User Defined Fields Best Design

hi all(happy raksha bandhan day)

we have one of Automation software for sales running for a
customer.He was cool for the first month of product, but later popped
with adding some extra fields.no problems i added in database , put a
seperate code in my application for that field.but later every 2 days
he was adding new fields.....so i thought to put in some inbuilt logic
user defined fields.second his user defined fields are like shoudl be
numeric,string , length validation.But do not know whats the best way
to acheive this.I mean should i make seperate table where i define
field name, data types , validation and then in my application code a
general logic for it in my application code.Any one has prooven design
for user defined fields,just thinking if i can even get a idea.....


When i die i die programming........