double dimensional dictionary in c#

hi all, i have visual studio c# winform

  my winform is connected to my phpmyadmin database. My database has a tbale which contains 4 columns(pincode,request number, service category and service type). 

 

how to buil a two-dimensional dictionary structure in c#?

 

In fact i need to build a collection of pincodes in which each pincode contains a collections of request numbers. And  each request number contains its personal records(service category and service type).

 

 The two-dimensional dictionary collection should look like:

pincode==>request number==>personal records--> service category and service type

 

Any help is welcome.