Как да добавяте контроли програмно при натискане на бутон?

Добре, по-долу съм поставил работещ пример. Неща, които трябва да знаете:

• За да избегнете грешка

Message=Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request. Source=System.Web ErrorCode=-2147467259

задайте това свойство на вашите динамични контроли

EnableViewState = false

Данните за този пример изглеждат така

Марка Цвят на двигателя Цена
VW, 1.2, Черен, 72 000
VW, 1.2, Бял, 70 000
VW, 1.6 TDI, Червен, 79 500
VW, 1.6 TDI, Бял, 78 800
Форд, 1.6, Черен, 57 600
Форд, 1.6, Зелен, 57 100
Форд, 2.0 TDCi, Черен, 87 300
Форд, 2.0 TDCi, Бяло, 86 600

По принцип това работи по следния начин:

• Изберете марка автомобил и изберете бутон (бутонът е нашият критерий за генериране на контроли) Разбира се, можете също да правите логика с падащи менюта и т.н.
• На втория екран имате едно или две падащи менюта според вашия избор
• Ако имате две падащи менюта, второто зависи от избора на първото падащо меню, така че имате пример за зависимост и обратно изпращане

Код на страницата:

<form id="form1" runat="server">
<div>
<asp:MultiView ID="mvPanels" runat="server">
    <asp:View ID="vPanel1" runat="server">
        <table>
            <tbody>
                <tr>
                    <td>Pick car</td>
                    <td><asp:DropDownList ID="ddlBrand" runat="server" /></td>
                    <td><asp:Button ID="btnPanel1Next" Text="Next" runat="server" 
                            onclick="btnPanel1Next_Click" /></td>
                    <td><asp:Button ID="btnPanel1NextEngineOnly" Text="Next (pick engine only)" runat="server" 
                            onclick="btnPanel1NextEngineOnly_Click" /></td>
                </tr>
            </tbody>
        </table>
    </asp:View>
    <asp:View ID="vPanel2" runat="server">
        <%--<table>
            <tbody>
                <tr>
                    <td>Pick engine</td>
                    <td><asp:DropDownList ID="ddlEngine" AutoPostBack="true" runat="server" 
                            onselectedindexchanged="ddlEngine_SelectedIndexChanged" /></td>
                    <td><asp:DropDownList ID="ddlColor" AutoPostBack="true" runat="server" 
                            onselectedindexchanged="ddlColor_SelectedIndexChanged" /></td>
                    <td><asp:Button ID="btnPanel2Prev" Text="Prev" runat="server" 
                            onclick="btnPanel2Prev_Click" /><asp:Button ID="btnPanel2Next" Text="Next" 
                            runat="server" Enabled="False" onclick="btnPanel2Next_Click" /></td>
                </tr>
            </tbody>
        </table>--%>
    </asp:View>
    <asp:View ID="vPanel3" runat="server">
        <table>
            <tbody>
                <tr>
                    <td><asp:Label ID="lblResult" runat="server" /></td>
                    <td><asp:Button ID="btnPanel3Prev" Text="Prev" runat="server" 
                            onclick="btnPanel3Prev_Click" /><asp:Button ID="btnPanel3Finish" 
                            Text="Confirm" runat="server" onclick="btnPanel3Finish_Click" /></td>
                </tr>
            </tbody>
        </table>
    </asp:View>
</asp:MultiView>
</div>
</form>

Код зад:

public partial class Default : System.Web.UI.Page
{
    public class CarConfiguration
    {
        public string Brand { get; set; }
        public string Engine { get; set; }
        public PaintColor Paint { get; set; }
        public string Cost { get; set; }
    }

