TRLS018 — Unsafe Result<T> deconstruction

Category: Usage — Result safety Severity: Warning Enabled by default: Yes

What it flags

Result<T> deconstruction that reads the value slot without gating on the success/error component. The value slot is only meaningful when the Result is successful; reading it on a failed Result yields default(T) silently.

// ❌ TRLS018
var (value, _) = GetOrderAsync(id).Result;   // value is default if GetOrderAsync failed
Console.WriteLine(value.Number);

// ✅ No warning
var result = await GetOrderAsync(id);
if (result.IsSuccess)
{
    var (value, _) = result;
    Console.WriteLine(value.Number);
}

// ✅ Preferred — use Match / TryGetValue / Unwrap
result.Match(
    onSuccess: order => Console.WriteLine(order.Number),
    onFailure: error => Console.WriteLine(error.Code));

Why

Result<T> is a discriminated union. The deconstruction pattern exposes both slots as tuple elements for ergonomics, but consuming the value slot without a success check defeats the whole point of the Result abstraction — the failure is silently swallowed into default(T) and subsequent code operates on a value that may never have existed.

How to fix

Use one of:

• if (result.IsSuccess) gate before deconstructing.
• result.TryGetValue(out var value) — returns false on failure.
• result.Match(onSuccess, onFailure) — exhaustive branching.
• result.Unwrap() — throws on failure (test helpers only).

Suppressing

At a sanctioned site (e.g. a test that has already asserted success):

[SuppressMessage("Trellis", TrellisDiagnosticIds.UnsafeResultDeconstruction,
    Justification = "Success already asserted above.")]

See also

• TRLS003 — Unsafe Maybe.Value access
• TRLS013 — Unsafe Maybe.Value in LINQ