What's the difference between struct and class in .NET?

Question

What\'s the difference between struct and class in .NET?

Answer 1

Every variable or field of a primitive value type or structure type holds a unique instance of that type, including all its fields (public and private). By contrast, variables or fields of reference types may hold null, or may refer to an object, stored elsewhere, to which any number of other references may also exist. The fields of a struct will be stored in the same place as the variable or field of that structure type, which may be either on the stack or may be part of another heap object.

Creating a variable or field of a primitive value type will create it with a default value; creating a variable or field of a structure type will create a new instance, creating all fields therein in the default manner. Creating a new instance of a reference type will start by creating all fields therein in the default manner, and then running optional additional code depending upon the type.

Copying one variable or field of a primitive type to another will copy the value. Copying one variable or field of structure type to another will copy all the fields (public and private) of the former instance to the latter instance. Copying one variable or field of reference type to another will cause the latter to refer to the same instance as the former (if any).

It's important to note that in some languages like C++, the semantic behavior of a type is independent of how it is stored, but that isn't true of .NET. If a type implements mutable value semantics, copying one variable of that type to another copies the properties of the first to another instance, referred to by the second, and using a member of the second to mutate it will cause that second instance to be changed, but not the first. If a type implements mutable reference semantics, copying one variable to another and using a member of the second to mutate the object will affect the object referred to by the first variable; types with immutable semantics do not allow mutation, so it doesn't matter semantically whether copying creates a new instance or creates another reference to the first.

In .NET, it is possible for value types to implement any of the above semantics, provided that all of their fields can do likewise. A reference type, however, can only implement mutable reference semantics or immutable semantics; value types with fields of mutable reference types are limited to either implementing mutable reference semantics or weird hybrid semantics.