DirectoryNotFoundException after Directory.Exists

Question

How can the following statement:

if (Directory.Exists(outputDestination) 
    && new DirectoryInfo(outputDestination).GetFiles().Count() > 0)

throw a DirectoryNotFoundException: Could not find a part of the path given I check directory exists before calling GetFiles()

Answer 1

How can the following statement:

if (Directory.Exists(outputDestination) 
   && new DirectoryInfo(outputDestination).GetFiles().Count() > 0)

throw a DirectoryNotFoundException?

Your code can throw a DirectoryNotFoundException because you've created a TOCTOU bug:

In software development, time of check to time of use (TOCTTOU or TOCTOU, pronounced "tock too") is a class of software bugs caused by changes in a system between the checking of a condition (such as a security credential) and the use of the results of that check. This is one example of a race condition.

Just because the directory exists for the call to Directory.Exists(), that does not mean that it still exists for the call to DirectoryInfo().

Answer 2

For a move operation across drive volumes, I encountered this. Even though the OP said that their answer was a race condition. There are other reasons that the exception can be thrown, such as too long of a file name. As per the docs:

The exception that is thrown when part of a file or directory cannot be found.

when part of a file is key. A check within a catch, will show that a long file name will give the DirectoryNotFoundException

try
{
    fromFile.CopyTo(toFile.FullName, toFile.Exists);
}
catch (DirectoryNotFoundException)
{
    // can occur for really long file names
    if (Math.Max(fromFile.FullName.Length, toFile.FullName.Length) >= 260)
    {
        try
        {
            // in 4.6.1 they added handling for long file names, but its weird
            string from = $"\\\\?\\{fromFile.FullName}";
            string to = $"\\\\?\\{toFile.FullName}";
            File.Copy(from, to, File.Exists(to));
            if (File.Exists(to))
            {
                File.Delete(from);
                Log("Success\n");
                continue;
            }
        }
        catch (DirectoryNotFoundException)
        {
            Log("Failed\n");
            continue;
        }
    }
    throw;
}