First diagnostic

After adding the analyzer package, build a project containing a linear lookup inside an iteration:

static List<int> FindMatches(List<int> items, List<int> candidates)
{
    var matches = new List<int>();
    foreach (var item in items)
    {
        if (candidates.Contains(item))
            matches.Add(item);
    }

    return matches;
}

List<T>.Contains is linear in the candidate count. Nested under a loop over items, the supported estimate composes both input variables and BIG1001 identifies the costly lookup.

Make the policy visible

Diagnostic severity uses standard analyzer configuration:

[*.cs]
dotnet_diagnostic.BIG1001.severity = warning

The diagnostic points to a pattern, not an automatic refactoring mandate. A HashSet<T> can offer expected constant lookup under documented hashing assumptions, but building that set also has cost and semantic implications. Interpret the evidence in the context of input sizes and call frequency.

See the diagnostic catalog and configuration reference.