Question about Intefraces ?

hi
i have doubt intefraces , place in real world applications.
i wrote 1 web app (3 layers):
bo : business objects , collections
dal : access sql server , returning either objects or lists.
dll: access dal , doing business rules.
pl: presentation layer access bll , format outputs

now did not use interfaces in whole application
but want know if design right or should add interfaces , if case , how

thanks

i think gernal useful use interface erverywhere not want have tied coupling. example

 

idatabaseaccessclass 1 (don't know how solved data-access, assuming something).

 

idatabaseaccessclass _db = new oracledb();

idatabaseaccessclass _db = new sqlserverdb();

 

where oracledb():idatabaseaccessclass or sqlserverdb()atabaseaccessclass. basicaly seperate layers exchangeable. can go crazy on that... pattern tell better code against interfaces instead of implementations....

 

 

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