But it can be quite long, especially when the file is on a network drive. If so you’ll have to wait for a time-out to know that the file is not reachable.

This was too slow for me so I coded this little utility which performs the same but with a customizable TimeOut. As it may interest you, here it is:
[csharp]private static bool VerifyFileExists(Uri uri, int timeout)
{
var task = new Task<bool>(() =>
{
var fi = new FileInfo(uri.LocalPath);
return fi.Exists;
});
task.Start();
return task.Wait(timeout) && task.Result;
}
[/csharp]

Enjoy !