Data chacing...

[ayuba asia]

i am developing a financial system using c sharp i have 1 master class that open all the master table required by all the form and store it in 1 static dataset that can be access from anywhere in the sytem.. but i want to change to have make it something like caching system where if the data required is not in the master class then i will get it from server.. the tricky part is that i want clear unwanted resource so i want to have to checking like if the dataset/datatable is not used in a certain timeframe let say 15 mins the i want to close the file any idea on how implement this idea.. sample MasterFile [Customer] when i do invoice transaction i will open the customer if the customer is not open at the master class and after i close the invoice screen the customer data still remain there for a cretain perriod of time.. so any form after that need to access the customer file dont have to fetch data from sql server again.. if not use in 30 mins it will closed... thanks