Walkthrough: Retrieve Selected Elements

[Autodesk.Revit.Attributes.Transaction(TransactionMode.ReadOnly)]    
public class Document_Selection : IExternalCommand
{
	public Autodesk.Revit.UI.Result Execute(ExternalCommandData commandData,
		ref string message, ElementSet elements)
	{
		try
		{
			// Select some elements in Revit before invoking this command
			
			// Get the handle of current document.
			UIDocument uidoc = commandData.Application.ActiveUIDocument;
			// Get the element selection of current document.
			Selection selection = uidoc.Selection;
			ElementSet collection = selection.Elements;
 
			if (0 == collection.Size)
			{
				// If no elements selected.
				TaskDialog.Show("Revit","You haven't selected any elements.");
			}
			else
			{
				String info = "Ids of selected elements in the document are: ";
				foreach (Element elem in collection)
				{
					info += "\n\t" + elem.Id.IntegerValue;
				}
				
				TaskDialog.Show("Revit",info);
		}
		}
		catch (Exception e)
		{
			message = e.Message;
			return Autodesk.Revit.UI.Result.Failed;
		}
		
		return Autodesk.Revit.UI.Result.Succeeded;
	}     
}