Learn unit testing, as it's pretty much a must for most jobs nowadays. Unit testing is essentially going through and isolating your functions from external dependencies and testing that they are doing what you want them to do. The art of unit testing with examples in C# by roy osherove is a nice read if you want to learn about it
www.amazon.com/Art-Unit-Testing-examples/dp/1617290890/ref=sr_1_1?ie=UTF8&qid=1523240970&sr=8-1&keywords=art+of+unit+testing
www.amazon.com/Art-Unit-Testing-examples/dp/1617290890/ref=sr_1_1?ie=UTF8&qid=1523240970&sr=8-1&keywords=art+of+unit+testing