First or default null (ARCH903)

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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

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

Null-check the result before using it.