C# Namespaces

Namespaces are a fundamental concept in C# that help organize and manage the scope of classes, structs, enums, delegates, and other namespaces. They prevent naming conflicts and make it easier to locate and use classes within large codebases.

1. Purpose of Namespaces

2. Declaring Namespaces

namespace NamespaceName
{
    // Types declarations
    class ClassName
    {
        // Class members
    }
}

using System;

namespace MyApplication.Utilities
{
    class Logger
    {
        public void Log(string message)
        {
            Console.WriteLine($"Log: {message}");
        }
    }
}

3. Using Namespaces

using MyApplication.Utilities;

class Program
{
    static void Main()
    {
        Logger logger = new Logger();
        logger.Log("Application started.");
    }
}

Output:
Log: Application started.

class Program
{
    static void Main()
    {
        MyApplication.Utilities.Logger logger = new MyApplication.Utilities.Logger();
        logger.Log("Application started.");
    }
}

Output:
Log: Application started.

4. Nested Namespaces

namespace MyApplication
{
    namespace DataAccess
    {
        class Repository
        {
            public void GetData()
            {
                Console.WriteLine("Fetching data from repository.");
            }
        }
    }

    namespace BusinessLogic
    {
        class Service
        {
            public void ProcessData()
            {
                Console.WriteLine("Processing data in service.");
            }
        }
    }
}

using MyApplication.DataAccess;
using MyApplication.BusinessLogic;

class Program
{
    static void Main()
    {
        Repository repo = new Repository();
        repo.GetData(); // Output: Fetching data from repository.

        Service service = new Service();
        service.ProcessData(); // Output: Processing data in service.
    }
}

Output:
Fetching data from repository.
Processing data in service.

5. Alias Directives

using DA = MyApplication.DataAccess;
using BL = MyApplication.BusinessLogic;

class Program
{
    static void Main()
    {
        DA.Repository repo = new DA.Repository();
        repo.GetData(); // Output: Fetching data from repository.

        BL.Service service = new BL.Service();
        service.ProcessData(); // Output: Processing data in service.
    }
}

Output:
Fetching data from repository.
Processing data in service.

using MyString = System.String;

class Program
{
    static void Main()
    {
        MyString greeting = "Hello, World!";
        Console.WriteLine(greeting);
    }
}

Output:
Hello, World!

6. Global Namespace

namespace MyApplication
{
    class String // This class shadows System.String
    {
        public string Value { get; set; }
    }

    class Program
    {
        static void Main()
        {
            // Using the custom String class
            String myString = new String { Value = "Custom String" };
            Console.WriteLine(myString.Value); // Output: Custom String

            // Using the System.String class
            global::System.String systemString = "System String";
            Console.WriteLine(systemString); // Output: System String
        }
    }
}

Output:
Custom String
System String

7. Best Practices for Using Namespaces

8. Common Mistakes and How to Avoid Them

9. Advanced Topics

namespace MyApplication.Utilities;

class Helper
{
  public void Assist()
  {
      Console.WriteLine("Assisting...");
  }
}

namespace MyApplication
{
  using System;

  class Program
  {
      static void Main()
      {
          Console.WriteLine("Hello from inside the namespace!");
      }
  }
}

Output:
Hello from inside the namespace!

10. Real-World Example: Organizing a Library System

using System;
using LibrarySystem.Books;
using LibrarySystem.Members;
using LibrarySystem.Lending;

namespace LibrarySystem
{
    namespace Books
    {
        class Book
        {
            public string Title { get; set; }
            public string Author { get; set; }

            public void DisplayInfo()
            {
                Console.WriteLine($"Title: {Title}, Author: {Author}");
            }
        }
    }

    namespace Members
    {
        class Member
        {
            public string Name { get; set; }
            public int MemberId { get; set; }

            public void DisplayInfo()
            {
                Console.WriteLine($"Name: {Name}, Member ID: {MemberId}");
            }
        }
    }

    namespace Lending
    {
        class Loan
        {
            public Books.Book Book { get; set; }
            public Members.Member Member { get; set; }
            public DateTime LoanDate { get; set; }

            public void DisplayLoanInfo()
            {
                Console.WriteLine($"Book: {Book.Title}, Borrowed by: {Member.Name}, On: {LoanDate.ToShortDateString()}");
            }
        }
    }

    class Program
    {
        static void Main()
        {
            Books.Book book = new Books.Book { Title = "1984", Author = "George Orwell" };
            Members.Member member = new Members.Member { Name = "John Doe", MemberId = 101 };
            Lending.Loan loan = new Lending.Loan
            {
                Book = book,
                Member = member,
                LoanDate = DateTime.Now
            };

            book.DisplayInfo(); // Output: Title: 1984, Author: George Orwell
            member.DisplayInfo(); // Output: Name: John Doe, Member ID: 101
            loan.DisplayLoanInfo(); // Output: Book: 1984, Borrowed by: John Doe, On: [Current Date]
        }
    }
}

Output:
Title: 1984, Author: George Orwell
Name: John Doe, Member ID: 101
Book: 1984, Borrowed by: John Doe, On: [Current Date]

11. Summary

Key points