[twitter-dev] Can I DM via the API with username and password?

[mchid]

I need my app to be able to send a direct message to a registered
users - so I know their username and the password they use to log in.
Do I need them to manually authorise this first (using oAuth) or can I
avoid this? For reference (and for my sins) the app is developed in
c#.net :) Thanks