потърси в МСДН за dynamic properties.
ето и кратък цитат:
Dynamic properties allow you to configure your application so that some or all of its property values are stored in an external configuration file rather than in the application's compiled code. By providing administrators with the means to update property values that may need to change over time, this can reduce the total cost of maintaining an application after the application has been deployed.

общо взето в UI designer -> избираш си control -> properties ->кликаш върху dynamic properties там има ... и си избираш кое propertyда ти е dynamic. после отиваш в app.config file и там си има за него

това е за WinForms естествено