Mahmoud Abas, Full-Stack Software Engineer & AI

Operating Systems 1 (2026-2027) - Windows Forms: The Code Behind the Form

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Operating Systems 1, Lab 2

Windows Forms: The Code Behind the Form

What the designer writes for you, why a form is a class, and how to start a project the right way.

Objectives

Explain rapid application development and when Windows Forms fits
Read InitializeComponent and fix a designer that will not open
Treat a form as a class and change properties at run time
Open a second form and pass it data through its constructor
Use a TabControl, a MenuStrip and a StatusStrip, and know which events fire
Start a project the right way: font, names, tab order, logic out of the form

Part 1

Why Windows Forms

Fast to build, easy to outgrow

Rapid Application Development

Windows Forms: One of the original .NET project types (2001 and 2002): drag controls, set properties, double-click for a handler.

Proof of concept: A quick program that tests whether an idea works. With Windows Forms it takes hours or days.

The Desktop Project Types

Project type	Window designed with	Status
Windows Forms	Drag and drop	Since the first .NET; common in companies
WPF	XAML markup	Usual choice for a new desktop app
UWP	XAML	Tied to new Windows versions; deprecated
.NET MAUI	XAML	Desktop and mobile; young

When to Use Windows Forms

A quick prototype to test an idea
A small utility for yourself or your team
A simple application one person must build and maintain
Existing business applications: you will meet them at work

Predict: Resize the Window

The form was designed at 420 x 235. What happens when the window is smaller, or maximized?

At 300 by 200 the text boxes and the button are cut off at the right edge

Maximized, the controls stay in the top-left corner and the rest is empty

Part 2

The Code Behind the Designer

A quick demo, then a look inside

A Quick Demo in Two Minutes

Drag a Label and a TextBox; set the label to First Name
Ctrl+click both, then Ctrl+C, Ctrl+V: more pairs, lined up by snaplines
Drag a Button and a CheckBox
Drag a ProgressBar and set Value to 45

The quick demo form: three labels and text boxes, button1, checkBox1 and a progress bar at 45

Wire the Button

Double-click button1: Visual Studio creates button1_Click

csharp

private void button1_Click(object sender, EventArgs e)
{
    MessageBox.Show($"Hello {textBox1.Text} {textBox2.Text}");
}

textBox1 is an object on the form
Text is a property: what the user typed
$"..." is string interpolation: each part in braces becomes its value

Predict: Lab and Student

The user types Lab and Student and clicks button1. What does the message say?

The form with Lab and Student typed and a message box saying Hello Lab Student

Where the Code Lives

partial class Form1: the other half is Form1.Designer.cs, method InitializeComponent

csharp

this.button1 = new System.Windows.Forms.Button();
// ...
this.button1.Location = new System.Drawing.Point(120, 140);
this.button1.Name = "button1";
this.button1.Size = new System.Drawing.Size(100, 30);
this.button1.Text = "button1";
this.button1.Click += new System.EventHandler(this.button1_Click);
// ...
this.progressBar1.Value = 45;
this.Controls.Add(this.button1);

Part of our Form1.Designer.cs: a new object, one line per property, the event wiring, Controls.Add

Predict: Delete Form1_Load Only

You double-clicked the form by mistake, then deleted the new method Form1_Load from Form1.cs. The wiring line is still in Form1.Designer.cs.

text

Form1.Designer.cs(129,55): error CS1061: 'Form1' does not contain a definition for 'Form1_Load' and no accessible extension method 'Form1_Load' accepting a first argument of type 'Form1' could be found (are you missing a using directive or an assembly reference?)

Our build (path shortened). The designer also refuses to open: The designer cannot process unknown name 'Form1_Load' ...

