in c # i need to write a program to measure bandwidth usage per day, week, month etc

How do I go about d开发者_运维知识库o this. What API (microsoft and non-microsoft are available) to achieve this?

Assuming you are talking IIS you can use log parser to process the logs and then store the restuls in some kind of DB. It has an API that can be accessed via c#, you just need to reference it in visual studio. There are examples in the documentation.