C# #

1. What is C#?

2. Basics

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What is C#?

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Basics

• basic example program

  using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Text;
  using System.Threading.Tasks;
  
  namespace basics
  {
      class Program
      {
          /* code execution start from Main()*/
          static void Main(string[] args)
          {
              Console.WriteLine("hello world");
              Console.ReadLine();
          }
      }
  }
  

Output

// print and stay on same line
Console.Write();

// print and go to new line
Console.WriteLine();

Input

// store input as string
Console.ReadLine();

Declare Variables

string x = "hello";
char c = ' ';
int y;
y = 2;
Console.WriteLine(x + c + y); // concatenation

Data Types

• char, string, int, bool

• float, double, decimal (most accurate)

String

string x = "hello world";
// x.Length, x.ToUpper(), x.ToLower()
// x.Contains('h'), x[2]
// x.IndexOf('o'), x.IndexOf("llo") returns index of starting 'l', -1 if not found
// x.Substring(2) start from index 2, inclusive, to end
// x.Substring(2, 4) start from index 2 and 3 characters behind

Numbers

// Math.Abs(-40), Math.Pow(2.2, 3), Math.Sqrt(4)
// Math.Max(4, 44), Math.Min(23, 12), Math.Round(4.12)
// Math.Floor(4.9), Math.Ceiling(4.2)
int x = Convert.ToInt32("13"); // convert string to int

Arrays

// type[] name = {values};
int[] x = { 1, 2, 3 };
string[] y = new string[10]; // create empty array with specific number of elements

• 2-dimensional array

  int[,] x = {
      {1, 2},
      {3, 4}
  }
  
  // declare empty 2D array
  int[,] y = new int[row, col]
  

Methods

namespace basics
{
    class Program
    {
        // method is declared outside Main() and name is capitalized by convention
        // void: no return
        static void Hello(string x)
        {
            Console.WriteLine("hello " + x);
        }

        // return an integer
        static int Number()
        {
            return 2;
        }

        static void Main(string[] args)
        {
            Hello("user");
            Console.WriteLine(Number());
        }
    }
}

Conditionals

• logical conditions: <, >, <=, >=, ==, !=, &&, ||

if(condition){ /* code */}
else if(condition){ /* code */ }
else { /* code */ }

ternary = (condition) ? True :  False;

switch (expression)
{
    case a:
        // code
        break;
    case b:
        // code
        break;
    default:
        // code
        break;
}

Loops

• while

  while(condition)
  {
      // code
  }
  
  do
  {
      // code
  } while(condition);
  

• for

  for (initial; condition; iterate)
  {
      // code
  }
  

Exceptions

try
{
    // code that'll break
}
catch (exceptionType e)
{
    // handle error
}
finally
{
    // default handle
}

Classes & Objects

• allow to create custom data types based on primitive ones

• class names are capitalized by convention

• Objects are instances of a Class

// MyClass.cs
class MyClass
{
    // attributes
}

• constructor

  • special method, called whenever an object is created

  • same name as the class

  // MyClass.cs
  class MyClass
  {
      // attributes
  
      public MyClass(arguments)
      {
          // code
      }
  }
  

• object methods

  • methods defined in the class and used by it

  class MyClass
  {
      // attributes
  
      public MyClass() { }
  
      private void method2()
      {
          // can only be called inside the class
      }
  
      public void method1()
      {
          // can be called outside of the class
      }
  
  }
  

• getter & setter

  • methods to control access to the attributes, make classes more secure

  • name is capitalized and same as attribute

  class MyClass
  {
      // attributes
  
      public MyClass(argument)
      {
          // use setter to set attribute value
          Attribute1 =  argument
      }
  
      public string Attribute1
      {
          get { return attribute1; }
          set {
              // validate data, value will be any passed value
              if(value == someValue)
              {
                  attribute1 = value;
              }
              else
              {
                  attribute1 = defaultValue;
              }
          }
      }
  }
  

• static attributes

  • shared by all objects and instances of the class

  • contained on the class and specific to it, instead of each object

  • not unique from object to object

  • can be directly accessed through the class

  • use methods to be accessed by objects

  class MyClass
  {
      // every object's attribute1 will be 99
      // can be accessed with MyClass.attribute1
      public static int attribute1 = 99;
  
      public MyClass(argument)
      {
          // can use constructor to modify value everytime an object is created
          attribute1++;
      }
  
      // method to be accessed by objects
      public int getAttribute1()
      {
          return attribute1;
      }
  }
  

• static methods

  • belong to the class itself

  • do not need to create object to use the method, e.g Math.Sqrt()

  • name is capitalized by convention

  • can be directly accessed through the class

  class MyClass
  {
      public MyClass(argument) { }
  
      public static void Method1()
      {
          // code
      }
  }
  

• static classes

  • prevent to create object of the class, e.g Math class

  static class MyClass {}
  
  // error
  MyClass class1 = new MyClass();
  

• inheritance

  • inherit all attributes and methods of the parent class or classes

  • do not inherit private ones

  • child class can override the parent’s

  class MyClass
  {
      // child can override
      public virtual void Method1() {}
  }
  
  class MyChildClass : MyClass
  {
      public override void Method1()
      {
          // new code
      }
  }
  

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