Event Handling ⚡

🔔 Real-World Analogy

Think of events like a doorbell:

• 🚪 The Doorbell = The event (something happened)
• 👂 You Listening = The event handler (waiting for it)
• 🚶 Opening the Door = The response (what you do)

💻 Code Time!

Basic Event Handler:

private void btnClickMe_Click(object sender, EventArgs e)
{
    // This code runs when the button is clicked!
    MessageBox.Show("You clicked me!");
}

Explanation:

• sender - Which control triggered the event
• EventArgs - Additional information about the event
• Inside { } - Your code that runs!

📝 The Event Pattern

Three things happen with every event:

1. Event happens - User clicks, types, etc.
2. Event fires - The control says "Hey, someone did something!"
3. Handler runs - Your code executes in response

// 1. Event is defined in the control
public event EventHandler Click;

// 2. Event is raised (fired)
if (Click != null)
    Click(this, EventArgs.Empty);

// 3. Your handler responds
private void btn_Click(object sender, EventArgs e)
{
    // Your code here
}

🔗 Wiring Up Events

Two ways to connect events:

1. Designer (Easy) - Double-click the control
2. Code (Manual) - Write the connection yourself

// Manual wiring (in Form1.cs)
public Form1()
{
    InitializeComponent();
    btnSave.Click += new EventHandler(btnSave_Click);
}

// Or even shorter (modern C#)
public Form1()
{
    InitializeComponent();
    btnSave.Click += btnSave_Click;
}

✨ Visual Studio does this for you automatically when you double-click!

📝 Basic Form Load

private void Form1_Load(object sender, EventArgs e)
{
    // Populate a ComboBox
    cmbDepartment.Items.AddRange(new string[] 
        { "IT", "HR", "Finance" });
    cmbDepartment.SelectedIndex = 0;
    
    // Set default values
    txtDate.Text = DateTime.Now.ToShortDateString();
    lblStatus.Text = "Ready";
    
    // Disable button until ready
    btnSave.Enabled = false;
}

✅ Common Setup Tasks

• 📋 Load data from database
• 📊 Populate dropdowns with options
• 📅 Set default values (today's date, etc.)
• 🎨 Configure UI (enable/disable controls)
• 🔍 Set focus to the first field

📝 Example: Email Validation

private void txtEmail_TextChanged(object sender, EventArgs e)
{
    // Check if email contains '@'
    bool isValid = txtEmail.Text.Contains("@");
    
    // Update UI based on validation
    btnSave.Enabled = isValid;
    lblEmailStatus.Text = isValid ? "✅ Valid email" : "❌ Invalid email";
    lblEmailStatus.ForeColor = isValid ? Color.Green : Color.Red;
}

✅ Common Uses

• ✉️ Email validation - Check for @ symbol
• 📱 Phone number - Format as you type
• 🔑 Password strength - Show strength indicator
• 📊 Search - Filter results as user types
• 📝 Character count - Show remaining characters

📝 Example: Product Selection

private void cmbProducts_SelectedIndexChanged(object sender, EventArgs e)
{
    // Get selected product
    string product = cmbProducts.SelectedItem?.ToString();
    
    if (product == null) return;
    
    // Update other controls based on selection
    switch (product)
    {
        case "Laptop":
            txtPrice.Text = "999.99";
            txtDescription.Text = "High-performance laptop";
            break;
        case "Mouse":
            txtPrice.Text = "29.99";
            txtDescription.Text = "Wireless mouse";
            break;
        default:
            txtPrice.Text = "0.00";
            txtDescription.Text = "Unknown product";
            break;
    }
}

✅ Common Uses

• 🛒 Product selection - Show price/description
• 🏙️ Country/City - Update city dropdown when country changes
• 📊 Filter data - Show different data based on selection
• 👤 User profile - Load user details when selected

📝 Confirm Before Closing

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    // Ask user if they want to close
    DialogResult result = MessageBox.Show(
        "Are you sure you want to exit?",
        "Confirm Exit",
        MessageBoxButtons.YesNo,
        MessageBoxIcon.Question
    );
    
    // Cancel closing if user clicks No
    if (result == DialogResult.No)
    {
        e.Cancel = true;  // ⛔ Stop the form from closing
    }
}

✅ Common Uses

• 💾 Save unsaved changes - "Save before closing?"
• 🔒 Confirm exit - "Are you sure?"
• 📋 Clean up - Close database connections
• 📊 Save state - Remember window position