Object reference not set to an instance of an object.

[JustRun]

Hi,

I'm developing a gridview its datasource is SqlDataSource. one of the Grid item is asp:Hyperlink ID = "lnkTopicTitle", I wanna deal with this Hyperlink from the C# code behind to send a parameter with the NavigateURL property.

    protected void grdTopicArch_DataBinding(object sender, EventArgs e)
    {
        HyperLink lnkTopicTitle = (HyperLink)grdTopicArch.FindControl("lnkTopicTitle");
        lnkTopicTitle.NavigateUrl = "~/ChurchSubjs/ArchiveTopics.aspx";
    }
 

I tried this code also with:

    protected void lnkTopicTitle_DataBinding(object sender, EventArgs e)
 

But i got this error
System.NullReferenceException: Object reference not set to an instance of an object.
In this line:
lnkTopicTitle.NavigateUrl = "~/ChurchSubjs/ArchiveTopics.aspx";

Any Ideas

[tlhintoq]

Line 3 makes a hyperlink named "lnkTopicTitle" based on the results of a .FindControl()
Line 4 uses that hyperlink.

So... What would happen if line 3 wasn't able to find any controls matching your search? It's result would be null. Null would be assigned to lnkTopicTitle. Thus lnkTopicTitle would not have been assigned to an object, which is your error.

Try putting a breakpoint on line 4. This will let you check the values of your variables before the line executes.

You want to get into the habit of assuming that everything is going to fail and be different than what you expect. Add enough error compensation and checking to handle failures. In this case something like...

 if (lnkTopicTitle != null)
{
// do your thing here
}
 

[Frinavale]

Could you please post the ASP code for your GridView (specifically the <columns> section).

[JustRun]

Yes I know, when i tried to put a break point , i found that lnkTopicTitle holds Null value, but i tried using FindControl() before and it was wotking very good.

I don't know why it gives me this null !!!

[JustRun]

<asp:GridView ID="grdTopicArch" runat="server" AllowPaging="True" AllowSorting="True"
                        AutoGenerateColumns="False" BackColor="White" BorderColor="#CCCCCC" BorderStyle="None"
                        BorderWidth="1px" CellPadding="3" DataKeyNames="TopicID" DataSourceID="sdsWeeklyTopic"
                        Width="520px" PageSize="20" OnDataBinding="grdTopicArch_DataBinding" OnDataBound="grdTopicArch_DataBound">
                        <Columns>
                            <asp:BoundField DataField="TopicID" HeaderText="TopicID" ReadOnly="True" SortExpression="TopicID"
                                Visible="False" />
                            <asp:TemplateField HeaderText="Title" SortExpression="TopicTitle">
                                <ItemTemplate>
                                    <asp:HyperLink ID="lnkTopicTitle" runat="server" Text='<%# Eval("TopicTitle") %>'
                                        OnDataBinding="lnkTopicTitle_DataBinding"></asp:HyperLink>
                                </ItemTemplate>
                                <HeaderStyle Font-Bold="True" Width="200px" />
                            </asp:TemplateField>
                            <asp:BoundField DataField="TopicDate" HeaderText="Date" SortExpression="TopicDate">
                                <HeaderStyle Width="150px" />
                            </asp:BoundField>
                            <asp:BoundField DataField="UserName" HeaderText="Editor" SortExpression="UserName">
                                <HeaderStyle Width="100px" />
                            </asp:BoundField>
                        </Columns>
                        <FooterStyle BackColor="White" ForeColor="#000066" />
                        <RowStyle ForeColor="#000066" />
                        <PagerStyle BackColor="White" ForeColor="#000066" HorizontalAlign="Left" />
                        <SelectedRowStyle BackColor="#669999" Font-Bold="True" ForeColor="White" />
                        <HeaderStyle Font-Bold="False" ForeColor="#00576D" HorizontalAlign="Center" VerticalAlign="Middle"
                            Height="50px" Font-Italic="False" />
                    </asp:GridView>
 

[Frinavale]

@JustRun
Is the "sender" parameter in this method the HyperLink itself?


Why don't you just set the NavaigateUrl directly in your asp code:

<asp:TemplateField HeaderText="Title" SortExpression="TopicTitle">
    <ItemTemplate>
        <asp:HyperLink 
                    ID="lnkTopicTitle" 
                    runat="server" 
                    Text='<%# Eval("TopicTitle") %>'
                    NavigateUrl = "~/ChurchSubjs/ArchiveTopics.aspx">
        </asp:HyperLink>
    </ItemTemplate>
    <HeaderStyle Font-Bold="True" Width="200px" />
</asp:TemplateField>                           
 

[JustRun]

I wanna send the parameter encrypted, like this:
lnkTopicTitle.NavigateUrl = "~/ChurchSubjs/ArchiveTopics.aspx?tid="+ Encryption.Encrypt(sdsWeeklyTopic.SelectCommand[0].ToString())

[tlhintoq]

@JustRun
.FindControl() has a couple overrides. One includes a bool telling the method whether or not to search child controls. So for example you could have it search just a panel... or the panel and all the controls placed on the panel such as tab pages etc.

Maybe when you first tested it, it worked on your testing control, but then you placed that on a panel, into a tab control, onto a page... somehow you nested it but you aren't looking inside the right control collection, so you get are getting null back now where you didn't before.

[bhupinder]

Hi,

You can find control on rowdatabound not in databinding.

 protected void gvdata_RowDataBound(object sender, GridViewRowEventArgs  e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            HyperLink lnkTopicTitle = (HyperLink)e.Row.Cells[1].FindControl("hyper");
            Response.Write(lnkTopicTitle.NavigateUrl);
            Response.End();
            //if (lnkTopicTitle != null)
            //{
 
 
            //    lnkTopicTitle.NavigateUrl = "~/default.aspx";
            //}
        }
    }

[Frinavale]

You know, I have a feeling that you don't need to use the FindControl method because I think that the "sender" parameter is the hyperlink......

I could be completely wrong here but please check it out.

[JustRun]

Yes, Thanks you bhupinder ,

It works now :)

[JustRun]

Thanks Frinavale, I tried the RowDataBound event, and it works.

Thank you.