I "field" sharpen my chain saw with a air powered die grinder with a proper size stone and then dress the depth tooth with an angle grinder equipped with a 320 sanding disc and firm rubber backer.
Don't laugh! I find I do as good a job or better than a professional jig job, it takes 10 minutes to do and once you get good at it you won't go without a sharp blade ever again.
I once did one for a friend of mine and he just about freaked when he saw how I was going to do the job when I pulled out the tools, this was his brand new Husqavarna and he had an unusual attachment to his long worn out original blade.
It took longer to get him to let me sharpen the blade than the actual sharpening but in the end even he admitted it looked and performed as good as new.
I think most people don't trust how accurately your Mark I eyeball can get an angle if all you do is move the blade to the next tooth and hand grind each gullet without moving your hand, the fact that you only spend a few seconds doing each tooth adds to the accuracy and leads to less metal removed from each individual tooth.
Once you can completely sharpen a blade in less than 10 minutes by hand you will never let a blade get dull as a touch up takes even less time. The blade lasts longer too and it won't cost anymore than a handfull of stones.
Some pointers for success;
1) Buy the white stones as they are pure alumina oxide and cut "softer" with less heat, they wear faster but also cut faster too...buy them by the bag.
2) Use a vise to hold your saw at table height, its easier to see what your doing and the rigidity of the vise makes the job easier.
3) If one tooth is badly chewed up touch it up like the last one, don't spend a bunch of time trying to fix it up and make it perfect. It will eventually get worn down like the others after a number of sharpenings.
4) Dress the top of each tooth with a fine hone to get rid of the burr, this stops "digging in" on the first cut and prevents the burr getting torn off during the first cut which will "rag" the tip causing excessive wear that you will have to grind back to during the next sharpen.
5) Keep a sponge handy to wet and cool each tooth before and after the grind, you should never grind so long that it turns blue from the heat.
6) Don't tell your friends how fast you can do it, cause it worth at least 10 bucks for 10 minutes work!
P.S. Throw away the file, they do a crappy job of sharpening a saw blade and are even harder to keep a good angle, the idea with a die grinder is light pressure which aids in the accuracy.