How to keep user input after closing ?

It depends on how much data and such. I am not sure what that data is; you are not clear about that.

A common way to do what you are asking is to store the data as a "Setting". In your project in the Solution Explorer there should be a "Properties" node; when you expand that, you can double-click on "Settings.settings". A window will be opened showing a grid with columns for names, types, scopes and values. You can create a name for a setting and then use the defaults. The default type is string and the default scope is User. Then at the beginning of your program (such as in the form's Load event) get the value from "Properties.Settings.Default.SettingName" where SettingName is the name you specified. Then at the end (such as the form's Closed event) save the settings. Then .Net will take care of writing the values to a XML file and getting them form it.

If you have only one form in your application, then you'll need to save the content of textboxes, comboboxes etc to a disk file and then reload the data from the file when the application is run again.

Unfortunately, forms are too complicated to be serialized so you'll have to save the data 'manually'. A simple text file or an XML file should suffice though you will, of course, need to devise a suitable format so you can easily recover the data.

If you have multiple files in your application and you only want to close one (which may be reopened subsequently in the same session) rather than the whole application then, as an alternative to a disk file, you could save the data within memory to a global variable or variables. I'd be inclined to design a custom class which you can create an instance of containing the data and then assign that to the global variable from the form's FormClosed eventhandler. Of course, when the application does eventually close, you will still need to save the data to disk.