不能隐式地将类型字符串转换为StreamWriter

我正在尝试将一个文件分成10个部分。我认为我可以这样做但我得到了题目中提到的错误。这个问题有简单的解决办法吗?

StreamWriter a = new StreamWriter(new FileStream(@"C:'work'missing" + num + ".txt");
using (var r = new StreamReader(readMissing))
                {
                    var rr = r.ReadLine();
                    while (rr != null)
                    {
                        a.WriteLine(rr);
                        count++;

                        if (count == 36139)
                        {
                            iNum = Int32.Parse(num);
                            iNum++;
                            num = iNum.ToString();
                            a = (@"C:'work'missing" + num + ".txt");  //problem line
                            count = 0;
                        }
                        rr = r.ReadLine();

                    }
                    a.Close();
                }

a.Close();
a = new StreamWriter(@"C:'work'missing" + num + ".txt");