the underlying data type of integer.

Strings

The built-in string type is much like Java’s string type. We can concatenate using the +

operator. Just like Java, there are a variety of methods available to find the index Of

matching strings or characters, generate substrings, compare for equality (if we use == on

strings we are comparing if the references are equal, just like Java), generate clones, trim,

etc. See the reference for more details.

Classes

We have already seen defining a class above. To specify inheritance use a colon after the

class name and then the base class. If we want to invoke the constructor for the base

class, we must use the keyword “base” after the constructor in the derived class. We

must also be explicit with virtual methods. The example illustrates basic usage:

public class BankAccount

public double m_amount;

BankAccount(double d) {

m_amount = d;

public virtual string GetInfo() {

return “Basic Account”;

public class SportsCar : IDriveable {

void Start() {

// Code here to implement start

void Stop() {

// Code here to implement stop

void Turn() {

// Code here to implement turn

We can declare methods that take as input an interface, which accept any object that

implements the interface, for example:

void GoForward(IDrivable d)

d.Start();

// wait

d.Stop();

Getting Input

To input data, we must read it as a string and then convert it to the desired type.

Console.ReadLine() will return a line of input text as a string.

We can then use type.Parse(string) to convert the string to the desired type. For example:

string s;

int i;

s = Console.ReadLine();

i = int.Parse(s);

we can also use double.Parse(s); float.Parse(s); etc.

There is also a useful Convert class, with methods such as Convert.ToDouble(val);

Convert.ToBoolean(val); Convert.ToDateTime(val); etc.

Procedural Code

We also have our familiar procedural constructs:

Arithmetic, relational, Boolean operators: all the same as Java/C++

For, While, Do, If : all the same as Java/C++

Switch statements: Like Java, except forces a break after a case. Code is not allowed to

“fall through” to the next case, but several case labels may mark the same location.

Math class: Math.Sin(), Math.Cos(), etc.

Random class:

Random r = new Random();

r.NextDouble(); // Returns random double between 0-

r.Next(10,20); // Random int, 10 ≤ int < 20

Passing Parameters

If we pass a value parameter to a method then by default we get the pass by value

behavior, just like Java. For example:

public static void foo(int a) { a=1; }

static void Main(string[] args) { int x=3; foo(x); Console.WriteLine(x); }

This outputs the value of 3 because x is passed by value to method foo, which gets a copy

of x’s value under the variable name of a.

However, C# allows a ref keyword to pass value types by reference:

public static void foo(int ref a) { a=1; }

static void Main(string[] args) { int x=3; foo(ref x); Console.WriteLine(x); }

The ref keyword must be used in both the parameter declaration of the method and also

when invoked, so it is clear what parameters are passed by reference and may be

changed. In this case, the program outputs the value of 1 since variable a in foo is really

a reference to where x is stored in Main.

int arr = new int[100];

This allocates a chunk of data off the heap large enough to store the array, and arr

references this chunk of data.

We can get the size of the array dynamically through the Length property:

Console.WriteLine(arr.Length); // Outputs 100 for above declaration

If we want to declare a method parameter to be of type array we would use:

public void foo(int[] data)

To return an array we can use:

public int[] foo()

Just like in Java, if we have two array variables and want to copy one to the other we

can’t do it with just an assignment. This would assign the reference, not make a copy of

the array. To copy the array we must copy each element one at a time, or use the Clone()

method to make a copy of the data and set a new reference to it (and garbage collect the

old array values).

Multidimensional Arrays, Vectors, foreach

We have two ways to declare multidimensional arrays. One is to create arrays of arrays,

as Java does. Another is to create a true multidimensional array.

The following defines a 30 x 3 array:

int[,] arr = new int[30][3];

Here we put a comma inside the [] to indicate two dimensions. This allocates a single

chunk of memory of size 303sizeof(int) and creates a reference to it. We use the

formulas for row major order to access each element of the array.

The following defines a 30 x 3 array using an array of arrays:

int[][] arr = new int[30][3];

To an end user this looks much like the previous declaration, but it creates an array of 30

elements, where each element is an array of 3 elements. This gives us the possibility of

creating ragged arrays but is slower to access since we must dereference each array

index.

Related to arrays, you may want to check out the ArrayList class defined in

System.Collections. It defines a class that behaves like a Java vector in that it allows

dynamic allocation of elements that can be accessed like an array or also by name using a

key.

Lastly, C# provides a new loop method, called foreach. Foreach will loop through each

element in an array or collection. For example:

string[] arr = {"hello", "world", "foo", "abracadabra"}; foreach (string x in arr) Console.WriteLine(x);

Will output each string in the array.

Delegates

C# uses delegates where languages such as C++ use function pointers. A delegate

defines a class that describes one or more methods. Another method can use this

definition, regardless of the actual code that implements it. C# uses this technique to pass

the EventHandler type to different methods, where the event may be handled in different

ways.

Here is an example of a delegate.

public delegate int CompareDelegate(string s1, string s2);

// Two different methods for comparison public static int compare1(string s1, string s2) { return (s1.CompareTo(s2)); } public static int compare2(string s1, string s2) { if (s1.Length <= s2.Length) return -1; else return 1; }

// A method that uses the delegate to find the minimum public static string FindMin(string[] arr, CompareDelegate compare) { int i, minIndex=0;

for (i=1; i<arr.Length; i++) { if (compare(arr[minIndex],arr[i])>0) minIndex=i; } return arr[minIndex]; }