how to use validation in window application

It depends on how much validation you need to do and when you need to do it.
You can validate when the user presses a button - to enter the information for example.
Or, you can validate when the user moves away from the TextBox.

To do the former, just do your checks in the Button.Click event.
To do the later, handle the TextBox.Leave event.

Personally, I would go for the Button.Click route: It allows the user to fill things in, in whatever order he chooses.


"can u explain it more using code"

private void myEnterButton_Click(object sender, EventArgs e)
    {
    if (!string.IsNullOrEmpty(myTextBox.Text) && myTextBox.Text != "Bad customer")
        {
        ... All ok.
        }
    else
        {
        ... bad Validation
        }
    }