C#FileStream复制大文件

private void CopyFile(string fromFile, string toFile, int lengthEachTime)
        {
            FileStream fileToCopy = new FileStream(fromFile, FileMode.Open, FileAccess.Read);
            FileStream copyToFile = new FileStream(toFile, FileMode.Append, FileAccess.Write);
            int lengthToCopy;
            if (lengthEachTime < fileToCopy.Length)//如果分段拷贝，即每次拷贝内容小于文件总长度
            {
                byte[] buffer = new byte[lengthEachTime];
                int copied = 0;
                while (copied <= ((int)fileToCopy.Length - lengthEachTime))//拷贝主体部分
                {
                    lengthToCopy = fileToCopy.Read(buffer, 0, lengthEachTime);
                    fileToCopy.Flush();
                    copyToFile.Write(buffer, 0, lengthEachTime);
                    copyToFile.Flush();
                    copyToFile.Position = fileToCopy.Position;
                    copied += lengthToCopy;
                }
                int left = (int)fileToCopy.Length - copied;//拷贝剩余部分
                lengthToCopy = fileToCopy.Read(buffer, 0, left);
                fileToCopy.Flush();
                copyToFile.Write(buffer, 0, left);
                copyToFile.Flush();
            }
            else//如果整体拷贝，即每次拷贝内容大于文件总长度
            {
                byte[] buffer = new byte[fileToCopy.Length];
                fileToCopy.Read(buffer,0,(int)fileToCopy.Length);
                fileToCopy.Flush();
                copyToFile.Write(buffer, 0, (int)fileToCopy.Length);
                copyToFile.Flush();
            }
            fileToCopy.Close();
            copyToFile.Close();
        }