Functions in VB.NET fall in two categories: Inbuilt functions and user defined function. The inbuilt functions are always in use thus common.
There are a lot of in build common functions too many to go into on a single post e.g. there are String functions like Split, lCase, Ucase, there are Maths functions like Sqrt, Log, Sin, conversion functions like CStr, CInt, CBool and many more.
Best thing you can do is get some good VB.net books or go to a site like this:
Or if youre stuck google it e.g. how do I convert to a string in VB.net? (answer Cstr)
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