using System;

// Program start class
class helloworld
{
// Main begins program!
static void Main()
{
// Write to console
Console.WriteLine("Hello, C# is fun");
// keep screen from going away
Console.ReadLine();
}
}

using System;

class Loop
{
public static void Main()
{
int myInt = 0;

while (myInt < 10) //int lessthen 10
{
Console.Write("{0} ", myInt);
myInt++;
}
Console.WriteLine();
}
}

// Namespace Declaration
using System;

// helper class
class Classes
{
string myString;

// Constructor
public OutputClass(string inputString)
{
myString = inputString;
}

// Instance Method
public void printString()
{
Console.WriteLine("{0}", myString);
}

// Destructor
~OutputClass()
{
// Some resource cleanup routines
}
}

// Program start class
class ExampleClass
{
// Main begins program execution.
public static void Main()
{
// Instance of OutputClass
OutputClass outCl = new OutputClass("This is printed by the output class.");

// Call Output class' method
outCl.printString();
}
}
More about classes:
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using System;

class ifelse
{
public static void Main()
{
int age;
Console.WriteLine("How Old are You?");
age = Console.ReadLine();
if (age < 18)
{
Console.WriteLine("YOU ARE TOO YOUNG!")
Console.Readline();
}
else
{
Console.WriteLine("WELCOME");
Console.ReadLine()
}
}
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace helloworld
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
label1.Text = "Hello"; //sets label1 text to Hello
button1.Text = "Set Name"; //sets button1 text to Set
textBox1.Visible = false; //hides textbox1
}

private void button1_Click(object sender, EventArgs e)
{
button1.Text = "Set";
textBox1.Visible = true; //shows textbox1
label1.Text = "Hello, " + textBox1.Text; //displays name in label1

}
}
}

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