The ErrorProvider control is quite useful when it comes to user input validation. The next code segment assumes an ErrorProvider control named errorProvider1 has been placed on a form along with a Textbox named usernameTextBox and a ComboBox named comboBox1.

private void usernameTextbox_Leave(object sender, EventArgs e)        {            if (String.IsNullOrEmpty(usernameTextbox.Text))            {                errorProvider1.SetError(usernameTextbox, "enter a value"); //Check For Empty Strings            }            else            {                if (usernameTextbox.Text.All(char.IsDigit))                {                    errorProvider1.SetError(usernameTextbox, "enter a string value!"); //Check for Numbers                }                else                {                    if (usernameTextbox.Text.Length  8)                    {                       errorProvider1.SetError(usernameTextbox, "can only be 8 characters long"); //Check Length                    }                }            }        }        private void comboBox1_Leave(object sender, EventArgs e)        {        }        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)        {            if (int.Parse(comboBox1.SelectedItem.ToString())              {                errorProvider1.SetError(usernameTextbox, "can only be 18 and above"); //Determine Selection            }        }