"e.Cancel" how causes that event canceled

As Olivier already stated for you, "First of all, MSDN states that OnClosing is obsolote and you should use OnFormClosing instead."

On top of that, the OnFormClosing is the event handler. Setting e.Cancel does not fire a new event, but rather sets a boolean property in the event arguments. When the event gets passed on to the base class using "base.OnClosing(e)", somewhere in the inheritance structure there would be code to read that property and then cancel the event processes if the value is true.

---deleted-- and recommend SAKryukov's solution.

And my updated answer here is an example how the e.cancel works with a custom example.

Here is a class with a custom event which has the CancelEventArgs type of argument.

When this event fires you can set its cancel property which signals the object about further action.

public class PersonWithEvent
{
    public event CancelEventHandler NameAcceptanceEvent;
    private string _name;
    public string ErrMessage{get;set;}
    public string Name
    {
        get { return _name; }
        set
        {
                CancelEventArgs cancelParams = new CancelEventArgs();
                NameAcceptanceEvent.DynamicInvoke(new object[] { this, cancelParams });
                if (cancelParams.Cancel != true)
                {
                    _name = value;
                    ErrMessage = "";
                }
                else
                {
                    ErrMessage = "This name not allowed";
                }
        }
    }

}

Now the acceptance of a name is not encapsulated in this object, but delegated to the user who can implement his own logic for that.

private string personName;
 private void button1_Click(object sender, EventArgs e)
{
    PersonWithEvent person = new PersonWithEvent();
    person.NameAcceptanceEvent+=new CancelEventHandler(person_NameAcceptanceEvent);
    personName = "Monster";
    person.Name = personName;
    MessageBox.Show(person.Name);
    MessageBox.Show(person.ErrMessage);
    personName = "Angel";
    person.Name = personName;
    MessageBox.Show(person.Name);
    MessageBox.Show(person.ErrMessage);
}
private void person_NameAcceptanceEvent(object sender, CancelEventArgs e)
{
    if (personName == "Monster")
    {
        e.Cancel = true;
    }
}

Good luck

Sure, OnFormClosing should be used instead.

The operation is pretty simple. No event is really "canceled", what canceled is the operation of closing form. Is the event argument's Cancel == true, form is not closed, so the event FormClosed in never fired. Most typical action used in the event handler is Form.Hide(). It is also good to check up event argument's CloseReason: normally, closing of the form should be prevented (again, replaced with Form.Hide) only for ClosingReason.UserClosing.

[EDIT]

Here is the code sample:

using System.Windows.Forms;

//...

protected override void OnClosing(FormClosingEventArgs e) {
   if (e.CloseReason == CloseReason.UserClosing) {
      this.Hide();
      e.Cancel = true;
   }
}

—SA