dropdownlist 显示日期

dropdownlist1代表年 autopostback=true

dropdownlist2代表月  autopostback=true

dropdownlist3代表日

    protected void Page_Load(object sender, EventArgs e)
    ...{
        string s = Request.QueryString["id"];
        Response.Write(Server.UrlDecode(s));

        DateTime tnow = DateTime.Now;
        ArrayList years = new ArrayList();
        ArrayList months = new ArrayList();
        int i;
        for (i = 1991; i >= 1958; i--)
        ...{
            years.Add(i);
        }
        for (i = 1; i <= 12; i++)
        ...{
            months.Add(i);
        }

        if (!IsPostBack)
        ...{
            DropDownList1.DataSource = years;
            DropDownList1.SelectedValue =years[8].ToString();
            DropDownList1.DataBind();

            DropDownList2.DataSource = months;
            DropDownList2.SelectedValue = tnow.Month.ToString();
            DropDownList2.DataBind();

            int year, month;
            Int32.TryParse(tnow.Year.ToString(), out year);
            Int32.TryParse(tnow.Month.ToString(), out month);

            binddropdownlist3(year, month);
            DropDownList3.SelectedValue = tnow.Day.ToString();
        }
    }

    protected bool checkLeap(int year)
    ...{
        if ((year % 4 != 0) && (year % 100 != 0) || (year % 400 == 0))
        ...{ 
            return true; 
        }
        else
        ...{
            return false;
        }
    }

    protected void binddropdownlist3(int year, int month)
    ...{
        int i;
        ArrayList days = new ArrayList();
        switch (month)
        ...{ 
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                for (i = 1; i <= 31; i++)
                ...{
                    days.Add(i);
                }
                break;
            case 2:
                if (checkLeap(year))
                ...{
                    for (i = 1; i <= 29; i++)
                    ...{
                        days.Add(i);
                    }
                }
                else
                ...{
                    for (i = 1; i <= 28; i++)
                    ...{
                        days.Add(i);
                    }                    
                }
                break;
            case 4:
            case 6:
            case 9:
            case 11:
                for (i = 1; i <= 30; i++)
                ...{
                    days.Add(i);
                }
                break;
        }
        DropDownList3.DataSource = days;
        DropDownList3.DataBind();

    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    ...{
        int year, month;
        Int32.TryParse(DropDownList1.SelectedValue.ToString(), out year);
        Int32.TryParse(DropDownList2.SelectedValue.ToString(),out month);
        binddropdownlist3(year, month);
    }
    protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
    ...{
        int year, month;
        Int32.TryParse(DropDownList1.SelectedValue.ToString(),out year);
        Int32.TryParse(DropDownList2.SelectedValue.ToString(),out month);
        binddropdownlist3(year, month);
    }

但是这种做法会刷新页面，不是很好！