Question

It is interop between C# and F#

In F#,

type test =
{
    value: int
}

type Wrapper (value: test) =
    member val Value = value with get, set

let trythis = new Wrapper(null)  // error as expected

However, in C#

 var trythis = new Wrapper(null);  //this runs fine

Answer 1

The non-nullable constraint on types is an F# specific feature and so it does not have any representation in .NET (and therefore C# does not respect it).

In fact, you can workaround this even in F# using an unsafe Unchecked.defaultof<_> value:

let trythis = new Wrapper(Unchecked.defaultof<_>) 

This is very useful if you want to check for null in an object that is exposed to C#:

type Wrapper (value: test) =
    if value = Unchecked.defaultof<_> then
      invalidArg "value" "Value should not be null."
    member val Value = value with get, set