I have a lot of forms need to switch, each form has a textbox control, but as long as I run the program, writing in the textbox, switching window cut back, there is no content in the textbox, I think it is the form content to preserve,

I know the reason may be that each button clicks are new to a new form, so there is no content,
But I don't know how to change...
Probably know method has, before switching to save the content of the form, when the new assignment, but I don't know how to save, save?
Another method is to make a judgment before the new, to see if this entity exists, so is not new, but I still can't,,
Just learning c # for a few days, did the project was, for bosses to help solve, thank!