Rory Primrose

Learn from my mistakes, you don't have time to make them yourself

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Class vs Struct

Posted on November 24, 2008

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: