VS 2010 how to read big txt file in chunk

i tried to read using io.file.readallines and streamreader.readallline but too big

so i tried linebyline using streamreader.readline into while EOS or Do loop until EOF but too slow to process entire file

so i wont to try read small chunk using .readblock method but it uses buffer and chararray so should be last line will be truncated.

have i add somethings to end of code to check if last char is cf/cr and read 1 char at time till cr/cf and join with read chunk or exist a different way?

cunk approximatively of 50-100 Mb