How to determine the size of a System.Type value?

Question

How to determine the size of a Type value?

let typ = if true then (123).GetType() else "asdf".GetType();;
let sz = sizeof<typ>

  let sz = sizeof<typ>;;
  ----------------^^^
error FS0039

One solution is to use Marshal.SizeOf(obj), but may be another solution exists?

Answer 1

I think Marshal.SizeOf is what you are looking for here. Note that there are two overloads of the method.

• Marshal.SizeOf(t) (MSDN) takes System.Type as an argument and returns the size of the type that is represented by the System.Type value (I believe this is what you want.)

• Marshal.SizeOf(structure) (MSDN) takes any object and returns the size of the structure based on the runtime type.

The sizeof<'T> construct in F# works only when the type is statically know, but then it generates nice single sizeof IL instruction (as you can see from the source code).