Lesson 38 - Understanding Object References and Object Lifetime

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

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In this lesson we're going to talk about the lifetime of objects - in terms of their references being kept in memory - and how the .NET Framework Runtime manages those memory allocations for you.

Step 1: Create a New Project

To illustrate this point, create a new ASP.NET project called “CS-ASP_038” and create a simple Car class with an object instance of it:


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When that last line of code executes creating a new Car() instance, the .NET Framework creates a spot in the computer's memory large enough to hold the new instance of the Car class. The computer's memory has addresses - much like how you have a home address – and these addresses are where the .NET Framework Runtime temporarily stores values, like objects or variables, during the lifetime of the variable or the object. You can summarize this .NET Framework memory allocation process as follows:

  1. Creating a variable/object creates a place in memory large enough for the particular data type (in this case a class instance of Car).

  2. The Framework keeps an address (or “pointer”) of where it put that new instance of car, and then it serves that address back to you - the programmer - so that you can get back to the information in memory whenever you need it (such as when referencing the variable somewhere else in code).

Step 2: Understanding Memory Addresses and Object References

In the illustration below, you can think of the color-coding for each property as the memory address storing that property’s value. You then use that address reference to look up that value whenever you get/set the value for the property in code:


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Whenever you see the new keyword, you can take that to mean there is a new instance with its own particular location being created in memory. This might become clearer if you split up the declaration/assignment steps on their own line of code:


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Here, the first line of code is creating (1) a reference in memory that is not yet pointing to any particular memory address storing particular values in memory (if given):


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It’s not until you use the new keyword that this reference (2) becomes set to a particular memory address. You can now “connect” to the memory reference - not directly the values in memory, but the reference to the values (if present) in memory – wherever you refer to it with the human-readable format provided in code:


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Step 3: Automated Memory Management via the Garbage Collector

One of the key features of C# is something called “Garbage Collection” which cleans up memory registers that no longer have a references pointing to them. This is an automatic process which is very different from earlier languages like C or C++ that required programmers to keep very close tabs on each and every item in memory. References can be dropped whenever an object falls out of scope (such as an object declared local to a method and therefore staying alive only as long as that method is running) or with a specific line of code that “destroys” the object. In this illustration, the .NET Garbage Collector would mark all of these memory registers for deletion – freeing up that space - because the reference no longer exists:


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Step 4: Implications of Reference vs Value Storage Types

This reference/value relationship has a variety of interesting implications when writing code in C#. One such implication is when you assign one object instance to another, you are actually copying the reference, rather than copying a new set of memory registers:


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Now, both myCar and myOtherCar point to the exact same memory registers:


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This means that if you change the value for myCar.Make you will also be changing it for myOtherCar.Make, and vice versa. This is very different from how value types work, which copy values to their own unique memory registers:


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Step 5: Explicit De-Referencing Using the ‘null’ Keyword

Also, if you de-reference one of these references (by setting it to null), the other reference still points to the memory registers. And since there is still a reference to those registers, the Garbage Collector keeps the memory intact unless myOtherCar also gets set to null:


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It’s important to note that object references are normally removed whenever the .NET Framework gets around to it. In some situations this is an indeterminate point in time. This can sometimes cause problems, especially when the object in memory is holding onto a system resource like a network connection or a file in the file system. Those are scenarios in which you would want to force the garbage collector to immediately do its clean-up process so that you can possibly use those resources for something else. This more deterministic approach to managing memory is somewhat more complicated than just setting all references to null but it’s worth keeping in mind as you move further down the path of your programming career.


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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