First or default null (ARCH903)

Key dotnet.linq.first-or-default-null · Severity warning · Category linq

These methods return null when there is no match; direct access throws NullReferenceException.

Incorrect

var user = users.FirstOrDefault(u => u.Id == id);
return user.Name; // NRE if not found

Correct

var user = users.FirstOrDefault(u => u.Id == id);
if (user is null)
    return null;
return user.Name;

Fix

Null-check the result before using it.