Film noir
owes a lot to the hard-boiled detective fiction of the 30s and to the
cycle of films this fiction inspired.  Like the gangster film,
this fiction mined a Depression-era fascination with the
underside of American life, examining it from a tough-minded point of
view that reflected the disillusionment of hard times.  But it
was, at bottom, a romantic genre — the detective, however, cynical,
had a code of honor that kept him untainted by the muck he had to slog
through.  He may not have trusted the police, or other
representatives of official society, but he was a law unto himself,
dispensing rough justice in spite of the failures of the established
order.  (Clearly there's a connection here, too, with the Western,
in which a lone-hand hero often must assert the values of decency and
order in the absence of official institutions dedicated to the purpose.)

This is a far cry from the existential estrangement of the classic noir
protagonist whose code of honor has broken down somewhere along the
line — whose chief problem is not doing the right thing but having no
clear sense of what the right thing is, or why it matters in a world
gone haywire.

The key to traditional hard-boiled detective fiction is a mystery to be solved,
which becomes emblematic of a moral imbalance that needs to be
righted.  Solving the mystery and righting the balance restore
hope.  In a true noir there's a sense, or at least a nagging suspicion, that hope is a fool's game.

The following detective thrillers are often identified as films noirs:

Murder, My Sweet

The Lady In the Lake

I Wake Up Screaming


The Big Sleep

Behind Closed Doors

The Mask Of the Dragon


They all have noirish elements, and often look like films noirs, but they belong to an older tradition, one in which atomic-age angst and despair ultimately have no place.

[The noir credentials of the films listed above are as follows . . .
Murder, My Sweet
and The Lady In the Lake are included in the Warner noir DVD series . . . 
I Wake Up Screaming
and Vicki are included in the Fox noir DVD series . . . The Big Sleep and Laura are included on almost all lists of films noirs . . . Behind Closed Doors is included in Kino's film noir DVD box set . . . and The Mask Of the Dragon is included in the VCI Forgotten Noir DVD series.]