Develop and add-in for Revit 2012

I need to develop an add-in for Autodesk Revit 2012. as you may know Revit is an object oriented modelling software. all of the components used in a Revit model has some attributes (such as height, width, length, start date, finish date, etc.). what i want to do is to develop an add in which makes query (for example select all the components with the start date of X) and automatically generate a bounding box ( a box which contains the whole object) around it. I also have a sample C# file which gives the size and coordinates of the bounding box by selecting a component. it can be used as a prototype. Thanks