What event of the textbox did you use?

This is pretty much doable and this functionality is called AutoComplete is Web Platform and Incremental Search in WinForms Arena.

The only issue I found with this approach is that, you have to load the entire xml file into your dataset. wich could be a problem when your database gets to a larger size. I would still use SQL...

Yes, it is possible to save and load Datasets to and from xml.
And after loading you can manipulate the dataset as you would after filling it with data from any database.

Here is how you can...

String values are possible using attributes not directly.

You can even store string values to the enum variables.

Read (http://www.coderewind.com/?categ&view_article=34) this article (http://www.coderewind.com/?categ&view_article=34) and you can even download a working sample which uses Connection to Access DB...