当DropDownList的SelectedValue为true时显示表

Question

I want to show a table when a SelectedValue of my DropDownList ddlKlasse is true. I'm using asp.net empty web forms with a masterpage. The idea is:

1. when ddlKlasse.SelectedValue = "2" table tblDubbelTwee must be shown
2. when ddlKlasse.SelectedValue = "5" table tblVierMet must be shown
3. when ddlKlasse.SelectedValue = "9" table tblAchtMet must be shown

I get the values in my dropdownlist from my database.

The code I have now is:

In my Page_Load:

protected void Page_Load(object sender, EventArgs e)
    {
        BindDropDownListKlasse();
        BindDropDownListVereniging();


        if (!Page.IsPostBack)
        {
            tblDubbelTwee.Visible = false;
            tblVierMet.Visible = false;
            tblAchtMet.Visible = false;


        }
    }

For binding the dropdown:

private void BindDropDownListKlasse()
    {
        try
        {
            using (SqlConnection conn = new SqlConnection(connString))
            {
                using (SqlCommand com = new SqlCommand())
                {
                    com.CommandText = "SELECT DISTINCT AantalDeelnemers, Naam FROM Klasse;";
                    com.Connection = conn;
                    conn.Open();
                    SqlDataAdapter da = new SqlDataAdapter(com);
                    DataTable dt = new DataTable();
                    da.Fill(dt);
                    ddlKlasse.DataSource = dt;
                    ddlKlasse.DataValueField = "AantalDeelnemers";
                    ddlKlasse.DataTextField = "Naam";
                    ddlKlasse.DataBind();
                    conn.Close();

                    //Adding "Kies de klasse" optie in dropdownlist voor validatie
                    ddlKlasse.Items.Insert(0, new ListItem("Kies de klasse", "0"));
                }
            }
        }
        catch
        {

        }
    }

For showing the tables:

protected void ddlKlasse_SelectedIndexChanged(object sender, EventArgs e)
    {
        ListItem selectedListItemDubbelTwee = ddlKlasse.Items.FindByValue("2");
        if (selectedListItemDubbelTwee != null)
        {
            selectedListItemDubbelTwee.Selected = true;
            tblDubbelTwee.Visible = true;
            tblVierMet.Visible = false;
            tblAchtMet.Visible = false;
        };

        ListItem selectedListItemVierMet = ddlKlasse.Items.FindByValue("5");
        if (selectedListItemVierMet != null)
        {
            selectedListItemVierMet.Selected = true;
            tblVierMet.Visible = true;
            tblDubbelTwee.Visible = false;
            tblAchtMet.Visible = false;
        };

        ListItem selectedListItemAchtMet = ddlKlasse.Items.FindByValue("9");
        if (selectedListItemAchtMet != null)
        {
            selectedListItemAchtMet.Selected = true;
            tblAchtMet.Visible = true;
            tblDubbelTwee.Visible = false;
            tblVierMet.Visible = false;
        };
    }

My DropDownList:

<div class="form-inline">
    <div class="form-group">
        <asp:DropDownList ID="ddlKlasse" class="form-control" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlKlasse_SelectedIndexChanged"></asp:DropDownList>
    </div>
</div>

One of my tables:

<asp:Table ID="tblDubbelTwee" runat="server" class="table">
    <asp:TableHeaderRow>
        <asp:TableHeaderCell>Naam</asp:TableHeaderCell>
        <asp:TableHeaderCell>Email</asp:TableHeaderCell>
        <asp:TableHeaderCell>Lidmaatschapsnr</asp:TableHeaderCell>
    </asp:TableHeaderRow>

    <asp:TableRow>
        <asp:TableCell>
            <asp:TextBox ID="txtNaam" type="text" class="form-control" runat="server"></asp:TextBox>
        </asp:TableCell>
        <asp:TableCell>
            <asp:TextBox ID="txtEmail" type="text" class="form-control" runat="server"></asp:TextBox>
        </asp:TableCell>
        <asp:TableCell>
            <asp:TextBox ID="txtLidmaatschapsnr" type="text" class="form-control" runat="server"></asp:TextBox>
        </asp:TableCell>
    </asp:TableRow>
    <asp:TableRow>
        <asp:TableCell>
            <asp:TextBox ID="txtNaam2" type="text" class="form-control" runat="server"></asp:TextBox>
        </asp:TableCell>
        <asp:TableCell>
            <asp:TextBox ID="txtEmail2" type="text" class="form-control" runat="server"></asp:TextBox>
        </asp:TableCell>
        <asp:TableCell>
            <asp:TextBox ID="txtLidmaatschapsnr2" type="text" class="form-control" runat="server"></asp:TextBox>
        </asp:TableCell>
    </asp:TableRow>
</asp:Table>

But when I run my project and select a field in my DropDownList it gives the error: System.Web.HttpException: Selecting multiple items in a DropDownList is not allowed.

Can you please help me to solve this problem?

Answer 1

Your current handler for DDL index change is doing something very different from what you have described. It basically checks if the list item exists in the ddl list (no matter selected or not) and then tries to select it. Since all of 3 items exist, you are effectively trying to select 3 items, which is not allowed.

What you might be looking for is a switch by selected value:

switch (ddlKlasse.SelectValue)
{
    case "2":
            tblDubbelTwee.Visible = true;
            tblVierMet.Visible = false;
            tblAchtMet.Visible = false;
            break;
    case "5": //similar here
    case "9": //similar here
    default:
    // exception or something
}

Answer 2

refere this code  
protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                bindgrid();

            }
        }

        void bindgrid()
        {
            con.Open();
            SqlCommand cmd = new SqlCommand("select DISTINCT Class from addmitionform1", con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
            con.Close();
            ddlclass.DataSource = ds;
            ddlclass.DataTextField = "Class";
            ddlclass.DataValueField = "Class";
            ddlclass.DataBind();
            ddlclass.Items.Insert(0, new ListItem("--Select--", "0"));

        }

protected void ddlKlasse_SelectedIndexChanged(object sender, EventArgs e)
    {
          string id = (ddlclass.SelectedValue.ToString());
            con.Close();
            SqlDataAdapter da1 = new SqlDataAdapter("Select id,FirstName from addmitionform1 where Class= '" + id + "'", con);
            //SqlDataAdapter da1 = new SqlDataAdapter("Select * from addmitionform1 ", con);
            DataSet ds1 = new DataSet();
            da1.Fill(ds1);
            GridView1.DataSource = ds1;
            GridView1.DataBind();
    }

Answer 3

First move BindDropDownListKlasse() and BindDropDownListVereniging() inside if (!IsPostBack) , as HarveySpecter suggested.
And change ddlKlasse_SelectedIndexChanged() to:

protected void ddlKlasse_SelectedIndexChanged(object sender, EventArgs e)
{
    bool showDubbelTwee = false, showVierMet = false, showAchtMet = false;
    switch (ddlKlasse.SelectedValue)
    {
        case "2":
            showDubbelTwee = true;
            break;
        case "5":
            showVierMet = true;
            break;
        case "9":
            showAchtMet = true;
            break;
    }

    tblAchtMet.Visible = showVierMet;
    tblDubbelTwee.Visible = showDubbelTwee;
    tblVierMet.Visible = showVierMet;
}

What you are doing now in ddlKlasse_SelectedIndexChanged() is check for each value if it's present in the dropdown, and if present select it and show the respective table.
Instead, you need to check which value is selected and show the respective table.