Tutorial Series: Free C# Fundamentals via ASP.NET Web Apps
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In this lesson, we're going to take a slight departure from pure C# and talk about something ASP.NET specific. While generally avoiding the minutiae of ASP.NET, the specific topic of this lesson will hopefully lead you towards building more interesting C# applications.
Either open the CS-ASP_017 Project from the provided code folder for this lesson, or setup your Default.aspx page as follows:
To illustrate this, create a new ASP.NET project called “CS-ASP_017.” This project is based on the previous lesson, and has a Default.aspx with the following Server Controls and programmatic IDs:
myTextBox
myCalendar
okButton
resultLabel
Double-click the okButton, and in Default.aspx.cs, write the following code in the Page_Load and okButton_Click events:
The Page_Load event simply assigns, upon the page being loaded, the myTextBox.Text property with a default string, as well as the selected date to two days ahead of the current date. Be careful to reference the Date property that is a part of the Now property - by writing Now.Date - otherwise myCalendar.SelectedDate will hold a specific time (including hour/minutes, which is not how calendars work). Meanwhile, the okButton_Click event simply outputs the value contained in myTextBox.Text concatenated with the value in myCalendar.SelectedDate.
Setting up a form, or calendar, with default values is a common task, however there is a problem with this code that we wrote. If you run the application, you will notice that when you change the values for the Calendar or TextBox, those changes are never reflected after submitting them upon clicking the button. That’s because the default values always reload after every submission before the okButton_Click event is processed. The easiest solution to this problem is to include code that does one thing (assign default values) if it’s the first time the page is loaded, and do another thing if it’s not (ignore the default values).
The easiest way to branch off these two separate cases is to determine whether or not the page was loaded by the bool IsPostBack property. Simply put, PostBack occurs when the okButton is clicked, and you can’t PostBack unless you first got to the page from some other means (from a direct-link, for example). This means that PostBack will never be the initial page load. Therefore, if the page was loaded via the okButton (PostBack) we can safely ignore the default values, and instead have the code read-in the user-input values.
Here is how we resolve the issue in code:
Noticed how the conditional checks if the page is loaded not by PostBack. Checking for the opposite would involve a bit more code and not improve readability:
Tip: Whenever you have an empty conditional (a block of code that essentially does nothing if the condition is met), you should consider inverting the expression being evaluated to check the opposite of that condition instead. If nothing else, it will make your code cleaner and easier to read.
You may be wondering when exactly, during the page load process, the IsPostBack property is set. This refers to code execution/event timing, and is touched upon at the following URL:
In particular, notice how the event handling occurs after the initial Load procedure:
You now know how to initialize form values, as well as how to ensure that those values are only set during the initial page load. Keep this in mind as it will be important moving forward in the series. Good job!
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
Lesson 7 - Variables and Data Types
Lesson 8 - Data Type Conversion
Lesson 9 - Arithmetic Operators
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
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
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 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