How to lock a text file for read and write?

[C# Programming]

I need to use an incremental counter, by means of retrieving and updating a value stored in a text file

I know how to use the StreamWriter and the StreamReader classes but what I need to do is to read an int value from a text file, then increment the value by one and write it back to the file.

Is there a way to place a lock in the file while doing both operations? The reason for that is to avoid multiple concurency.
Also, how can the file be checked if it is locked?

Thanks,

CBenac