Event Handlers in C# WinForms

Understanding sender, event arguments, and how events work

Windows Forms apps are event-driven — they sit and wait for the user to do something (like click a button), then respond with code. This is where event handlers come in.

🧰 What Is an Event Handler?

An event handler is a method that runs when something happens — like a button being clicked, the mouse moving, or a textbox losing focus.

myButton.Click += MyClickHandler;

Here, you’re telling the button to run MyClickHandler when it’s clicked.

🚀 Getting Started

🛠 Common Events in WinForms:

Control	Event	What it means
`Button`	`Click`	User clicks the button
`TextBox`	`TextChanged`	Text content has changed
`Form`	`Load`	The form has finished loading
`ComboBox`	`SelectedIndexChanged`	New item selected

🧩 Event Handler Syntax

Every event has a standard method signature:

private void button1_Click(object sender, EventArgs e)
{
    // This code runs when the button is clicked
}

Part	What it means
`sender`	The control that triggered the event
`EventArgs`	Extra info about the event (can be subclassed)

✅ You can assign it manually or use the Visual Studio designer to double-click the control and auto-generate it.

🧪 Example: Show Which Button Was Clicked

private void button_Click(object sender, EventArgs e)
{
    Button clicked = sender as Button;
    if (clicked != null)
    {
        MessageBox.Show("You clicked: " + clicked.Text);
    }
}

Set it up:

button1.Click += button_Click;
button2.Click += button_Click;

🧠 sender helps you reuse one event handler for multiple controls.

🔍 Custom EventArgs

Sometimes built-in EventArgs isn’t enough. You can define your own:

public class ScoreEventArgs : EventArgs
{
    public int Score { get; set; }
}

Useful when creating custom controls or raising your own events.

🧱 Full Example: Text Change Logger

private void textBox1_TextChanged(object sender, EventArgs e)
{
    TextBox changedBox = sender as TextBox;
    Console.WriteLine("New text: " + changedBox.Text);
}

✅ Works with multiple textboxes — use sender to identify which was changed.

📚 Summary

Concept	Description
`Event Handler`	A method that runs when an event occurs
`sender`	The object that triggered the event
`EventArgs`	Extra data passed with the event
`+=`	Used to connect an event to a handler
`as`	Type-casts the sender to a specific control

🧪 Quick Challenge

🧩 Create 3 buttons. When any button is clicked, display a message like:

You clicked: Button 1

💡 Hint: Use a shared event handler and check ((Button)sender).Text.

🎓 Want to Go Deeper?

✅ Use MouseEventArgs for mouse clicks & positions
✅ Handle form-level events like FormClosing
✅ Raise your own custom events
✅ Use KeyPress and KeyDown for keyboard input

💬 Still unsure how sender or EventArgs works in your project?
We can help you wire up events or debug handlers anytime!