Check if a file Exists async?

Question

I wish there was a File.ExistsAsync()

I have:

bool exists = await Task.Run(() => File.Exists(fileName));

Using a thread for this feels like an antipattern. Is there a cleaner way?

Answer 1

There isn't a File.ExistsAsync probably for good reason; because it makes absolutely no sense to have one; File.Exists is not going to take very long; I tested it as 0.006255ms when the file exists and 0.010925ms when the file does not exist.

There are a few times when it is sensible to call File.Exists; however usually I think the correct solution would be to open the file (thus preventing deletion), catching any exceptions - as there is no guarantee that the file will continue to exist after the call to File.Exists.

When you want to create a new file and not overwrite the old one :

File.Open("fn", FileMode.CreateNew)

For most of the use cases I can think of File.Open() (whether for existing or create new) is going to be better because once the call succeeds you will have a handle to the file and be able to do something with it. Even when using the file's existence as a flag I think I'd still open and close it. The only time I've really used File.Exists is when checking to see if a local HTML file is there before calling the browser so I can show a nice error message when it isn't.

The is no guarantee that something else won't delete the file after File.Exists; so if you did open it after checking with File.Exists the open call could still fail.

In my tests using a FileExists on network drive takes longer than File.Open, File.Exists takes 1.5967ms whereas File.OpenRead takes 0.3927ms)

Maybe if you could expand upon why you're doing this we'd be better able to answer; until then I'd say that you shouldn't do this