How to Subscribe Multiple Buttons to the Same Event Handler and Act According to What Button Was Clicked

How can I subscribe multiple buttons to the same event handler and act according to what button was clicked?

When you subscribe to the event on a button, it's just a standard event handler:

button1.Click += myEventHandler;

You can use the same code to add handlers for every button:

button1.Click += myEventHandler;
button2.Click += myEventHandler;
button3.Click += myEventHandler;
button4.Click += myEventHandler;
button5.Click += myEventHandler;
button6.Click += myEventHandler;

This will cause your handler in myEventHandler to be run when any of the buttons are clicked.

How to add one event listener for all buttons

You don't really need to add listeners to all the buttons. There is such thing in JS as Bubbling and capturing so you can wrap all your buttons with a div and catch all the events there. But make sure you check that the click was on a button and not on the div.

const wrapper = document.getElementById('wrapper');

wrapper.addEventListener('click', (event) => {

  const isButton = event.target.nodeName === 'BUTTON';

  if (!isButton) {

    return;

  }

  console.dir(event.target.id);

})

div {

  padding: 20px;

  border: 1px solid black;

}

<div id='wrapper'>

  <button id='but1'>

  Button1

  </button>

  <button id='but2'>

  Button2

  </button>

  <button id='but3'>

  Button3

  </button>

  <button id='but4'>

  Button4

  </button>

</div>

How to check if multiple buttons where clicked in an alphabet soup game

You can try to create buttons and assign event hadlers in a loop:

  for (int line = 0; line < 7; ++line) {
    for (int column = 0; column < 10; ++column) {
      Button button = new Button() {
        Parent = this,
        Text = "?", // Here you to generate button's text, e.g. with Random   
        Location = new Point(50 + line * 40, 50 + column * 40),
        Size = new Size(30, 30),
      };

      button.Click += (ss, ee) => {
        Button myButton = ss as Button;

        myButton.ForeColor = System.Drawing.Color.Red;
      };
    }
  }

If you put all the buttons on the form manually, you can assign event hadlers in a loop as well:

  foreach (var button in Controls.OfType<Button>()) {
    button.Click += (ss, ee) => {
      Button myButton = ss as Button;

      myButton.ForeColor = System.Drawing.Color.Red;
    };
  }

Common event handler for multiple controls of different types in C#

If you add name of the index to the Tag of control, then you can set bChangedValues too.

private void Control_ValueChanged(object sender, EventArgs e)
{
    var control = (Control)sender;
    control.BackColor = CHANGED_COLOUR;
    bChangedValues[control.Tag] = true;
}

How can I apply oops in the code below to make it less redundant code?

Did you know you can link multiple buttons to the same event handler? See this answer to learn how.

Once you have all the buttons pointed at the same handler, you can use the same code, except the part that decides whether to add, multiple, divide, or subtract. For that you will need a series of if statements, checking the sender to see which button raised the event.

private void MyClickHandler(object sender, EventArgs e)
{
    int num1 = 0;
    int num2 = 0;
    String num = firstNum.Text;
    String sNum = secondNum.Text;
    if (num == "")
    {
        MessageBox.Show("error type both num");
    }
    else
    {
        num1 = int.Parse(num);
        num2 = int.Parse(sNum);
    }
    int total = 0;
    if (sender == addBtn) total = num1 + num2;
    if (sender == subBtn) total = num1 - num2;
    if (sender == multBtn) total = num1 * num2;
    if (sender == divideBtn) total = num1 / num2;
   
    MessageBox.Show(total.ToString());

    firstNum.Text = string.Empty;
    secondNum.Text = string.Empty;
}

Extra credit

If you really want to be fancy you can get rid of the messy if statements by using a lookup table with delegates for each operation.

private Dictionary<Control,Func<int,int,int>> _operations = new Dictionary<Control,Func<int,int,int>>
{
    { addBtn,    (x,y) => x + y },
    { subBtn,    (x,y) => x - y },
    { multBtn,   (x,y) => x * y },
    { divideBtn, (x,y) => x / y }
};

private void MyClickHandler(object sender, EventArgs e)
{
    int num1 = 0;
    int num2 = 0;
    String num = firstNum.Text;
    String sNum = secondNum.Text;
    if (num == "")
    {
        MessageBox.Show("error type both num");
    }
    else
    {
        num1 = int.Parse(num);
        num2 = int.Parse(sNum);
    }
    int total = _operations[sender](num1,num2);
   
    MessageBox.Show(total.ToString());

    firstNum.Text = string.Empty;
    secondNum.Text = string.Empty;
}