Practical .NET

Retain State by Serializing to Disk

If you're looking for an easy way to save the user's current state, the simplest solution is use the Serialize and Deserialize method.

The reality is that users often don't complete a task without being interrupted. As a decent human being, you should allow your users to save their work so that they can continue later. You could use a database to save the user's state but often you'll want to save unstructured data: a bizarre combination of current internal variables and lists of data displayed on the screen that represent "where the user is right now."

A simple solution is to create a "state object" with whatever properties you need to hold stat data, shove the current data into those properties, and then save that object to a file on the user's disk (it is, after all, the user's data and I'd rather take advantage of the user's storage than mine). This is the code to write out your state object:

Dim so As New StateObject
...set properties on state object...
Dim str As New FileStream("c:\status\status.st", FileMode.Create, FileAccess.Write)
Dim bf = New BinaryFormatter()
bf.Serialize(str, so)

And here's the code to read it back in:

Dim str As New FileStream("c:\status\status.st",FileMode.Open, FileAccess.Read)
Dim bf = new BinaryFormatter()
Dim so As StateObject
so = DirectCast(bf.Deserialize(str), StateObject)

This code works equally well if you're writing out a List of objects as it does with a single object: Just pass the whole List to the Serialize method.

Two things to remember: You'll need add the Serializable attribute to your state object class; If the user wants to resume from another computer, they're going to be disappointed -- this only works if the user restarts on the same computer they saved their state object on.

About the Author

Peter Vogel is a system architect and principal in PH&V Information Services. PH&V provides full-stack consulting from UX design through object modeling to database design. Peter tweets about his VSM columns with the hashtag #vogelarticles. His blog posts on user experience design can be found at http://blog.learningtree.com/tag/ui/.

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