Question

CompilationRepresentationFlags.UseNullAsTrueValue can be used to

Permit the use of null as a representation for nullary discriminators in a discriminated union

Option.None is the most prominent example of this.

Why is this useful? How is a null check better than the traditional mechanism for checking union cases (the generated Tag property)?

It leads to perhaps unexpected behavior:

Some(1).ToString() //"Some(1)"
None.ToString()    //NullReferenceException

EDIT

I tested Jack’s assertion that comparing to null instead of a static readonly field is faster.

[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
type T<'T> =
  | Z
  | X of 'T

let t = Z

Using ILSpy, I can see t compiles to null (as expected):

public static Test.T<a> t<a>()
{
    return null;
}

The test:

let mutable i = 0
for _ in 1 .. 10000000 do
  match t with
  | Z -> i <- i + 1
  | _ -> ()

The results:

Real: 00:00:00.036, CPU: 00:00:00.046, GC gen0: 0, gen1: 0, gen2: 0

If the CompilationRepresentation attribute is removed, t becomes a static readonly field:

public static Test.T<a> t<a>()
{
    return Test.T<a>.Z;
}

public static Test.T<T> Z
{
    [CompilationMapping(SourceConstructFlags.UnionCase, 0)]
    get
    {
        return Test.T<T>._unique_Z;
    }
}

internal static readonly Test.T<T> _unique_Z = new Test.T<T>._Z();

And the results are the same:

Real: 00:00:00.036, CPU: 00:00:00.031, GC gen0: 0, gen1: 0, gen2: 0

The pattern match is compiled as t == null in the former case and t is Z in the latter.

Answer 1

Jack’s answer seems good, but to expand a little bit, at the IL level the CLR provides a specific opcode for loading null values (ldnull) and efficient means of testing for them (ldnull followed by beq/bne.un/ceq/cgt.un). When JITted, these should be more efficient than dereferencing a Tag property and branching accordingly. While the per-call savings are probably small, option types are used frequently enough that the cumulative savings may be significant.

Of course, as you note there is a tradeoff: methods inherited from obj may throw null reference exceptions. This is one good reason to use string x/hash x/x=y instead of x.ToString()/x.GetHashCode()/x.Equals(y) when dealing with F# values. Sadly, there is no (possible) equivalent of x.GetType() for values represented by null.