Questions about creating application related to Twitter account.

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C#

Hello,

I have a task to create an application that can do something of followed:

1) Search Twitter for specific hashtag, and drag that post to some local database
2) Make a local application that logs in to your Twitter account and automatically send messages from some local file or something else
3) Or application that will follow tweets, and when the tweet with specific content is posted, application automatically write answer

First of all, I want to know does Twitter allows this kind of actions. And if it does, any code or advice will be very useful.

Regards