Lesson 17 - Page_Load and Page.IsPostBack

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.

Step 1: Create a New Project

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:

  1. myTextBox

  2. myCalendar

  3. okButton

  4. resultLabel

cs-asp-017---page_load-and-page.ispostback.001

Step 2: Setting Up the PostBack Problem

Double-click the okButton, and in Default.aspx.cs, write the following code in the Page_Load and okButton_Click events:

cs-asp-017---page_load-and-page.ispostback.002

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.

Step 3: Understanding the PostBack Problem

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).

Step 4: Referencing the IsPostBack Property

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.

Step 5: Resolving the Problem with a Conditional Check

Here is how we resolve the issue in code:

cs-asp-017---page_load-and page.ispostback.003

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:

cs-asp-017---page_load-and-page.ispostback.004

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.

Step 6: Understanding Code Execution and Event Timing

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:

http://is.gd/postbackcycle

In particular, notice how the event handling occurs after the initial Load procedure:

cs-asp-017---page_load-and-page.ispostback.007

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!


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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