程序代码：

static void Main(string[] args)
{
    string document = File.ReadAllText(@"d:\temp\a.txt");
    int index = GetIndex(3, document);
    if (index == -1)
    {
        throw new Exception("...");
    }
    else
    {
        document = document.Insert(index, "Insert text");
    }
    File.WriteAllText(@"d:\temp\a.txt", document);
}

程序代码：

static int GetIndex(int no, string document)
        {
            int lineNo = 0;
            for (int i = 0; i < document.Length; i++)
            {
                if (document[i] == '\n')
                {
                    lineNo++;
                    if (lineNo == no)
                        return i;
                }
            }
            return -1;
        }