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 } }