F# nullness at interop boundaries

F# nullness checking makes .NET reference types that can be null visible as T | null. The compiler reports most problems. The risk is the wrong fix: a fix that compiles and hides a real null. Convert each T | null to an F# type once, at the boundary. Then the rest of the code uses T or T option.

scripts/run.ps1 verifies the null results and diagnostics below. Verify lists the rules that it does not verify.

Turn on the checks

<PropertyGroup>
  <Nullable>enable</Nullable>
  <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
</PropertyGroup>

<Nullable>enable</Nullable> passes --checknulls+. Nullness diagnostics are warnings (FS3261, FS3262). TreatWarningsAsErrors makes them fail the build.

Members that return | null

These members return T | null. Using the result as T gives FS3261.

Handle null at the boundary

let tryHost (s: string) : string option =
    match Uri.TryCreate(s, UriKind.Absolute) with
    | true, NonNull u -> Some u.Host
    | _ -> None

let rec rootMessage (ex: exn) : string =
    match ex.InnerException with
    | null -> ex.Message
    | inner -> rootMessage inner

Rules that the build does not tell you in advance:

• isNull x and String.IsNullOrEmpty x do not narrow string | null to string. The else branch still gives FS3261. Use match.
• Do not mix | null -> with | NonNull v ->. After | null, the value is already non-null. NonNull then gives FS3262 and the match gives FS0025. Use | v ->, or use Null and NonNull together.
• Annotate public return types. Without an annotation, the compiler infers T | null and moves the null to the caller.
• A string binding cannot hold null. let mutable x: string = null gives FS3261. Declare string | null, or better, use string option.
• Passing ex.InnerException to a parameter of type exn gives FS3261 with the text The types 'exn' and 'exn | null' do not have compatible nullability. Match on the value before the call, or declare the parameter as exn | null and match inside.

Null leaks that compile clean

The compiler does not report these. Each one compiles with nullness and TreatWarningsAsErrors on.

Use Option.ofObj instead of Some on a .NET result. Use string x only when an empty string is the correct value for null.

When suppression is acceptable

Use nonNull or Unchecked.nonNull only when the API cannot return null in this call, and the type does not say so. Write the reason in a comment on the same line or the line above. Example: a value that a Try method returned with true, when you cannot use NonNull in the pattern. Never suppress FS3261 for a file or a project to get a build to pass. NonNullQuick and nullArgCheck throw on null like nonNull. They need the same written reason.

// GetEntryAssembly is null only in unmanaged hosts. This exe always has one.
let entry = Unchecked.nonNull (Assembly.GetEntryAssembly())

New BCL overloads (FS0041)

.NET 9 added int64 overloads to TimeSpan.FromSeconds, FromMinutes, FromMilliseconds, and FromMicroseconds. An int argument used to convert to float. Now it matches no overload exactly, so the compiler reports FS0041. FromHours and FromDays have int overloads and still compile.

Convert the argument at the call. Do not change the type of a public parameter or a record field to fix FS0041. For other FS0041 errors, read the candidate list in the message and annotate the argument with one candidate type.

Verify

1. Build with Nullable and TreatWarningsAsErrors on. Do not add NoWarn for 3261.
2. Search the changed code for nonNull, #nowarn, Some, and string used as a function (string x, x |> string). Check each use on a .NET result against the tables above.
3. Run the code with null inputs at each boundary: a JSON null, an unset environment variable, an exception with no inner exception, and an empty reader.

To check the facts in this skill, run the bundled proof:

pwsh ./scripts/run.ps1

The runner builds scripts/Clean with nullness and warnings as errors, runs it, and compares 19 results. It then builds each file in scripts/breaks and checks that it fails with only the diagnostic codes in its // expect: lines and with the message text in its // expect-text: lines. The clean lines it prints are the results in the tables above.

These rules come from the F# compiler documentation and are not bundled proofs: <Nullable>enable</Nullable> passes --checknulls+, #nowarn "3261" and NoWarn hide later nullness warnings, and an unannotated function infers T | null from its body.