2.2 Using — Importing Namespaces

Let’s go through using in C# — the keyword that lets you import namespaces for easier access to types.

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📌 1. What Is using?

• The using directive allows you to reference types from other namespaces without writing the full, fully-qualified name.

• Helps reduce clutter and make code more readable.

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📌 2. Basic Syntax

using System;
 
class Program
{
    static void Main()
    {
        Console.WriteLine("Hello, World!");
    }
}

• using System; imports the System namespace.

• Without it, you would need to write:

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

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📌 3. Importing Multiple Namespaces

• You can include multiple using directives at the top of a file:

using System;
using System.Collections.Generic;
 
class Program
{
    static void Main()
    {
        List<string> names = new List<string> { "Alice", "Bob" };
        Console.WriteLine(string.Join(", ", names));
    }
}

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📌 4. Aliasing a Namespace or Type

• You can create shorter names or avoid conflicts using the using alias = namespace_or_type; syntax.

Example: Namespace alias

using Utils = MyApplication.Utilities;
 
class Program
{
    static void Main()
    {
        Utils.Logger log = new Utils.Logger();
        log.Log("Hello!");
    }
}

Example: Type alias

using IntList = System.Collections.Generic.List<int>;
 
class Program
{
    static void Main()
    {
        IntList numbers = new IntList { 1, 2, 3 };
        Console.WriteLine(string.Join(", ", numbers));
    }
}

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📌 5. Global Using (C# 10)

• global using applies the import to all files in the project, so you don’t have to repeat it in every file.

// File: GlobalUsings.cs
global using System;
global using System.Collections.Generic;
 
// Now Console and List<T> are available in all files

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📌 6. Key Points

Feature	Notes
using directive	Import a namespace for easier access to types
Aliases	Avoid conflicts or shorten long names (using alias = ...)
global using	Available in C# 10+, applies project-wide
Reduces fully-qualified names	Improves code readability

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📌 7. Analogy

• namespace = folder

• using = shortcut that lets you access files (types) in that folder without typing the full path

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✅ Summary:

• using imports namespaces or creates aliases.

• Simplifies code, improves readability, and avoids fully-qualified names.

• global using is a modern feature for project-wide imports.

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