    [Serializable]
    public class CarConfigurationFilter
    {
        public string Brand { get; set; }
        public string Engine { get; set; }
        public PaintColor? Paint { get; set; }
        public bool EngineOnly { get; set; }
    }

    public enum PaintColor
    {
        Black,
        Red,
        White,
        Green,
    }

    public List<CarConfiguration> availableCars = new List<CarConfiguration>
    {
        new CarConfiguration{
            Brand = "VW",
            Engine = "1.2",
            Paint = PaintColor.Black,
            Cost = "72 000",
        },
        new CarConfiguration{
            Brand = "VW",
            Engine = "1.2",
            Paint = PaintColor.White,
            Cost = "70 000",
        },
        new CarConfiguration{
            Brand = "VW",
            Engine = "1.6 TDI",
            Paint = PaintColor.Red,
            Cost = "79 500"
        },
        new CarConfiguration{
            Brand = "VW",
            Engine = "1.6 TDI",
            Paint = PaintColor.White,
            Cost = "78 800",
        },
        new CarConfiguration{
            Brand = "Ford",
            Engine = "1.6",
            Paint = PaintColor.Black,
            Cost = "57 600"
        },
        new CarConfiguration{
            Brand = "Ford",
            Engine = "1.6",
            Paint = PaintColor.Green,
            Cost = "57 100"
        },
        new CarConfiguration{
            Brand = "Ford",
            Engine = "2.0 TDCi",
            Paint = PaintColor.Black,
            Cost = "87 300"
        },
        new CarConfiguration{
            Brand = "Ford",
            Engine = "2.0 TDCi",
            Paint = PaintColor.White,
            Cost = "86 600"
        },
    };

    CarConfigurationFilter filter
    {
        get { return (CarConfigurationFilter)ViewState["Filter"]; }
        set { ViewState["Filter"] = value; }
    }

    //If you have multiview in control you need to create this in event before event PageLoad like  LoadViewState, LoadControlState, LoadControlState
    protected override void OnPreLoad(EventArgs e)
    {
        if (IsPostBack && mvPanels.ActiveViewIndex == 1)
        {
            CreateControlsOnPanel2(true, !filter.EngineOnly);
        }
        base.OnPreLoad(e);
    }

    protected override void OnLoad(EventArgs e)
    {
        if (!IsPostBack)
        {
            ddlBrand.DataSource = availableCars.Select(x => x.Brand).Distinct();
            ddlBrand.DataBind();
            filter = new CarConfigurationFilter();
            mvPanels.ActiveViewIndex = 0;
        }
        base.OnLoad(e);
    }

    protected void btnPanel1Next_Click(object sender, EventArgs e)
    {
        filter.Brand = ddlBrand.SelectedValue;
        filter.EngineOnly = false;

        CreateControlsOnPanel2(true, true);

        mvPanels.ActiveViewIndex++; 
    }

    protected void btnPanel1NextEngineOnly_Click(object sender, EventArgs e)
    {
        filter.Brand = ddlBrand.SelectedValue;
        filter.EngineOnly = true;
        CreateControlsOnPanel2(true, false);

        mvPanels.ActiveViewIndex++; 
    }

    void CreateControlsOnPanel2(bool enginePickerEnabled, bool colorPickerEnabled)
    {
        var btnPanel2Prev = new Button();
        btnPanel2Prev.ID = "btnPanel1Prev";
        btnPanel2Prev.EnableViewState = false;    
        btnPanel2Prev.Text = "Prev";
        btnPanel2Prev.Click += btnPanel2Prev_Click;

        var btnPanel2Next = new Button();
        btnPanel2Next.ID="btnPanel2Next";
        btnPanel2Next.Text = "Next";
        btnPanel2Next.EnableViewState = false;
        btnPanel2Next.Enabled = false;
        btnPanel2Next.Click += btnPanel2Next_Click;


        if (enginePickerEnabled)
        {
            var ddlEngine = new DropDownList();
            ddlEngine.ID = "ddlEngine";
            ddlEngine.AutoPostBack = true;
            ddlEngine.EnableViewState = false;

            var engines = availableCars.Where(x => x.Brand == filter.Brand).Select(found => found.Engine).Distinct().ToList();
            engines.Insert(0, String.Empty);
            ddlEngine.DataSource = engines;
            ddlEngine.DataBind();
            if (!String.IsNullOrEmpty(filter.Engine))
            {
                ddlEngine.SelectedValue = filter.Engine;
                if (!colorPickerEnabled)
                    btnPanel2Next.Enabled = true;
            }
            else
                ddlEngine.SelectedIndex = 0;
            ddlEngine.SelectedIndexChanged += ddlEngine_SelectedIndexChanged;
            vPanel2.Controls.Add(ddlEngine);
        }
        //remember to add ID to all dynamic controls or there might be an error on postback
        if (colorPickerEnabled)
        {
            var ddlColor = new DropDownList();
            ddlColor.ID = "ddlColor";
            ddlColor.AutoPostBack = true;
            ddlColor.EnableViewState = false;
            ddlColor.SelectedIndexChanged += ddlColor_SelectedIndexChanged;
            vPanel2.Controls.Add(ddlColor);

