Lesson 40 - Namespaces and Using Directives

Tutorial Series: Free C# Fundamentals via ASP.NET Web Apps

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The last few lessons have been covering a lot of details involving classes because they're so integral to the .NET Framework. In fact, just about everything you do in .NET involves a class in some way. The .NET Framework Class Library itself is really just a collection of classes with pre-built functionality that you can utilize in your projects. This library was built – by Microsoft – to make your job easier, and so that you don’t have to “re-invent the wheel” when creating software. The aim of this lesson is to help you understand how the .NET Framework Class Library was put together and how you can get the most out of it. This should help you better understand how the pre-built code is organized and how to access code that's not part of your project by default. Much of this hinges on a deeply related coding topic that you have been seeing on the periphery throughout these lessons and that is the topic of namespaces along with the using directive.

Step 1: Understanding How Namespaces and Using Directives Relate

To understand how namespaces work, let’s briefly look back at the concepts of scope and locality. Consider having two different properties in your code that, logically, should have identical names. This is not possible when those properties are declared in the same scope (the class, in this case). However, it is perfectly possible to share the same name if these properties were in separate classes:


The compiler easily distinguishes between these properties because they each belong to very different scopes. By the same token, a namespace is yet another scope that simply acts as a container for a group of classes. So if you have two classes with the same name, it is forbidden if they are both part of the same scope since the compiler has no way of distinguishing between them:


However, if each of these classes belong in different namespaces, they belong within different scopes entirely:


Step 2: Referencing With and Without Using Directives

It becomes more obvious how the compiler can tell these classes apart when you reference them using their “full names,” that include the namespace they respectively belong to:


The CS_ASP_040 namespace here is color-coded different from the other namespace because it is being used within the current namespace:


We don’t have to reference the namespace “full name” for the first Car object as the compiler intuits that we are referring to the Car class from within the current namespace “CS_ASP_040”. However, the SomeOtherGroupOfClasses.Car object is different because it’s referencing a class from an outside namespace. In this case, you have to either reference the namespace on each line of code that utilizes this class, or as a shorthand you can add this outside namespace to the current one through a using directive:


Step 3: Resolving Class Name Ambiguities

Under normal circumstances this would adequately solve the problem of avoiding typing out the “full name” of the Class, but in this particular example we still have two Car classes without a way for the compiler to distinguish between them. In the example below, we know that car2 is supposed to be of the type found in SomeOtherGroupOfClasses, but the compiler will think it’s referring to the type found in the current CS_ASP_040 namespace even with the using directive:



If this is confusing, think about the concept of first and last names in the real world. If you were told, "Bob likes coffee", you'd probably have a difficult time understanding which Bob was being referred to. However, if you were told that "Robert Tharon Tabor likes coffee", you'd likely be able to distinguish that Bob from the other Bobs you know. In a similar manner, scope gives context to shared names, such as the example with "Make" given above. If you were told to modify the Make property, but it was given no context, you wouldn't know which property to act on. In the same way, the compiler needs context to distinguish between shared names.

Step 4: Using Namespaces to Import Code Libraries

It’s important to note that while a namespace can just be seen as a way of grouping classes into different areas in your codebase, the biggest benefit of a namespace is to be able to import into your project (and make reference to) a library of code that is outside of it. We saw this earlier when we wanted to access the StringBuilder class that originates from the System.Text namespace:


If we didn’t include the using directive to this namespace, we would have had to use the full name for this reference, making a less clean looking result:


It should be fairly clear that the System.Text namespace contains within it a collection of classes – one of which is the StringBuilder class. You can see, with Intellisense, all of the constituent classes within this namespace:


Step 5: Visual Studio Defaults to Importing Common Namespaces

You will typically only want to import libraries that you intend to use. However, projects that are based on a template (such as the ASP.NET projects we have been working with) will anticipate common libraries that are useful – or in some cases necessary – by adding them ahead of time as using directives within each file that references them:


Step 6: Understanding Nested Namespaces

Another aspect of namespaces is that they can be nested within each other. In the example above, the Text namespace is nested within the System namespace. You can create nested classes just as you would expect:


And then you can reference them using the dot accessor in the way that you would expect:



Because classes can share the same name - but belong to a different namespace - be careful to ensure when researching a class that it is from the namespace you intend on using. When in doubt, enter the "full name" of the class including its namespace in the search engine.

Related Articles in this Tutorial:

