找一下错误
public static void login(string table,string uid,string pw,string email)
{
SqlConnection con=DB.createcon();
con.Open();
SqlCommand cmd=new SqlCommand( "insert into "+table+ "(uid,pw,email)values( ' "+uid+ " ', ' "+pw+ " ', ' "+email+ " ') ");
cmd.ExecuteNonQuery();
con.Close();
}

ExecuteNonQuery: Connection 属性尚未初始化。
cmd.ExecuteNonQuery();

[解决办法]
SqlCommand cmd=new SqlCommand( "insert into "+table+ "(uid,pw,email)values( ' "+uid+ " ', ' "+pw+ " ', ' "+email+ " ') ",con);
cmd.ExecuteNonQuery();