I would like to make a list where you can book one or more resources and it should not be possible to book the same resource twice in the same timespan.
How can this be done?
Any suggestions apart from adding third party solutions...? Would like to make my own...
You might want to look at the template that was designed in the Fantastic 40 for booking meeting rooms. I would not use this template in production, but you could install it and play with it in a dev environment and utilize elements of it for your project. http://technet.microsoft.com/en-us/windowsserver/sharepoint/bb407286.aspx
You need to create EventReceivers for ItemAdding and ItemUpdating.
In these events, run query to check the given Start and End DateTime are not overlapping to any item already in the list. This can be achieved by CAML query using DateRangesOverlap and ExpandRecurrence properties.
Please check the following link for a code example: http://sridharu.blogspot.com/2008/01/how-to-read-recurring-events-using-caml.html