Lesson 1 - Series Introduction

Lesson 2 - Installing Visual Studio 2015

Lesson 3 - Building Your First Web App

Lesson 4 - Understanding What You Just Did

Lesson 5 - Working with Projects in Visual Studio

Lesson 6 - Simple Web Page Formatting in Visual Studio

Challenge 1

Solution 1

Lesson 7 - Variables and Data Types

Lesson 8 - Data Type Conversion

Lesson 9 - Arithmetic Operators

Lesson 10 - C# Syntax Basics

Challenge 2 - ChallengeSimpleCalculator

Solution - ChallengeSimpleCalculator

Lesson 11 - Conditional If Statements

Lesson 12 - The Conditional Ternary Operator

Challenge 3 - ChallengeConditionalRadioButton

Solution - Challenge Conditional RadioButton

Lesson 13 - Comparison and Logical Operators

Lesson 13 Challenge - First Papa Bob's Website

Solution - Challenge First Papa Bob's Website

Lesson 14 - Working with Dates and Times

Lesson 15 - Working With Spans of Time

Lesson 16 - Working with the Calendar Server Control

Challenge 4 - Challenge Days Between Dates

Solution - Challenge Days Between Dates

Lesson 17 - Page_Load and Page.IsPostBack

Lesson 18 - Setting a Break Point and Debugging

Lesson 19 - Formatting Strings

Challenge 5 - Challenge Epic Spies Assignment

Solution - Challenge Epic Spies Assignment

Lesson 20 - Maintaining State with ViewState

Lesson 21 - Storing Values in Arrays

Lesson 22 - Understanding Multidimensional Arrays

Lesson 23 - Changing the Length of an Array

Challenge 6 - Challenge Epic Spies Asset Tracker

Solution - Challenge Epic Spies Asset Tracker

Lesson 24 - Understanding Variable Scope

Lesson 25 - Code Blocks and Nested If Statements

Lesson 26 - Looping with the For Iteration Statement

Challenge 7 - Challenge For Xmen Battle Count

Solution - Challenge For Xmen Battle Count

Lesson 27 - Looping with the while() & do...while() Iteration Statements

Lesson 28 - Creating and Calling Simple Helper Methods

Lesson 29 - Creating Methods with Input Parameters

Lesson 30 - Returning Values from Methods

Lesson 31 - Creating Overloaded Methods

Lesson 32 - Creating Optional Parameters

Lesson 33 - Creating Names Parameters

Lesson 34 - Creating Methods with Output Parameters

Challenge 8 - Challenge Postal Calculator Helper Methods

Solution - Challenge Postal Calculator Helper Methods

Mega Challenge Casino

Solution - Mega Challenge Casino

Lesson 35 - Manipulating Strings

Challenge 9 - Phun With Strings

Solution - Challenge Phun With Strings

Lesson 36 - Introduction to Classes and Objects

Challenge - Hero Monster Classes Part 1

Solution - Hero Monster Classes Part 1

Challenge - Hero Monster Classes Part 2

Solution - Challenge Hero Monster Classes Part 2

Lesson 37 - Creating Class Files Creating Cohesive Classes and Code Navigation

Lesson 38 - Understanding Object References and Object Lifetime

Lesson 39 - Understanding the .NET Framework and Compilation

Lesson 40 - Namespaces and Using Directives

Lesson 41 - Creating Class Libraries and Adding References to Assemblies

Lesson 42 - Accessibility Modifiers, Fields and Properties

Lesson 43 - Creating Constructor Methods

Lesson 44 - Naming Conventions for Identifiers

Lesson 45 - Static vs Instance Members

Challenge 10 - Challenge Simple Darts

Solution - Challenge Simple Darts

Lesson 46 - Working with the List Collection

Lesson 47 - Object Initializers

Lesson 48 - Collection Initializers

Lesson 49 - Working with the Dictionary Collection

Lesson 50 - Looping with the foreach Iteration Statement

Lesson 51 - Implicitly-Typed Variables with the var Keyword

Challenge 11 - Challenge Student Courses

Solution - Challenge Student Courses

Mega Challenge War

Solution - Mega Challenge War

Lesson 52 - Creating GUIDs

Lesson 53 - Working with Enumerations

Lesson 54 - Understanding the switch() Statement

Lesson 55 - First Pass at the Separation of Concerns Principle

Lesson 56 - Understanding Exception Handling

Lesson 57 - Understanding Global Exception Handling

Lesson 58 - Understanding Custom Exceptions

Lesson 59 - Creating a Database in Visual Studio

Lesson 60 - Creating an Entity Data Model

Lesson 61 - Displaying the DbSet Result in an ASP.NET GridView

Lesson 62 - Implementing a Button Command in a GridView

Lesson 63 - Using a Tools-Centric Approach to Building a Database Application

Lesson 64 - Using a Maintenance-Driven Approach to Building a Database Application

Lesson 65 - Creating a New Instance of an Entity and Persisting it to the Database

Lesson 66 - Package Management with NuGet

Lesson 67 - NuGet No-Commit Workflow

Lesson 68 - Introduction the Twitter Bootstrap CSS Framework

Lesson 69 - Mapping Enum Types to Entity Properties in the Framework Designer

Lesson 70 - Deploying the App to Microsoft Azure Web Services Web Apps

Papa Bob's Mega Challenge

Papa Bob's Mega Solution Part 1 - Setting up the Solution

Papa Bob's Mega Solution Part 2 - Adding an Order to the Database

Papa Bob's Mega Solution Part 3 - Passing an Order from the Presentation Layer

Papa Bob's Mega Solution Part 4 - Creating the Order Form

Papa Bob's Mega Solution Part 5 - Adding Enums

Papa Bob's Mega Solution Part 6 - Creating an Order with Validation

Papa Bob's Mega Solution Part 7 - Calculating the Order Price

Papa Bob's Mega Solution Part 8 - Displaying the Price to the User

Papa Bob's Mega Solution Part 9 - Creating the Order Management Page


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