动态控件消失了ASP.NET C＃（加载控件取决于DropDownList选择）

Question

I'm new in ASP.NET; I have a DropDownList in a page (with a masterpage):

<asp:DropDownList ID="cmbPrueba" runat="server" OnSelectedIndexChanged="cmbPrueba_SelectedIndexChanged" AutoPostBack="true">
    <asp:ListItem Value="0">Compresor de Aire</asp:ListItem>
    <asp:ListItem Value="1">Compresor/Unidad de Refrigeración</asp:ListItem>
</asp:DropDownList>
<asp:Button ID="btnActualizar" runat="server" Text="Actualizar" OnClick="btnActualizar_Click" />
<asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>

Depending of the DropDownList (cmbPrueba) the placeHolder creates controls using the string array; (I made the string arrays simulating string result of database).

So, if I take itemIndex=0 ("CompresorDeAire) I will create: "TextBox", "Calendar", "TextBox";

if I take index=1 (CompresorUnidadDeRefrigeracion ) the controls are: "DropDownList", "TextBox", "Calendar", "Calendar", "TextBox"... but there is a "DropDownList" control, so I will full it with this info:

private string[] CompresorUnidadDeRefrigeracionTipoCompresor = new string[] { "Compresor Alternativo", "Compresor de Tornillo", "Unidad de Refrigeración" };

And so on. Here is the code:

public partial class Controles : System.Web.UI.Page
{
    private Label _Label;
    private TextBox _TextBox = new TextBox();
    private Calendar _Calendar = new Calendar();
    private DropDownList _DropDownList = new DropDownList();

    private string[] CompresorDeAire = new string[] { "TextBox", "Calendar", "TextBox" };
    private string[] CompresorUnidadDeRefrigeracion = new string[] { "DropDownList", "TextBox", "Calendar", "Calendar", "TextBox" };
    private string[] CompresorUnidadDeRefrigeracionTipoCompresor = new string[] { "Compresor Alternativo", "Compresor de Tornillo", "Unidad de Refrigeración" };
    private string[] BombaElectrica = new string[] { "TextBox", "TextBox", "TextBox", "TextBox", "TextBox", "TextBox" };

    protected void Page_Load(object sender, EventArgs e)
    {
       LoadInfo(CompresorDeAire);
    }

    private void LoadInfo(string[] Arreglo)
    {
        for (int i = 0; i < Arreglo.Length; i++)
        {
            _Label = new Label();
            _TextBox = new TextBox();
            _Calendar = new Calendar();
            _DropDownList = new DropDownList();

            _Label.Text = Arreglo[i].ToString() + i.ToString();
            _Label.ID = _Label.Text;
            PlaceHolder1.Controls.Add(_Label);
            PlaceHolder1.Controls.Add(new LiteralControl("<br />"));

            if (Arreglo[i] == _TextBox.GetType().Name.ToString())
            {
                _TextBox.ID = "txt" + _Label.ID;
                //_TextBox.AutoPostBack = true;
                PlaceHolder1.Controls.Add(_TextBox);
            }
            else if (Arreglo[i] == _Calendar.GetType().Name.ToString())
            {
                _Calendar.ID = "cln" + _Label.ID;
                PlaceHolder1.Controls.Add(_Calendar);
            }
            else if (Arreglo[i] == _DropDownList.GetType().Name.ToString())
            {
                _DropDownList.ID = "cmb" + _Label.ID;

                //_DropDownList.AutoPostBack = true;
                foreach (var item in CompresorUnidadDeRefrigeracionTipoCompresor)
                {
                    int j = 0;
                    _DropDownList.Items.Add(item);
                    j++;
                }

                PlaceHolder1.Controls.Add(_DropDownList);
            }

            PlaceHolder1.Controls.Add(new LiteralControl("<br /><br />"));
        }
    }

    protected void cmbPrueba_SelectedIndexChanged(object sender, EventArgs e)
    {
        txtMensaje.Text = "";
        PlaceHolder1.Controls.Clear();

        switch (cmbPrueba.SelectedIndex)
        {
            case 0:
                this.LoadInfo(CompresorDeAire);
                break;

            case 1:
                this.LoadInfo(CompresorUnidadDeRefrigeracion);
                break;

            case 2:
                this.LoadInfo(BombaElectrica);
                break;
        }
    }

    protected void btnActualizar_Click(object sender, EventArgs e)
    {
        txtMensaje.Text = "";

        for (int i = 0; i < PlaceHolder1.Controls.Count; i++)
        {
            switch (PlaceHolder1.Controls[i].GetType().Name.ToString())
            {
                case "TextBox":
                    TextBox TB = PlaceHolder1.FindControl(PlaceHolder1.Controls[i].ID) as TextBox;
                    txtMensaje.Text += PlaceHolder1.Controls[i].GetType().Name + " " + PlaceHolder1.Controls[i].ID + " " + TB.Text + "\n";
                    TB.Text += "*";

                    break;

                case "Calendar":
                    Calendar Cal = PlaceHolder1.FindControl(PlaceHolder1.Controls[i].ID) as Calendar;
                    txtMensaje.Text += PlaceHolder1.Controls[i].GetType().Name + " " + PlaceHolder1.Controls[i].ID + " " + Cal.SelectedDate.ToShortDateString() + "\n";
                    break;

                case "DropDownList":
                    DropDownList DD = PlaceHolder1.FindControl(PlaceHolder1.Controls[i].ID) as DropDownList;

                    txtMensaje.Text += PlaceHolder1.Controls[i].GetType().Name + " " + PlaceHolder1.Controls[i].ID + " " + DD.Text + "\n";
                    break;
            }
        }
    }

    protected void btnLimpiar_Click(object sender, EventArgs e)
    {
        PlaceHolder1.Controls.Clear();
        txtMensaje.Text = "";
    }
}

When I run the code by default is Index = 0 , I use the textbox and calendar, and click "Actualizar" and I can see the info in the text box, when I choose Index=1 (and load the 2nd array) all the new controls show up, but if I choose a date or I write in the textbox and click in the buttom "Actualizar" the page return to the previous page (array 1).

I appreciate your help! thanks.

Answer 1

I am assuming that when you say “the page return to the previous page (array 1).” That you mean the first array (in the zero-th element)

The problem is that .NET will not automatically re-create the dynamic controls for you on the post back. You have to handle that.

Here are the basic steps for the first page request:

• Execute the Page_load event, which calls LoadInfo for CompresorDeAir.

Then when you selected a different entry in the dropdown and then click Actualizer button then it does a post back with these basic steps:

• Execute the Page_load event, which calls LoadInfo for CompresorDeAir.

• Execute cmbPrueba_SelectedIndexChanged which throws away the dynamic controls that were added in the page load and loads the control for the selected index.

• Execute btnActualizer_Click event which shows the controls in the dynamic place holder, these being the ones for the selected dropdown value.

Then when you change the text or date and then click Actualizer button it does these steps:

• Execute the Page_load event, which calls LoadInfo for CompresorDeAir.

• Execute btnActualizer_Click event which shows the controls in the dynamic place holder. In this case the ones from the page load are shown. the controls from the prior selected dropdown list item do not get created.

The only time the controls from the selected item in the dropdown list are added to the place holder is when the selected item changes for the dropdown.

The solution is put a hidden variable in the form to hold the last selected item from the drop down. Everytime the selected index changes then update this hidden value. In the page load event, on a postback, load the appropriate array based on that hidden value.