作为过程启动后写入文件会导致UnauthorizedAccessException（拒绝访问路径XXX。）

Question

The following C# code fails at line "File.WriteAllText("test_a.cmd", "timeout 15")" sometimes, anyone who knows why and how to solve it? Thanks.

        for (int i = 0; i < 100; i++)
        {
            File.WriteAllText("test_a.cmd", "rem timeout 1");
            var p = Process.Start("test_a.cmd");

            p.WaitForExit();
            p.Dispose();
            p = null;

            //GC.Collect();
            //GC.WaitForPendingFinalizers();

            File.Delete("test_a.cmd");

            // Somtetimes it causes System.UnauthorizedAccessException: Access to the path 'XXX\test_a.cmd' is denied.
            File.WriteAllText("test_a.cmd", "timeout 15");
        }

Answer 1

There's another process that has the file opened with delete sharing. Which allows your File.Delete() call to succeed. But not the subsequent File.WriteAllText(). Because the file still exists after you deleted it. It will not disappear until the other process has closed its handle on the file. Any attempt to overwrite the file while it is still in limbo like this will fail with an "access denied" error.

Such are the vagaries of running a program on a multi-tasking operating system where other processes may be interested in the files that you manipulate. There's always a good candidate for such a process on most any Windows install, disable your anti-virus program to see if that solves the problem.

Beyond just sleeping for a while and trying again, there's a decent way to minimize the lossage. You can rename the file, much lower odds that you'll be hit with this exception:

File.Delete("test_a.cmd.bak");
File.Move("test_a.cmd", "test_a.cmd.bak");
for (int attempt = 0; ; ++attempt) {
    try {
       File.WriteAllText("test_a.cmd", "timeout 15");
       break;
    }
    catch (System.UnauthorizedAccessException ex) {
       if (attempt > 10) throw;
       System.Threading.Thread.Sleep(1000);
    }
}
try {
    File.Delete("test_a.cmd.bak");
}
catch (Exception ex) {}

Answer 2

Since File.WriteAllText() creates/overwrites a file, the call to Delete is not needed. In fact, that is what, probably, causes your issue.

So, my solution is to:

• Remote File.Delete
• Add Thread.Sleep(100) before last WriteAllText

The sleep part is to allow Windows to do processing and that is what basically causes your exception.