Object reference not set to an instance of an object. - Visual Basic (Web)

Question

So, I'm having a slight problem with my code.

I was working on a small school project (recreation of book library) and encountered a problem which I cannot get a grasp of.

So, the user can search for a list of books, and after the list is populated (via DataList control), a small button "Book now" (Reserve) appears which user can click and reserve his book.

This is the code for the DataList control that resides in "Search.aspx"

<asp:DataList ID="DataList1" runat="server" DataKeyField="knjigaId" DataSourceID="SearchSqlDataSource" CssClass="searchControl">
                <ItemTemplate>
                <div class="pictureOfFoundBook">
                    <asp:Image ID="pictureOfFoundBook_imageLink" runat="server" ImageUrl='<%#"~/GetImage.aspx?knjigaId=" & Eval("knjigaId") & "&img=naslovnica" %>' />
                </div>
                <div class="descriptionOfFoundBook">
                    Naziv: <asp:Label ID="Label1" runat="server" Text='<%# Eval("naziv") %>' /><br />
                    Godina izdanja: <asp:Label ID="Label2" runat="server" Text='<%# Eval("godinaIzdanja") %>' /><br />
                    Broj stranica : <asp:Label ID="brojStranicaLabel" runat="server" Text='<%# Eval("brojStranica") %>' /><br />
                    Izdava&#269;: <asp:Label ID="NazivIzdavacaLabel" runat="server" Text='<%# Eval("NazivIzdavaca") %>' /><br />
                    Vrsta tiskovine : <asp:Label ID="NazivVrsteTiskovineLabel" runat="server" Text='<%# Eval("NazivVrsteTiskovine") %>' /><br />
                    Kategorija: <asp:Label ID="NazivKategorijeLabel" runat="server" Text='<%# Eval("NazivKategorije") %>' /><br /><br />
                    <asp:HyperLink ID="foundBookEditHL_adminOnly" runat="server" NavigateUrl='<%# "~/admin/knjigeEdit.aspx?knjigaId=" & Eval("knjigaId") %>'>Uredi knjigu</asp:HyperLink><br />
                    <asp:Button ID="rezervacijeButton" runat="server" Text="Rezerviraj" OnClick="rezervacijaClick" CommandArgument='<%# Eval("knjigaId") %>'/><br />
                    <asp:Label ID="rezStatusLabel" runat="server"></asp:Label>
                    <asp:PlaceHolder ID="rezStatusPlaceholder" runat="server"></asp:PlaceHolder>
                </div>
                <hr />
                </ItemTemplate>
            </asp:DataList>

I've set the DataList1 control as a Friend sub so I can access the controls in it from another sub;

Friend Sub DataList1_ItemCreated(sender As Object, e As System.Web.UI.WebControls.DataListItemEventArgs) Handles DataList1.ItemCreated
    End Sub

I was trying to do the following; on the click of a button "rezervacijeButton", a function "rezervacijaClick" runs, which populates the table in the database.

Protected Sub rezervacijaClick(sender As Object, e As System.EventArgs)
            Dim Conn As New System.Data.SqlClient.SqlConnection(ConfigurationManager.ConnectionStrings("LocalSqlServer").ToString)
            Dim cmd As New System.Data.SqlClient.SqlCommand
            Dim sql As New StringBuilder
            Dim rezstatus As Label = DataList1.FindControl("rezStatusLabel")

            sql.Append("INSERT INTO rezervacije(UserName, knjigaId) VALUES (@UserName, @knjigaId)")

            Dim buttonsender As Button = sender
            cmd.Parameters.AddWithValue("UserName", User.Identity.Name)
            cmd.Parameters.AddWithValue("knjigaId", buttonsender.CommandArgument)

            Conn.Open()
            cmd.CommandText = sql.ToString
            cmd.Connection = Conn
            cmd.ExecuteNonQuery()
            Conn.Close()

            buttonsender.Visible = False
            rezstatus.Text = "aaa"

            'Try
            '    rezstatus.Text = "testing..."
            'Catch ex As Exception
            '    exlabel.Text = "POGREŠKA"
            '    exlabel.ForeColor = Drawing.Color.Red
            'End 
        End Sub

The next thing I wanted to do in the function "rezervacijaClick" was to set the text value of the Label (with ID "rezStatusLabel", which resides inside the DataList1 control) to "SOME TEXT" after the "Reserve" button is clicked.

