Generating button in backend ASP Repeater and then finding the control in c#

Question

So I create a button in the behind code using a function in an Repeater. And then, there's a button that should take these automatically generated buttons (if I check the checkbox in button A, I take the values of row A in repeater,same goes for button B and so on...).

I can see checking the code while debugging that the checkbox are generated correctly. So I should just be able to find it, but can not. Even tho I use the findControl but have not been able to do it.

Here is the code from the front:

<asp:Panel ID="mainPanel" runat="server">
<asp:Repeater ID="repStudents" runat="server">
....
 <%# GetButton(Eval("Role").ToString()) %>
.... 
 <asp:LinkButton runat="server" OnClick="showChosen" ClientIDMode="Static"
CausesValidation="true">Save</asp:LinkButton>

And here I generate the button and then try to show the chosen value:

    protected string GetButton(string status)
            {
                string love="";
    
                if(status == "Rel")
                {
                    love = "<input id='relBtn' type='checkbox' runat='server'/>";
                }
                else
                {
                    love = "<input id='rfBtn' checked type='checkbox' runat='server'/>";
            }
            return love;
        }


protected void showChosen(object sender, EventArgs e)
        {
            CheckBox cb = (CheckBox)(mainPanel.FindControl("relBtn"));
            if(cb.Checked)
                lblError.Text = "Checkbox is checked";
else
                lblError.Text = "Checkbox is not checked";
            divError.Visible = true;

All I keep getting is the Null Reference Exception, even tho, there is just one relBtn in the whole page. If I look into the page generated code, I can see the relBtn, but for some reason, I can not find the Checkbox.

Answer 1

Ok, unless you crossed the desert, climbed mountains, exhausted every "reasonable" alternative?

You have to ask WHY you want to write code to inject a button when THAT is the WHOLE idea of quite near EVERY control from repeater, listview, Gridview and more!

In other words, the need for writing code code, and a GOOD strong case has not been made yet.

and why do we need a huge spectacular earth shaking narrative as to why the approach is being used?

Because in 99% of cases, you don't need to do this, and the data aware controls and darn near the WHOLE asp.net system is designed around you NOT having to take that road.

So, if you have some button, or whatever? Just drop it into the repeater control, and it will "repeat" over and over for you!!!

So, say this simple repeater:

<asp:Repeater ID="Repeater1" runat="server"  >
    <ItemTemplate>                
        <div style="border-style:solid;color:black;width:320px;height:450px;float:left;margin-right:10px;margin-bottom:10px">
            <div style="text-align:center;padding:2px 10px 2px 10px" class="cards" >
                <asp:Button ID="cmdMyView" runat="server" Text="View" 
                    CssClass="btn-info" style="float:right"
                    CommandArgument = '<%# Eval("ID") %>'
                    OnClick="cmdMyView_Click" />
                <br />

                <h3 id="hFighter" runat="server"><%# Eval("Fighter") %></h3>
                <asp:Image ID="Image2" runat="server" 
                    ImageUrl = '<%# Eval("ImagePath") %>' Width="170" />
                <h4>Engine</h4>
                <asp:Label ID="EngineLabel2" runat="server" Text='<%# Eval("Engine") %>' />
                <h4>Description</h4>
                <asp:Label ID="DescLabel" runat="server" Text='<%# Eval("Description") %>' />
                <br /> 
            </div>
        </div> 
        </ItemTemplate>
</asp:Repeater>

Note the button and the plain jane click event for that button.

So, to fill the repeater, we have this:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Not IsPostBack Then
        LoadData()
    End If

End Sub

Sub LoadData()

    Using conn As New SqlConnection(My.Settings.TEST4)
        Dim strSQL As String =
            "SELECT * FROM Fighters"

        Using cmdSQL As New SqlCommand(strSQL, conn)
            conn.Open()
            Dim rstData As New DataTable
            rstData.Load(cmdSQL.ExecuteReader)
            Repeater1.DataSource = rstData
            Repeater1.DataBind()
        End Using
    End Using

End Sub

And now we see/get this:

And now that button click from above (the view button).

Protected Sub cmdMyView_Click(sender As Object, e As EventArgs)

    Dim btn As Button = sender
    Dim rRow As RepeaterItem = btn.NamingContainer

    Debug.Print("Row index = " & rRow.ItemIndex)
    Debug.Print("DATA ID pk = " & btn.CommandArgument)
    Dim hFighter As HtmlGenericControl = rRow.FindControl("hFighter")
    Debug.Print("Figher name = " & hFighter.InnerText)

End Sub

output:

Row index = 3
DATA ID pk = 4
Figher name = Lockheed Martin F-35 Lightning II

So, note how the the simple button click picks up the current row of data.

From that, we can use find control, or get the "row index" of the click, or in our case, we also included the database PK id in the button command arugment.

So, VERY hard to make a case to do all those "hand stands" to write code to "inject" a button when darn near ALL of the data aware controls will repeat the data AND the controls for you over and over with great ease, and in fact ZERO code to have such buttons or controls repeat for you.

there are RARE cases to write code to inject, but they should be the last resort, since in 99% of cases, no such code is required, and worse yet, when you post-back, such injected controls will NOT persist, and you have to re-inject on every post back for such pages to work.

Edit: So, this could be used

markup:

<asp:Repeater ID="Repeater1" runat="server">
    <ItemTemplate>
        <i id="iHotel" runat="server"><%# Eval("HotelName") %></i>
        <br />
        <asp:CheckBox ID="chkREL" runat="server"
            Checked='<%# Eval("Status").ToString() == "Rel" ? true : false %>' />
        <br />
        <asp:Button ID="Button1" runat="server" Text="Button"
            OnClick="Button1_Click" />
        <hr />
    </ItemTemplate>
</asp:Repeater>

Say code to load:

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            LoadData();
        }
    }

    void LoadData()
    {
        Repeater1.DataSource = General.MyRst("SELECT * FROM tblHotelsA");
        Repeater1.DataBind();   

    }

result is this:

Button click:

protected void Button1_Click(object sender, EventArgs e)
{
    Button btn = (Button)sender;
    RepeaterItem iRow = (RepeaterItem)btn.NamingContainer;
    Debug.Print($"Row click index = {iRow.ItemIndex}");

    CheckBox chkRel = (CheckBox)iRow.FindControl("chkREL");
    Debug.Print($"Check box checked = {chkRel.Checked}");

    HtmlGenericControl iHotel = (HtmlGenericControl)iRow.FindControl("iHotel");
    Debug.Print($"Hotel name from 'p' item = {iHotel.InnerText}");

}

output:

Row click index = 1
Check box checked = True
Hotel name from 'p' item = Ramada Lodge