Types (C# Reference)
The C# typing system contains the following categories:
Variables that are value types store data, and those that are reference types store references to the actual data. Reference types are also referred to as objects. Pointer types can be used only in unsafe mode.
It is possible to convert a value type to a reference type, and back again to a value type, by using boxing and unboxing. With the exception of a boxed value type, you cannot convert a reference type to a value type.
This section also introduces void.
Value types are also nullable, which means they can store an additional non-value state. For more information, see Nullable Types.
See Also
Concepts
Reference
Casting and Type Conversions (C# Programming Guide)