BCL (Base Class Library): Now, if you want to communicate with the database. It is not required to write each and every line like how the compilation works in depth. If you want to read or write any file stored in the local disk, you are not required to write the complete C level coding. There are some classes available like files. Just create object for these classes and use it. So, these are some reusable libraries available in BCL in .Net Framework itself. We can use them and proceed with our work. We are not required to waste our time which is already been done.