Your debug output seems perfectly fine.
GET : https://api.twitter.com/1.1/search/tweets.json?q=tweetinvi&result_type=Mixed
Header OAuth oauth_consumer_key="xxxxx",oauth_nonce="4364041",oauth_signature_method="HMAC-SHA1",oauth_timestamp="1410968448",oauth_token="xxxxx",oauth_version="1.0",oauth_signature="ULpyAXJd7H6vPsxIYlkx5rOhp70%3D"
System.Collections.Generic.List`1[Tweetinvi.Core.Interfaces.ITweet]
The thread 0x1e08 has exited with code 259 (0x103)
Your
GET : https://api.twitter.com/1.1/search/tweets.json?q=tweetinvi&result_type=Mixed
Header OAuth oauth_consumer_key="xxxxx",oauth_nonce="4364041",oauth_signature_method="HMAC-SHA1",oauth_timestamp="1410968448",oauth_token="xxxxx",oauth_version="1.0",oauth_signature="ULpyAXJd7H6vPsxIYlkx5rOhp70%3D"
System.Collections.Generic.List`1[Tweetinvi.Core.Interfaces.ITweet]
The thread 0x1e08 has exited with code 259 (0x103)
Your
Console.WriteLine(tweets); is also working as expected here. Can you do a MessageBox.Show(tweets.Count())? I feel you're missing something very obvious here.