λ1008: Prefer ToNullable over Match

A Match that returns null for None and the value itself for Some should be expressed as ToNullable.

Cause

Match is called on an Option<T> where the none branch is null (or a function returning null) and the some branch is the identity function, resulting in a T?.

Reason for rule

Converting an Option<T> to a nullable value is a common interop step at the boundary to code that does not use Option. ToNullable names this conversion. The Match form requires a cast such as (string?)null to make the type inference work, which is noise that is easy to get wrong.

How to fix violations

Replace the Match call with ToNullable(). A code fix is available.

Examples

Disallowed

static string? Example(Option<string> option)
    => option.Match(none: (string?)null, some: x => x);

static int? Example(Option<int> option)
    => option.Match(none: () => (int?)null, some: Identity);

Allowed

static string? Example(Option<string> option)
    => option.ToNullable();

static int? Example(Option<int> option)
    => option.ToNullable();

// The some branch transforms the value, so this is a legitimate Match.
static string? Example(Option<string> option)
    => option.Match(none: (string?)null, some: x => x + "!");