如何知道写文件的时间已经完成？

I'm writing a program which operates on a file (only reads the file), while another program is writing that file (I've no control over it to use events and I don't know the content of the file). I want a way to know when that program finished writing, to stop my program operating on the file. I used these two method but I don't know which one is reliable and more performance:I'm writing a program which operates on a file