Lesson 51 - Implicitly-Typed Variables with the var Keyword

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

Previous Article  |  Next Article


This lesson will talk about implicitly typed local variables using the var keyword. The term “local” refers to variables declared within a method scope, or narrower, that are not available at a class level. And “implicitly typed” means the compiler can determine what the data type of the variable is based on context, and set it appropriately. You can let the compiler interpret the type by using the var keyword in place of the actual type declaration.


cs-asp-051---implicitly-typed-local-variables-with-the-var-keyword.001


Step 1: Understanding How the ‘var’ Keyword Works

This works because the compiler references the actual type when compiling to Intermediate Language (IL). You can see that IntelliSense has no trouble understanding that this is supposed to be an int since it was already initialized to an integer value:


cs-asp-051---implicitly-typed-local-variables-with-the-var-keyword.002


This extends beyond simple value types and works with objects as well:


cs-asp-051---implicitly-typed-local-variables-with-the-var-keyword.003


Step 2: Caveats When Using the ‘var’ Keyword

There are a few caveats when dealing with implicitly typed variables:

  1. You must initialize the variable with an assigned value type or object reference.

  2. It is still type-strict. It takes on the implied data type, and cannot be changed any more than any other explicitly typed variable.


cs-asp-051---implicitly-typed-local-variables-with-the-var-keyword.004


cs-asp-051---implicitly-typed-local-variables-with-the-var-keyword.005


You may be wondering about the benefit of having implicitly typed variables since the var keyword adheres to the same strongly typed guidelines, and just obscures what should otherwise be obvious when you look at code at a glance. You will begin to see the benefits later on where, in some cases, it’s difficult to determine the data type ahead of time, the data type is unknown, or the data type becomes generated automatically at runtime. Just keep its function in mind as you will often see it being used in code examples provided on the internet. And, eventually, you will come across a case in which it is necessary.

Step 3: Dynamically Setting an Implicit Variable

To see how an implicitly typed variable can be used in code, make us of the following example. We're going to create a mock application in which we present the user with a group of RadioButtons to select from. Depending on their selection, we will display which data type was chosen, either String, Integer or Bool.

In your CS-ASP_051 Project, add a Default.aspx page, and add the following Server Controls:


ServerControls


Give these Server Controls the following ID's:

  1. stringRadioButton
  2. intRadioButton
  3. boolRadioButton
  4. okButton
  5. resultLabel

Set the GroupName property for the RadioButtons to myGroup.

Next, double-click the okButton generate the okButton_Click EventHandler. We'll deal with the code that goes in here later. For now, create a new public method called determineUserInput() that returns an object:


determineUserInput


Tip:

You may notice that this method is returning an Object rather than a string, int, or bool as we've used previously. The reason for this is that this method needs to return one of these based on what the user selects, so it needs the ability to be set to any of them. Because an Object can store the value of any of these variables, that is what we will return from this method.

Inside this method, declare three variables: One for string, bool and int:


Variables


In addition, create an Object called myObject, but do not set its value yet. Next, create a series of conditional statements that determine which RadioButton was selected. Depending on the RadioButton, set myObject equal to the appropriate value:


ConditionalChecks


The purpose of this code is to assign myObject either the value of string, int or bool, then return it to the caller, which we will now create.

In the okButton_Click, create an implicitly typed variable called userInput and set it equal to the value of determineUserInput():


userInput


Now, our variable will take on the value of whatever the user has passed in, which at this point is unknown until runtime. In order to display this, we'll set the resultLabel to the following:


resultLabel


The particulars of how this code functions isn't important right now, but in essence, this uses a method to retrieve the type, not value, of the userInput variable. Then, we display it in the resultLabel. Save and run your project to see the result:


Result


While convoluted and impractical, hopefully this example gives you an idea of the use of implicitly typed variables when dealing with unknown data types. Even though the user could select one of multiple data types (or even type, depending on the application), you are able to store their selection without having to create several different variables in your code.


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


Comments

Please login or register to add a comment