使用C#不能在DropDownList中选择多个项目

当我尝试从下拉框“不能在DropDownList中选择多个项目”中选择项目时,我收到此错误.有人可以帮助我,我不知道为什么我得到这个.这是我的代码：

private void Bind_GridView()
{
this.BindGroupNameList(DropDownList1);
}

 private void GetGroupNameList(DropDownList DropDownList1)
    {
        String strConnString = System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;
        sqlConnection con2 = new sqlConnection(strConnString);
        sqlDataAdapter sda = new sqlDataAdapter();
        sqlCommand cmd1 = new sqlCommand("select distinct Name" +
                        " from MyTable");

        cmd1.Connection = con2;
        con2.open();

        DropDownList1.DataSource = cmd1.ExecuteReader();
        DropDownList1.DataTextField = "Name";
        DropDownList1.DataValueField = "Name";
        DropDownList1.DataBind();
        con2.Close();
        DropDownList1.Items.FindByValue(ViewState["MyFilter"].ToString())
                .Selected = true;
    }

   //on item change
    protected void NameChanged(object sender,EventArgs e)
    {
        DropDownList DropDownList1 = (DropDownList)sender;
        ViewState["MyFilter"] = DropDownList1.SelectedValue;
        this.Bind_GridView();
    }

这是我在aspx中的下拉框

<asp:DropDownList ID="DropDownList1" runat="server" AutopostBack="True" OnSelectedindexChanged="NameChanged"
                        DataTextField="Name" DataValueField="Name" 
                        AppendDataBoundItems="true">
                        <asp:ListItem Text="ALL" Value="ALL"></asp:ListItem>
                        <asp:ListItem Text="Top 10" Value="10"></asp:ListItem>
                    </asp:DropDownList>

这是页面加载的代码：

protected void Page_Load(object sender,EventArgs e)
    {

        if (!Page.IsPostBack)
        {

            ViewState["MyFilter"] = "ALL";
            this.Bind_GridView();


        }

}

这是调用GetGroupNameList的方法：

private void Bind_GridView()
    {
        DataTable dt = new DataTable();
        String strConnString = System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;
        sqlConnection con = new sqlConnection(strConnString);
        sqlDataAdapter sda = new sqlDataAdapter();
        sqlCommand cmd = new sqlCommand("sp_filter_Names");
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("@MyFilter",ViewState["MyFilter"].ToString());
        cmd.Connection = con;
        sda.SelectCommand = cmd;
        sda.Fill(dt);
        GV_Test.DataSource = dt;
        GV_Test.DataBind();
        GetGroupNameList();

    }

解决方法

改变这一行：

DropDownList1.Items.FindByValue(ViewState["MyFilter"].ToString())
              .Selected = true;

对此：

DropDownList1.SelectedValue = ViewState["MyFilter"].ToString();

问题是你已经有了一个选定的项目(可能是列表中的第一个),而你正在搜索另一个项目也是如此.请记住,拥有多个选定项对ListBox和CheckListBox有效,但对DropDownList无效.

选择其中一个ListItem不会自动取消选择ListItemColletion中的其他项.