The Fix: Delete the Wiring Line

csharp

this.Load += new System.EventHandler(this.Form1_Load);

Line 129 of our Form1.Designer.cs

Click Go to code on the designer's error page
Delete this one line
Save, and open the designer again

Remove a Handler the Right Way

Select the control, or the form, in the designer
In Properties, click the lightning bolt: the Events list
Select the handler next to the event (here Load) and delete it
The wiring line is gone; delete the method too if it is still there

Try It: Break and Fix the Designer

Part 3

A Form Is a Class

Inheritance, properties and objects

What Form Gives You

Press F12 on Form, then on its base class, and again

text

WinFormsDemo.Form1
System.Windows.Forms.Form
System.Windows.Forms.ContainerControl
System.Windows.Forms.ScrollableControl
System.Windows.Forms.Control
System.ComponentModel.Component
System.MarshalByRefObject
System.Object

The chain, printed by our running program

Title bar, icon, minimize, maximize, close
Resizing and the background
Drawing the controls you add
All inherited: nobody writes it again

Predict: Properties at Run Time

The user types Lab and Student, then clicks button1 once

csharp

public Form1()
{
    InitializeComponent();
    label2.Text = "Last Name";
    label3.Text = "Full Name";
}

The designer's value is only the starting value
Code can read and change any property
The designer still shows label2

Properties at Run Time: button1_Click

csharp

private void button1_Click(object sender, EventArgs e)
{
    textBox3.Text = $"{textBox1.Text} {textBox2.Text}";
    progressBar1.Value += 10;
}

Predict: The Sixth Click

The bar starts at 45 and each click adds 10. Minimum is 0, Maximum is 100.

The unhandled exception window: Value of 105 is not valid for Value

Predict: A Form Is an Object

Type Lab and Student in the first window, then click button1 twice

csharp

Form1 frm = new Form1();
frm.Show();
frm.Left = this.Left + 430;

button1_Click. The last line only moves the new window so both are visible.

The first window shows Lab and Student; two new Form1 windows at the right are empty

Part 4

Forms, Menus and the Status Strip

More than one form, and more than the common controls

Program.cs Picks the First Form

csharp

static void Main()
{
    ApplicationConfiguration.Initialize();
    Application.Run(new MainForm());
}

Program.cs

Application.Run(new MainForm()) creates the first form
Put another form's class there, and that form opens first
The first form is the main form: the application lives as long as it does

Pass Data Through the Constructor

csharp

public Form2(string message)
{
    InitializeComponent();
    formMessage = message;
    messageLabel.Text = formMessage;
}

Form2.cs, with a field private string formMessage;

csharp

Form2 frm = new Form2("Hello from Form 1");
frm.Show();

The main form's button

Predict: Open Form 2, Then Exit

Click Open Form 2. Then choose File, Exit on the main form, which runs this.Close().

Form2 shows Hello from Form 1 in large letters above the main form

TabControl: Pages in One Form

From the Toolbox group Containers
Holds several tab pages: a click on a tab shows another page
Still one form: each page is a container, drag controls onto it
Ours: a Main page with two buttons, an Options page with a check box

The Options tab selected, showing the Show tips check box

Predict: Three Clicks on Work

A StatusStrip (Menus & Toolbars) with a status label and a progress bar. The label starts as Ready, the bar at 0.

csharp

private void workButton_Click(object sender, EventArgs e)
{
    toolStripProgressBar1.Value += 5;
    toolStripStatusLabel1.Text = "Working...";
}

The status strip reads Working... and its bar is at 15

MenuStrip and the & Key

You type	The menu shows	Keyboard
`&File`	File, F underlined	Alt+F opens it
`E&xit`	Exit, x underlined	X, inside File
`&Help`	Help, H underlined	Alt+H opens it
`&About`	About, A underlined	A, inside Help

The Menu Handlers

csharp

private void exitToolStripMenuItem_Click(object sender, EventArgs e)
{
    this.Close();
}

private void aboutToolStripMenuItem_Click(object sender, EventArgs e)
{
    MessageBox.Show("Created in the OS1 lab");
}

Help, About shows a message box that says Created in the OS1 lab

Predict: Which Events Fire?

