Fire event on textbox lose focus

There is a Control.Leave in C#, which I think is perfect for your purpose.

you can go to events of the textbox in visual studio, and find the Leave event.

The code generated will be like :

private void txtbox_Leave(object sender, EventArgs e)
{
            //Check for available operation Code
} 

Please explore the LostFocus event in a textbox. Hope it helps