C# 시작하기 - Namespace : System

The System namespace is the core namespace in the .NET framework, providing fundamental classes and base classes that define commonly-used value and reference data types, events and event handlers, interfaces, attributes, and processing exceptions. Here's an overview of some of the key classes and their purposes:

Key Classes:

1. Object:
   • The root of the class hierarchy. Every class in C# directly or indirectly derives from this class.
   • Example methods: Equals(), GetHashCode(), ToString().
2. String:
   • Represents text as a series of Unicode characters.
   • Example methods: Substring(), Replace(), IndexOf(), ToUpper(), ToLower().
   • Example:

     string greeting = "Hello, World!";
     string upperGreeting = greeting.ToUpper();

3. Array:
   • Provides methods for creating, manipulating, searching, and sorting arrays.
   • Example methods: Sort(), Copy(), Clear().
   • Example:

     int[] numbers = { 1, 2, 3, 4, 5 };
     Array.Reverse(numbers);

4. Console:
   • Represents the standard input, output, and error streams for console applications.
   • Example methods: WriteLine(), ReadLine().
   • Example:

     Console.WriteLine("Enter your name:");
     string name = Console.ReadLine();
     Console.WriteLine("Hello, " + name);

5. Math:
   • Provides constants and static methods for trigonometric, logarithmic, and other common mathematical functions.
   • Example methods: Abs(), Ceiling(), Floor(), Pow(), Sqrt().
   • Example:

     double result = Math.Sqrt(25);

6. DateTime:
   • Represents an instant in time, typically expressed as a date and time of day.
   • Example methods: Now, Today, AddDays(), ToString().
   • Example:

     DateTime now = DateTime.Now;
     Console.WriteLine("Current Date and Time: " + now);

7. Random:
   • Represents a pseudo-random number generator, which is a device that produces a sequence of numbers that meet certain statistical requirements for randomness.
   • Example methods: Next(), NextDouble(), NextBytes().
   • Example:

     Random random = new Random();
     int randomNumber = random.Next(1, 100);

Example Usage:

Here's a simple example demonstrating the use of some of these classes:

using System;

class Program
{
    static void Main()
    {
        // Using the String class
        string greeting = "Hello, World!";
        string upperGreeting = greeting.ToUpper();
        Console.WriteLine(upperGreeting);

        // Using the Array class
        int[] numbers = { 1, 2, 3, 4, 5 };
        Array.Reverse(numbers);
        Console.WriteLine("Reversed Array: " + string.Join(", ", numbers));

        // Using the DateTime class
        DateTime now = DateTime.Now;
        Console.WriteLine("Current Date and Time: " + now);

        // Using the Math class
        double result = Math.Sqrt(25);
        Console.WriteLine("Square Root of 25: " + result);

        // Using the Random class
        Random random = new Random();
        int randomNumber = random.Next(1, 100);
        Console.WriteLine("Random Number: " + randomNumber);
    }
}