UnauthorizedAccessException StreamWriter

Question

I have the following code:

public WriteToFile(string path, List<string> text)
{
    File.Delete(path);
    using (TextWriter writer = new StreamWriter(path, true))
    {
        foreach(string t in text)
        {
            writer.WriteLine(text);
        }
    }
}

Most of the time it works fine, the file is deleted and then created again with the text inside. However every so often the using statement throws an UnauthorizedAccessException. Any idea why? I have admin rights and the program is run as admin.

Answer 1

This is normal, it became undiagnosable because you used File.Delete(). Which is unnecessary, just use the StreamWriter(string) constructor.

This goes wrong because deleting a file doesn't provide a guarantee that the file will actually be deleted. It may be opened by another process. Which has opened the file with delete sharing, programs like virus scanners and file indexers commonly do this. Which makes the Delete() call succeed but the file doesn't disappear until all handles on the file are closed. You got the UnauthorizedAccessException exception because the file didn't get deleted yet.

Get ahead by removing the File.Delete() call. You still need to assume that the StreamReader() constructor can fail. Less often, it is bound to happen sooner or later. You'll get a better exception message. Such are the vagaries of a multi-tasking operating system.