There is a lot of information around that discusses the differences between classes and structs. Unfortunately there isn’t a lot of information available about when to use one over the other.

MSDN has a good resource which provides guidance on how to choose between classes and structs. It starts by describing the differences between the two and then provides the following advice.

Consider defining a structure instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects. Do not define a structure unless the type has all of the following characteristics:

• It logically represents a single value, similar to primitive types (integer, double, and so on).
• It has an instance size smaller than 16 bytes.
• It is immutable.
• It will not have to be boxed frequently.

If one or more of these conditions are not met, create a reference type instead of a structure. Failure to adhere to this guideline can negatively impact performance.

The one that got me was having an instance size of 16 bytes or smaller. Several of the classes that I wanted to convert into structs defined string properties. Initially, I thought that a string would almost always be over 16 bytes making it inappropriate for a struct.

It later occurred to me that strings are reference types not value types. Any string variable is simply a pointer to the memory location that holds the data for that reference type. This means that the size of a string property in a struct is the size of IntPtr.

Structs are back on the menu.

Some useful links are:

• MSDN - Type Design Guidelines
• MSDN - Choosing Between Classes And Structures
• MSDN - Structure Design
• DotNetPearls - Struct Examples And Tricks