A logging handler on each event, driven on Windows with the real keyboard and mouse

Space on the focused button
Enter on the focused button
Mouse click on the button
Mouse click on a check box
Mouse click on a list item

Click
Click
MouseDown, Click, MouseClick, MouseUp
CheckedChanged, Click, MouseClick
Click, MouseClick, SelectedIndexChanged

Try It: The Events Playground

Part 5

Start a Project the Right Way

Six habits before you write any code

Six Habits, in This Order

Rename Form1 to MainForm (F2, and Yes to rename the references)
Set the form's Text: the title is a property, not the class name
Set the Font first, on the empty form
Name every control, before you double-click it
Check the tab order
Keep the logic in a class library

Predict: Font 9 to 18

A 360 x 150 form with a 100 x 30 button. We set the form's font to 18 points.

What	9 pt	18 pt
Form inside (ClientSize)	360 x 150	669 x 320
Button	100 x 30	186 x 64
Label using the form's font	9 pt	18 pt
Label with its own 10 pt font	10 pt	10 pt

Name Controls Like Variables

Old style (Hungarian)	This section
`lblFirstName`	`firstNameLabel`
`txtFirstName`	`firstNameText`
`btnSayHello`	`sayHelloButton`
`lstCity`	`cityList`

Predict: Rename After Double-Click

You double-click button1, then rename it sayHelloButton. What is the handler called now?

csharp

private void button1_Click(object sender, EventArgs e)

this.sayHelloButton.Click += new System.EventHandler(this.button1_Click);

Renaming the control does not rename the method

Predict: The Tab Order

The cursor is in the first name. Where does each Tab go?

Control	TabIndex
firstNameText	1
sayHelloButton	2
lastNameText	4
the two labels	0 and 3 (TabStop False)

Predict: The Object Instead of Its Text

The user types Lab and Student

csharp

MessageBox.Show($"Hello {firstNameText} {lastNameText}");

The message says Hello System.Windows.Forms.TextBox, Text: Lab System.Windows.Forms.TextBox, Text: Student

Keep the Logic Out of the Form

csharp

namespace GreetingLibrary;

public static class Greeter
{
    public static string Greet(string firstName, string lastName)
    {
        return $"Hello {firstName} {lastName}";
    }
}

A Class Library project

csharp

using GreetingLibrary;

Console.WriteLine(Greeter.Greet("Lab", "Student"));
Console.WriteLine(Greeter.Greet("Operating", "Systems"));

A console program using it

text

Hello Lab Student
Hello Operating Systems

Its real output

The Better Project

Main Form at 14 points with First Name, Last Name, Say Hello and a message box Hello Lab Student

csharp

private void sayHelloButton_Click(object sender, EventArgs e)
{
    string message = Greeter.Greet(
        firstNameText.Text, lastNameText.Text);
    MessageBox.Show(message);
}

MainForm.cs: the form only talks to the user

Quick Check

Which file decides the first form?
You double-clicked button1, then renamed it saveButton. The handler's name?
Space on a focused button: Click, MouseClick, or both?
Rename txtEmail in this section's style.
You deleted button2_Click and the designer broke. Which line goes, and from which file?

Part 6

Practice

About 30 minutes; answer first, then build and run

About 6 minutes

Practice 1: Predict the Tab Order

Control	TabIndex	TabStop
nameText	3	True
emailText	7	True
phoneText	5	True
newsCheck	1	False
saveButton	12	True

Practice 1: Answer

On open: nameText (TabIndex 3, the lowest tab stop)
Tab 1: phoneText (5)
Tab 2: emailText (7)
Tab 3: saveButton (12), then Tab 4: back to nameText

About 6 minutes

Practice 2: The Designer Will Not Open

csharp

this.checkBox1.CheckedChanged += new System.EventHandler(this.checkBox1_CheckedChanged);

Still in Form1.Designer.cs; the method was deleted from Form1.cs

Which error does the build report?
Give two ways to get a working project back.
How should the handler have been removed?

Practice 2: Answer

text

Form1.Designer.cs(104,75): error CS1061: 'Form1' does not contain a definition for 'checkBox1_CheckedChanged' ...

