static int GetGongLing()
{
    int GongLing = 0;
    string RuChangRiQi ="2020-01-30";
    string HeSuanYueFen= "2020-6-1"; 
    DateTime zhouQiStart = DateTime.Parse(HeSuanYueFen).AddDays(-1);
    DateTime RuChang;
    if (DateTime.TryParse(RuChangRiQi, out RuChang))
    {
        Console.WriteLine(RuChang);
        int index = 0;
        var startRuChang = RuChang;
        while (true)
        {
            if (startRuChang.AddMonths(index) < zhouQiStart)
            {
                index++;
            }
            else
                break;
        }
        GongLing = index-1;
    }
    else
    {
        GongLing = 0;
    }
    return GongLing;
}

注意不能寫成下面這種（不能一個月一個月累加判斷，要一次相加N次來判斷，起始index可以自己優化到一個接近範圍）

static int GetGongLing()
{
    int GongLing = 0;
    string RuChangRiQi ="2020-01-30";
    string HeSuanYueFen= "2020-7-1"; 
    DateTime zhouQiStart = DateTime.Parse(HeSuanYueFen).AddDays(-1);
    DateTime zhouQiEnd = DateTime.Parse(HeSuanYueFen).AddMonths(1).AddD
    DateTime RuChang;

    if (DateTime.TryParse(RuChangRiQi, out RuChang))
    {
        Console.WriteLine(RuChang);
        int index = 0;
        while (true)
        {
            RuChang=RuChang.AddMonths(index);
            if (RuChang < zhouQiStart)
            {
                index++;
                GongLing++;
            }
            else
                break;
        }
    }
    else
    {
        GongLing = 0;
    }
    return GongLing;
}