            if (!String.IsNullOrEmpty(filter.Engine))
            {
                var colors = availableCars.Where(x => x.Brand == filter.Brand && x.Engine == filter.Engine).Select(found => found.Paint.ToString()).Distinct().ToList();
                colors.Insert(0, String.Empty);
                ddlColor.DataSource = colors;
                if (filter.Paint.HasValue)
                {
                    ddlColor.SelectedValue = filter.Paint.Value.ToString();
                    btnPanel2Next.Enabled = true;
                }

            }
            else
            {
                ddlColor.DataSource = null;
                ddlColor.SelectedIndex = 0;
            }
            ddlColor.DataBind();
        }

        vPanel2.Controls.Add(btnPanel2Prev);
        vPanel2.Controls.Add(btnPanel2Next);
    }

    protected void ddlEngine_SelectedIndexChanged(object sender, EventArgs e)
    {
        var ddlEngine = sender as DropDownList;
        var btnPanel2Next = (Button)vPanel2.FindControl("btnPanel2Next");

        if (!String.IsNullOrEmpty(ddlEngine.SelectedValue))
        {
            filter.Engine = ddlEngine.SelectedValue;
            var colors = availableCars.Where(x => x.Brand == filter.Brand && x.Engine == filter.Engine).Select(found => found.Paint.ToString()).Distinct().ToList();
            colors.Insert(0, String.Empty);
            filter.Paint = null;

            var ddlColor = (DropDownList)vPanel2.FindControl("ddlColor");
            if (ddlColor != null)
            {
                ddlColor.DataSource = colors;
                ddlColor.DataBind();
                ddlColor.SelectedIndex = 0;

                btnPanel2Next.Enabled = false;
            }
            else
                btnPanel2Next.Enabled = true;
        }
        else
        {
            var ddlColor = (DropDownList)vPanel2.FindControl("ddlColor");
            if (ddlColor != null)
            {
                ddlColor.Items.Clear();
                ddlColor.SelectedIndex = -1;
            }
            filter.Engine = null;
            btnPanel2Next.Enabled = false;
        }
    }

    protected void ddlColor_SelectedIndexChanged(object sender, EventArgs e)
    {
        var ddlColor = (DropDownList)vPanel2.FindControl("ddlColor");
        var btnPanel2Next = (Button)vPanel2.FindControl("btnPanel2Next");

        if (!String.IsNullOrEmpty(ddlColor.SelectedValue))
        {
            filter.Paint = (PaintColor)Enum.Parse(typeof(PaintColor), ddlColor.SelectedValue);
            btnPanel2Next.Enabled = true;
        }
        else
        {
            filter.Paint = null;
            btnPanel2Next.Enabled = false;
        }
    }

    protected void btnPanel2Prev_Click(object sender, EventArgs e)
    {
        filter.Engine = null;
        filter.Paint = null;
        mvPanels.ActiveViewIndex--;
    }

    protected void btnPanel2Next_Click(object sender, EventArgs e)
    {
        mvPanels.ActiveViewIndex++;

        var selectedConfiguration = availableCars.Where(x => x.Brand == filter.Brand && x.Engine == filter.Engine
        && (!filter.Paint.HasValue || x.Paint == filter.Paint)).Distinct().FirstOrDefault();
        if (selectedConfiguration != null)
            lblResult.Text = String.Format("You have selected {0} {1} {2} for {3} PLN", selectedConfiguration.Brand,
                selectedConfiguration.Engine, selectedConfiguration.Paint, selectedConfiguration.Cost);
    }

    protected void btnPanel3Prev_Click(object sender, EventArgs e)
    {
        mvPanels.ActiveViewIndex--;
        CreateControlsOnPanel2(true, !filter.EngineOnly);

    }

    protected void btnPanel3Finish_Click(object sender, EventArgs e)
    {
    }
}

Ако имате допълнителни въпроси, ще се радвам да помогна