C# is a statically typed language and variable types are checked at compile time, but it does have some dynamic features. You can use the dynamic type and objects such as ExpandObject to get some of the features of a dynamically typed language. The var type is not in fact dynamic, and is simply inferred at compile time.
Dynamic typing has a performance and safety penalty, so it is best avoided if you can find another way to solve your problem. For example, the ViewBag in ASP.NET MVC is dynamic, so it is best not to use ViewBag, and use a well-defined view model instead. This has many other benefits apart from performance, such as safety and convenience.
