Key Events

You can look into the key events... I will simulate you with an example If you use the KeyDown event, when you press 'a' in the texbox, it will display 'a' in the textbox and then say just display a message box. If you use the KeyPress event, when you press 'a' in the textbox, it will display a message box first before display 'a' in the textbox There is also KeyUp. Again it depends on what you need. Cheers.

Say for example you want to prevent the user from entering any value other than numeric values and back space in a text box, then catch KeyPress event for that TextBox and write following lines in the event code block: private void checkNumeric_KeyPressed(object sender, System.Windows.Forms.KeyPressEventArgs e) { string theStr = "-0123456789\b"; //only numeric values and back slash allowed if (theStr.IndexOf(e.KeyChar.ToString(),0) <= 0) { e.Handled=true; } } You can change the logic in code block to suit your requirement. You can also use RegEx to achieve the same. Cheers, Yasser