Types, classes and interfaces

C# is statically typed: every variable has a type the compiler checks before the program runs. Value types such as int, decimal, DateTime and structs are copied when you assign them. Reference types such as classes are shared: two variables can point at the same object.

Classes hold data and behaviour, and interfaces describe a contract, such as IEmailSender, that several classes can implement. Interfaces are everywhere in .NET because dependency injection hands you an implementation of an interface. Learn properties, constructors, access modifiers and static members, plus decimal for money and DateTimeOffset or UTC for times. Generics such as List<Order> keep collections type-safe.