WinForm 文件夹Copy覆盖不了相同文件夹 求解

比如: 我要把D盘的user文件夹覆盖E盘的 user文件夹,  如果E盘的user文件夹里面含有D盘user文件夹相同内容,他就覆盖不了,相反的是D盘user文件夹内容是空的或者没有没有E盘的user文件夹同内容,他就能覆盖掉了是怎么回事? 代码如下:请高手帮忙看下
 private void CopyDirectory(string srcdir, string desdir)
         {             try
             {
                 string folderName = srcdir.Substring(srcdir.LastIndexOf("\\") + 1);                 string desfolderdir = desdir + "\\" + folderName;                 if (desdir.LastIndexOf("\\") == (desdir.Length - 1))
                 {
                     desfolderdir = desdir + folderName;
                 }
                 string[] filenames = Directory.GetFileSystemEntries(srcdir);                 foreach (string file in filenames)// 遍历所有的文件和目录
                 {
                     if (Directory.Exists(file))// 先当作目录处理如果存在这个目录就递归Copy该目录下面的文件
                     {                         string currentdir = desfolderdir + "\\" + file.Substring(file.LastIndexOf("\\") + 1);
                         if (!Directory.Exists(currentdir))
                         {
                             Directory.CreateDirectory(currentdir);
                         }                         CopyDirectory(file, desfolderdir);
                     }                     else // 否则直接copy文件
                     {
                         string srcfileName = file.Substring(file.LastIndexOf("\\") + 1);                         srcfileName = desfolderdir + "\\" + srcfileName;
                         if (!Directory.Exists(desfolderdir))
                         {
                             Directory.CreateDirectory(desfolderdir);
                         }
                         File.Copy(file, srcfileName);
                     }
                 }//foreach 
             }
             catch (Exception ex)
             {
                
               //  MessageBox.Show("该文件已存在!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
             }
             return;
            
             
         }