Handling Null Values in F#

Question

I need to interop with some C# code with F#. Null is a possible value that it is given so I need to check if the value was null. The docs suggest using pattern matching as s

Answer 1

If you have a type that has been declared in C# or a .NET library in general (not in F#) then null is a proper value of that type and you can easily compare the value against null as posted by kvb. For example, assume that C# caller gives you an instance of Random:

let foo (arg:System.Random) =
  if arg <> null then 
    // do something

Things become more tricky if the C# caller gives you a type that was declared in F#. Types declared in F# do not have null as a value and F# compiler will not allow you to assign them null or to check them against null. The problem is that C# doesn't do this check and a C# caller could still give you null. For example:

type MyType(n:int) =
  member x.Number = n

In that case, you need either boxing or Unchecked.defaultOf<_>:

let foo (m:MyType) =
  if (box m) <> null then
    // do something

let foo (m:MyType) =
  if m <> Unchecked.defaultOf<_> then
    // do something