ListBox选择的值显示为null

Question

我有一个填充了多年的列表框并设置了多个选项，但是当发送提交按钮时，值不会延续。我已经研究过，我发现的唯一答案是确保不再重新绑定并在!= postback中找到它 代码

          if (!IsPostBack)
        {
            CData[] Data = new Conduit().ReadObject(Global.Identifier);
            TForm VerifyAct = lClient.GetData(1stId, 2ndid);
            ddlPers.DataSource = Enum.GetNames(typeof(PersTaxDocInfoEnum));
            ddlPers.DataBind();
            ddlCorp.DataSource = Enum.GetNames(typeof(CorpTaxDocInfoEnum));
            ddlCorp.DataBind();
            var curDate = DateTime.Now.Year;
            Int32[] myYears = new Int32[5];
            myYears[0] = curDate - 1;
            myYears[1] = curDate - 2;
            myYears[2] = curDate - 3;
            myYears[3] = curDate - 4;
            myYears[4] = curDate - 5;


            DataTable yearsPast = new DataTable();
            yearsPast.Columns.Add("years");
            //yearsPast.Rows.Add(myYears);
            foreach (Int32 item in myYears)
            {
                yearsPast.Rows.Add(item);
            }
            lbYearsDataSource = yearsPast;
            lbYearsDataTextField = yearsPast.Columns[0].ToString();
            lbYearsDataValueField = yearsPast.Columns[0].ToString();
            lbYears.DataBind();
        ddlCorp.Visible = false;
        ddlPers.Visible = false;
        lbYears.Visible = false;
        btnSubmit.Enabled = false;
        }

    }

实际代码

int i = 0;

foreach (ListItem item in lbYears.Items)
{

    if (item.Selected)
    {
        fooYears[0] = item.Text;
    }
}
if (ddltype.SelectedValue=="2")
{
    taxForm = ddlPers.SelectedValue.ToString();
}
else
{
    Form = ddlCorp.SelectedValue.ToString();
}

这会出现一个null异常。我和可能永远都是开发的新手，但我已经使用Visual Studio进行调试，并且没有遇到值，我无法弄明白。任何帮助将不胜感激

Answer 1

您可以通过多种方式将数据填充到 ListBox 。但是，创建表只是为了将项目分配给 ListBox 有点奇怪。

您可以查看以下示例 -

ASPX

<asp:ListBox ID="YearListBox" runat="server" 
    SelectionMode="Multiple" />
<asp:Button runat="server" ID="SubmitButton" 
    OnClick="SubmitButton_Click" Text="Submit" />

代码背后

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        int year = DateTime.Now.Year;
        for (int i = 0; i < 5; i ++)
        {
            string item = (year - i).ToString();
            YearListBox.Items.Add(new ListItem(item, item));
        }
    }
}

protected void SubmitButton_Click(object sender, EventArgs e)
{
    IList<string> selectedValues = YearListBox.Items.Cast<ListItem>()
        .Where(x => x.Selected)
        .Select(x => x.Value)
        .ToList();
}