删除c#中的文件夹和子文件夹

我写了这段代码：

static void Main(string[] args)
{        
    DirectoryInfo mm = new DirectoryInfo(@"c:ex");
    string aa = Convert.ToString(mm);
    string[] allFileNames = 
        System.IO.Directory.GetFiles(aa,"*.*",System.IO.SearchOption.AllDirectories);
    string[] alldirNames = 
       System.IO.Directory.GetDirectories(aa,"*",System.IO.SearchOption.AllDirectories);

    foreach (string filename in allFileNames)
    {
        FileAttributes attr = File.GetAttributes(filename);
        File.SetAttributes(filename,attr & ~FileAttributes.ReadOnly);

    }

    foreach (string dirname in alldirNames)
    {
        FileAttributes attr = File.GetAttributes(dirname);
        File.SetAttributes(dirname,attr & ~FileAttributes.ReadOnly);
        Directory.Delete(dirname,true);
    }

    FileInfo[] list = mm.GetFiles();

    foreach (FileInfo k in list)
    {
        k.Delete();
    }
    mm.Delete();
    Console.ReadKey();
}

现在的问题是每当我运行程序时它会给我以下错误：

Could not find a part of the path ‘c:exxxbb’.

这个错误是什么意思？