How to prevent or block closing a WinForms window?

How can I prevent window closing by showing a MessageBox? (Technology:WinForms with C#)

When the close event occurs I want the following code to be run:

private void addFile_FormClosing( object sender, FormClosingEventArgs e ) {
    var closeMsg = MessageBox.Show( "Do you really want to close?", "Warning", MessageBoxButtons.YesNo, MessageBoxIcon.Question );

    if (closeMsg == DialogResult.Yes) {
        //close addFile form
    } else {
        //ignore closing event
    }
}

private void Form1_FormClosing(object sender, FormClosingEventArgs e) {
    var window = MessageBox.Show(
        "Close the window?", 
        "Are you sure?", 
        MessageBoxButtons.YesNo);

    e.Cancel = (window == DialogResult.No);
}

Catch FormClosing event and set e.Cancel = true

private void AdminFrame_FormClosing(object sender, FormClosingEventArgs e)
{
    var res = MessageBox.Show(this, "You really want to quit?", "Exit",
            MessageBoxButtons.YesNo, MessageBoxIcon.Warning, MessageBoxDefaultButton.Button2);
    if (res != DialogResult.Yes)
    {
      e.Cancel = true;
      return;
    }
}

A special twist might be to always prevent just the user closing the form:

private void Frm_FormClosing(object sender, FormClosingEventArgs e)
{
    e.Cancel = (e.CloseReason == CloseReason.UserClosing); 
    // disable user closing the form, but no one else
}

Within your OnFormClosing event you can show the dialog and if answer is false (to not show) then set the Cancel property of the EventArgs to true.

For prevent or block the form closing in particular situation you can use this strategy:

private void MyForm_FormClosing(object sender, FormClosingEventArgs e)
{

    if (FLAG_CONDITION == true)
    {
        MessageBox.Show("To exit save the change!!");
        e.Cancel = true;
    }

}

Straight from MSDN:

private void Form1_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
   // Determine if text has changed in the textbox by comparing to original text. 
   if (textBox1.Text != strMyOriginalText)
   {
      // Display a MsgBox asking the user to save changes or abort. 
      if(MessageBox.Show("Do you want to save changes to your text?", "My Application",
         MessageBoxButtons.YesNo) ==  DialogResult.Yes)
      {
         // Cancel the Closing event from closing the form.
         e.Cancel = true;
         // Call method to save file...
      }
   }
}

In your case you don't need to do anything to explicitly close the form. Unless you cancel it it will close automatically, so your code would be:

private void addFile_FormClosing( object sender, FormClosingEventArgs e ) {
    var closeMsg = MessageBox.Show( "Do you really want to close?", "Warning", MessageBoxButtons.YesNo, MessageBoxIcon.Question );

    if (closeMsg == DialogResult.Yes) {
        // do nothing
    } else {
        e.Cancel = true;
    }
}

You can run any code you want when closing the form then, hide the form instead of closing it to prevent it from being disposed

yourFormName.FormClosing += (s, e) =>
{
   // any code you want
   yourFormName.Hide(); // this hides the form
   e.Cancel = true;  // this cancels the close event, you can put any boolean exprission 
};