But after the button click, I get the following error :

 Object reference not set to an instance of an object. 
Line 21: 
Line 22:         buttonsender.Visible = False
Line 23:         rezstatus.Text = "aaa"
Line 24: 
Line 25:         'Try

Sean Airey · Accepted Answer · 2013-10-03 13:48:14Z

1

Your rezstatus object is Nothing (null).

This is happening because you aren't looking in the right place for your label.

Each record of data you bind to the DataList will create a new hierarchy of controls, containers that hold the other controls you have defined in your ItemTemplate.

The immediate descendants of DataList1 will be a collection of DataListItem objects and then you will have your controls inside those.

Since we don't know for sure (unless you know you are only binding one record to the DataList) which DataListItem the desired label will be in, we simply walk backwards up the control tree and find the label from there.

Because you are responding to a button click event in your rezervacijaClick method, the parameter sender will be of type Button and will come from rezervacijeButton, so we can use that information to find your label:

Dim clickedButton As Button = CType(sender, Button) 'convert the sender parameter to the correct type: Button
Dim buttonParent As Control = clickedButton.Parent 'get a reference to the button's parent control
Dim rezstatus As Label = CType(buttonParent.FindControl(""), Label) 'find the label by ID and convert it to a Label as the return type of FindControl is Control

answered Oct 3, 2013 at 13:48

Sean Airey

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6 Comments

Josh Darnell Over a year ago

Good explanation of the OPs misunderstanding how the structure of this control.

Sean Airey Over a year ago

Thanks, I learned this the hard way, it can be confusing at times.

valaya Over a year ago

Excellent, wasn't even aware I could trace back to parent control :/

Sean Airey Over a year ago

@valaya A habit I've got into, which helps immensely when trying to figure problems out, is to open up the documentation page for the specific class I'm working with on MSDN and have a scroll down the list of properties, methods and events to get an idea of what's available to me.

valaya Over a year ago

@Sean Yea, I was trying to do that, but I gotta admit I'm kinda lost whilst searching, especially since I've started with ASP a week ago.

|

Josh Darnell · Accepted Answer · 2013-10-03 14:05:08Z

I would recommend that, instead of using the Button click event, you use the DataList.ItemCommand event. It will make your life a lot easier. This event fires whenever a Button is clicked within a row of your DataList control.

This way, you get the index passed in through the DataListCommandEventArgs parameter. then you would just need to update your DataList markup to add the event handler:

<asp:DataList ID="DataList1" runat="server" DataKeyField="knjigaId" 
    DataSourceID="SearchSqlDataSource" CssClass="searchControl"
    ItemCommand="DataList1_ItemCommand" >

And your handler code would look like this:

Protected Sub DataList1_ItemCommand(sender As Object, e As System.EventArgs)
    Dim Conn As New System.Data.SqlClient.SqlConnection(ConfigurationManager.ConnectionStrings("LocalSqlServer").ToString)
    Dim cmd As New System.Data.SqlClient.SqlCommand
    Dim sql As New StringBuilder

    Dim rezstatus As Label = e.Item.FindControl("rezStatusLabel")

    sql.Append("INSERT INTO rezervacije(UserName, knjigaId) VALUES (@UserName, @knjigaId)")

    Dim buttonsender As Button = e.Item.FindControl("rezervacijeButton")
    cmd.Parameters.AddWithValue("UserName", User.Identity.Name)
    cmd.Parameters.AddWithValue("knjigaId", buttonsender.CommandArgument)

    Conn.Open()
    cmd.CommandText = sql.ToString
    cmd.Connection = Conn
    cmd.ExecuteNonQuery()
    Conn.Close()

    buttonsender.Visible = False
    rezstatus.Text = "aaa"
End Sub

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