struct - what is it for?

Question

I know something about struct type. But I can\'t understand: what is it for? when have I use it? Classes, simple value-types and enums - that\'s all that I need

Answer 1

structs are for objects that represent something whose identity is defined by the values stored in it's properties rather than by an Id or key. These are called "Value types" as opposed tyo objects called "Entity Types", whose identity persists over time and is not dependant on the values of the properties of the object.

As an example, a Person class (an Entity) has an identity that persists from one session to another, even from year to year, in spite of how the Person's address, phone number, employer, etc might change from one instance to another. If you inadvertently have two instances of a Person class in memory at the same time, which represent the same individual/entity, then it is important that they have the same values for their properties.

A CalendarMonth object otoh, (a value type) only has identity defined by the value which specifies which calendar month it is... no matter how many instances of "March 2009" you might be using, they are all interchangeable and equivilent. Another example might be an object representing a FiscalYear designation in a tax program. A great example is an address object. (Not talking here about the asociation of an address with a person or a business, or any other entity, but just the address itself). Changing just about any property of an address makes it a different address. Once you create an address object in memory, it is equivilent and interchangeable with every other address object that has the same properties. This is why these value types should generally be immutable. When you need one, create a new one with the property value(s) you need, and throw it away when you're done using it.