Tip: Performing Basic Validation with the Error Provider Control in C#

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
            }
        }