Public Class Textbox

     Inherits MicroFour.StrataFrame.UI.Web.TextBox

     .

     .

     .

End Class

Will it cause any errors when i try to add, update or delete record?

2. What will happen if I add code in Page.PreLoad and Page.PreRender event? If error may occur, what I should do to prevent the error?

3. Page.Load and Page.PreRenderComplete are the safe places for me to add some codes?

That is correct.

You could add code there.  You should handle errors just like any other errors that appear within an ASP.NET page, including the custom error pages if necessary.

Correct, those are the safest places to add code.