Our build of exactly this case (message shortened)

Delete the wiring line, or add the method back to Form1.cs
Next time: the Events list (lightning bolt), delete the handler next to CheckedChanged

About 6 minutes

Practice 3: A Bar With a Smaller Maximum

csharp

// progressBar1: Maximum = 50, Value = 0
private void button1_Click(object sender, EventArgs e)
{
    progressBar1.Value += 15;
}

What is the Value after three clicks?
What happens on the fourth? Which number does the error name?

Practice 3: Answer

Value of 60 is not valid for Value. Value should be between minimum and maximum

0, 15, 30, 45
Click 4 tries 60 > 50
Value of '60' is not valid for 'Value'

About 12 minutes

Practice 4: A Welcome Form

MainForm (Main Form): nameLabel, nameText, welcomeButton
WelcomeForm (Welcome): a large welcomeLabel
WelcomeForm's constructor takes the user name and shows Welcome, + name + !
Welcome opens a WelcomeForm with the name from nameText
What does it show for Student? What does new WelcomeForm() do?

Practice 4: Answer

csharp

public WelcomeForm(string userName)
{
    InitializeComponent();
    welcomeLabel.Text = $"Welcome, {userName}!";
}

private void welcomeButton_Click(object sender, EventArgs e)
{
    WelcomeForm frm = new WelcomeForm(nameText.Text);
    frm.Show();
}

The Welcome form shows Welcome, Student!

Key Takeaways

The designer only writes and renders code: read its errors, do not start over
Remove handlers through the Events list, never by deleting the method alone
A form is a class; its controls are objects whose properties code can change
Click fires for the mouse and the keyboard; MouseClick only for the mouse
Font first, names first, tab order checked, logic in a class library

Open this lesson

Mahmoud AbasWindows Forms: The Code Behind the Form

Project type

Window designed with

Status

Windows Forms

Drag and drop

Since the first .NET; common in companies

WPF

XAML markup

Usual choice for a new desktop app

UWP

XAML

Tied to new Windows versions; deprecated

.NET MAUI

XAML

Desktop and mobile; young

this.button1 = new System.Windows.Forms.Button(); // ... this.button1.Location = new System.Drawing.Point(120, 140); this.button1.Name = "button1"; this.button1.Size = new System.Drawing.Size(100, 30); this.button1.Text = "button1"; this.button1.Click += new System.EventHandler(this.button1_Click); // ... this.progressBar1.Value = 45; this.Controls.Add(this.button1);

Form1.Designer.cs(129,55): error CS1061: 'Form1' does not contain a definition for 'Form1_Load' and no accessible extension method 'Form1_Load' accepting a first argument of type 'Form1' could be found (are you missing a using directive or an assembly reference?)

WinFormsDemo.Form1 System.Windows.Forms.Form System.Windows.Forms.ContainerControl System.Windows.Forms.ScrollableControl System.Windows.Forms.Control System.ComponentModel.Component System.MarshalByRefObject System.Object

You type

The menu shows

Keyboard

&File

File, F underlined

Alt+F opens it

E&xit

Exit, x underlined

X, inside File

&Help

Help, H underlined

Alt+H opens it

&About

About, A underlined

A, inside Help

private void exitToolStripMenuItem_Click(object sender, EventArgs e) { this.Close(); } private void aboutToolStripMenuItem_Click(object sender, EventArgs e) { MessageBox.Show("Created in the OS1 lab"); }

What

9 pt

18 pt

Form inside (ClientSize)

360 x 150

669 x 320

Button

100 x 30

186 x 64

Label using the form's font

9 pt

18 pt

Label with its own 10 pt font

10 pt

Old style (Hungarian)

This section

lblFirstName

firstNameLabel

txtFirstName

firstNameText

btnSayHello

sayHelloButton

lstCity

cityList

Control

TabIndex

firstNameText

sayHelloButton

lastNameText

the two labels

0 and 3 (TabStop False)

Control

TabIndex

TabStop

nameText

True

emailText

True

phoneText

True

newsCheck

False

saveButton

True

public WelcomeForm(string userName) { InitializeComponent(); welcomeLabel.Text = $"Welcome, {userName}!"; } private void welcomeButton_Click(object sender, EventArgs e) { WelcomeForm frm = new WelcomeForm(nameText.Text); frm.Show(); }