Namespaces

Namespaces in C# are used to organize code and prevent naming collisions. They
provide a way to group related types (classes, structs, interfaces, enums,
delegates) under a common name. This makes code more manageable, especially in
larger projects where multiple developers might be working on different parts of
the application.

Declaring Namespaces

You declare a namespace using the namespace keyword, followed by the namespace
name and a block of code enclosed in curly braces.

namespace MyApplication {
    // Classes, structs, etc., go here
    class MyClass {
        // ...
    }
 
    namespace DataAccess {
        class UserRepository {
            // ...
        }
    }
}

In the example above, MyApplication is the parent namespace, and DataAccess
is a nested namespace. To refer to MyClass, you would use
MyApplication.MyClass. To refer to UserRepository, you would use
MyApplication.DataAccess.UserRepository.

Using Namespaces

To avoid typing the fully qualified name of a type every time you use it, you
can import a namespace using the using directive.

using System;
using MyApplication.DataAccess;
 
namespace MyApplication {
    class Program {
        static void Main(string[] args) {
            Console.WriteLine("Hello from MyApplication!");
            UserRepository repository = new UserRepository();
            // ...
        }
    }
}

The using System; directive allows you to use types from the System
namespace (like Console) without fully qualifying them. Similarly, using MyApplication.DataAccess; allows you to use UserRepository directly.

Global Namespace

Every C# file implicitly belongs to the global namespace if no explicit
namespace is declared. Types declared directly in a file without a namespace
keyword reside in the global namespace.

Namespace Aliases

You can create an alias for a namespace to simplify long namespace names or
resolve ambiguities when two different namespaces contain types with the same
name.

using DAL = MyApplication.DataAccess;
 
namespace MyApplication {
    class Program {
        static void Main(string[] args)
        {
            DAL.UserRepository repository = new DAL.UserRepository();
            // ...
        }
    }
}

Here, DAL is an alias for MyApplication.DataAccess.

File-Scoped Namespaces (C# 10 and later)

C# 10 introduced file-scoped namespaces, which simplify namespace declarations
by applying the namespace to the entire file without needing curly braces.

// MyFile.cs
namespace MyApplication.Utilities;
 
class Helper {
    // This class is automatically part of MyApplication.Utilities
}

This syntax applies the MyApplication.Utilities namespace to all types declared within MyFile.cs.