How to recover from the error?

I am retrieving data from database and bind to a gridview & listview controls and here i need to display each "tags"(i.e, comma separated values) as hyperlinks.

string tags = string.Empty;
using (DataClass1DataContext DcContext = new DataClass1DataContext())
{
GridView1(or)ListView1.DataSource = DcContext.tblArticles
.Join(DcContext.tblCategories,
x => x.category,
y => y.categoryId,
(x, y) => new { x, y })
.Join(DcContext.tblSubCategories,
p => p.x.subcategory,
q => q.subcategoryId,
(p, q) => new
{
tags = p.x.keywords,
Article = p.x.articleId,
Published = p.x.articleDate,
Title = p.x.title,
Summary = p.x.description,
Labels = p.x.keywords,
Poster = p.x.image,
Category = p.y.Category,
Subcategory = q.subcategory,
});

What I need is?
The tags which i retrieved will need to display as related keywords in the form of hyperlinks and bind to the gridview/Listview.

<asp:GridView ID="GridView1" runat="server" Width="100%" AutoGenerateColumns="false">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:Image ID="PosterImg" runat="server" ImageUrl='<%#Eval("Poster") %>' Width="50" Height="50" />
ItemTemplate>
asp:TemplateField>
<asp:BoundField DataField="Published" HeaderText="Published on" />
<asp:BoundField DataField="Title" HeaderText="Article" />
<asp:BoundField DataField="Summary" HeaderText="Summary about your articles" />
<asp:TemplateField>
<ItemTemplate>
<asp:PlaceHolder ID="PlaceHolder1" runat="server">asp:PlaceHolder>
ItemTemplate>
asp:TemplateField>
<asp:BoundField DataField="Category" HeaderText="Category" />
<asp:BoundField DataField="Subcategory" HeaderText="Subcategory" />
Columns>
asp:GridView>
<asp:ListView ID="ListView1" runat="server">
<ItemTemplate>
<asp:Image ID="PosterImg" CssClass="thumb" runat="server" ImageUrl='<%#Eval("Poster") %>' Width="120" Height="150" />
<h4>
<asp:HyperLink ID="HyperLink2" NavigateUrl='<%#"?p="+Eval("Article") %>' runat="server"><%#Eval("Title") %>asp:HyperLink>
h4>
<cite>
Posted by
<asp:HyperLink ID="HyperLink3" NavigateUrl='<%#"?user="+Eval("Article") %>' runat="server">Usernameasp:HyperLink>
under
<span>
<asp:HyperLink ID="HyperLink4" NavigateUrl='<%#"?subcategory="+Eval("Subcategory") %>' runat="server"><%#Eval("Subcategory") %>asp:HyperLink>
·
<asp:HyperLink ID="HyperLink5" NavigateUrl='<%#"?category="+Eval("Category") %>' runat="server"><%#Eval("Category") %>asp:HyperLink>
span>
on
<time><%#Eval("Published") %>time>
cite><br />
<p>
<%#Eval("Summary") %>
p>
<span>
<asp:PlaceHolder ID="PlaceHolder1" runat="server">asp:PlaceHolder>
span>
ItemTemplate>
asp:ListView>

What I have tried:

Added a placeholder inside the item template of each control(i.e, GRIDVIEW and LISTVIEW)
and by using LINQ I retrieved it from database and assign it to local string variable.

string[] strArray = tags.Split(',');
foreach (var obj in strArray)
{
HyperLink TagLinks = new HyperLink();
TagLinks.NavigateUrl = "?tags=" + obj;
TagLinks.Text = obj;
PlaceHolder PlaceHolder1 = (PlaceHolder)GridView1.FindControl("PlaceHolder1");
PlaceHolder1.Controls.Add(TagLinks);
}

I received an error:

Object reference not set to an instance of an object.

How to resolve this?

Hi Guru Prakash,

At which line you are receiving NullReference Exception?

To me it looks like you should be getting error at line 07 in the last code snippet.

You can not find control by id directly in the grid view if the control is added in the template field because there can be multiple rows in the grid views. Finding control directly this way will return Null for sure.

You need to get hold of the row in the grid view and find control inside the row.

foreach(GridViewRow row in GridView1.Rows)
{
PlaceHolder placeHolder = row.FindControl("PlaceHolder1") as PlaceHolder;
if(placeHolder != null)
{
//Do your operation.
}
}

The same thing applies for finding control in ListView also. Instead of Rows, ListView will have Items property which you need to loop thru and find the control in that.

foreach(DataListItem item in ListView1.Items) {
{
PlaceHolder placeHolder = item.FindControl("PlaceHolder1") as PlaceHolder;
if(placeHolder != null)
{
//Do your operation.
}
}

Thanks and regards,

Chetan Ranpariya

2 Replies

Know the answer? Post it — somebody with the same question will find it here.

Nigel Fernandes9y ago
Can you share entire error message ? Along with stack trace ?

It also could be some database value is NULL and that why its showing object reference error.
0
Chetan Ranpariya9y ago
Hi Guru Prakash,

At which line you are receiving NullReference Exception?

To me it looks like you should be getting error at line 07 in the last code snippet.

You can not find control by id directly in the grid view if the control is added in the template field because there can be multiple rows in the grid views. Finding control directly this way will return Null for sure.

You need to get hold of the row in the grid view and find control inside the row.
foreach(GridViewRow row in GridView1.Rows)
{
PlaceHolder placeHolder = row.FindControl("PlaceHolder1") as PlaceHolder;
if(placeHolder != null)
{
//Do your operation.
}
}
The same thing applies for finding control in ListView also. Instead of Rows, ListView will have Items property which you need to loop thru and find the control in that.
foreach(DataListItem item in ListView1.Items) {
{
PlaceHolder placeHolder = item.FindControl("PlaceHolder1") as PlaceHolder;
if(placeHolder != null)
{
            //Do your operation.
}
}
Thanks and regards,
Chetan Ranpariya
0