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Probably needs casting...


if(dr.Read())
username = (String)dr["username"];

Use square brackets...


dr["username"]

Returning to my earlier point about disposing your database objects, it is bad practice to leave database connections lingering, which is what you are...

OleDbDataReader dr = cmd.ExecuteReader();

And


if (dr.Read())

The clue was in the exception.

Also don't forget to Dispose cmd, db and dr, or better yet, use 'using' statements.