DirectoryInfo dir = new DirectoryInfo(@"C:\DotNet");
 FileInfo [] fo = dir.GetFiles();
           Dictionary fileInfo = new Dictionary();
foreach (FileInfo f in fo)
{
     DateTime dt = f.LastAccessTimeUtc;
     fileInfo.Add(f.Name, dt);
}

//With Dictionary 
var result1 = from fileIn in fileInfo.Keys orderby fileInfo[fileIn] descending select fileIn;
            
//With Dictionary to get the Values and File Name. 
foreach (var ele in result1)
{
    MessageBox.Show(ele + "  " + fileInfo[ele]);
}