Consider this Asp.net page code:
<head runat="server">
<title></title>
<script type="text/javascript">
function showhide(master, detail) {
var src = $(master).children()[0].src;
if (src.endsWith("plus.png"))
src = src.replace('plus.png', 'minus.png');
else
src = src.replace('minus.png', 'plus.png');
$(master).children()[0].src = src;
$(detail).slideToggle("normal");
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
<Scripts>
<asp:ScriptReference Path="~/scripts/jquery-1.6.2.min.js" ScriptMode="Release" />
</Scripts>
</asp:ScriptManager>
<div>
<asp:SqlDataSource ID="sqlDsCustomers" runat="server" ConnectionString="<%$ ConnectionStrings:Northwind %>"
SelectCommand="SELECT [Customers].[CustomerID], [Customers].[CompanyName], COUNT([OrderID]) TotalOrders FROM [Customers] INNER JOIN [Orders] ON [Customers].[CustomerID]=[Orders].[CustomerID] Group By [Customers].[CustomerID], [Customers].[CompanyName]">
</asp:SqlDataSource>
<asp:GridView Width="100%" AllowPaging="True" ID="gvCustomers" AutoGenerateColumns="False"
DataSourceID="sqlDsCustomers" runat="server" ShowHeader="False" OnRowCreated="gvCustomers_RowCreated">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<div class="group" id='<%#String.Format("customer{0}",Container.DataItemIndex) %>'
onclick='showhide(<%#String.Format("\"#customer{0}\"",Container.DataItemIndex) %>,<%#String.Format("\"#order{0}\"",Container.DataItemIndex) %>)'>
<asp:Image ID="imgCollapsible" CssClass="first" ImageUrl="~/Assets/img/plus.png"
Style="margin-right: 5px;" runat="server" /><span class="header">
<%#Eval("CustomerID")%>
:
<%#Eval("CompanyName")%>
(<%#Eval("TotalOrders")%>Orders) </span>
</div>
<div id='<%#String.Format("order{0}",Container.DataItemIndex) %>' class="order">
<asp:GridView AutoGenerateColumns="false" CssClass="grid" ID="ddd" runat="server"
ShowHeader="true" EnableViewState="false">
<RowStyle CssClass="row" />
<AlternatingRowStyle CssClass="altrow" />
<Columns>
<asp:TemplateField ItemStyle-CssClass="rownum">
<ItemTemplate>
<%# Container.DataItemIndex + 1 %>
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField HeaderText="Order ID" DataField="OrderID" ItemStyle-Width="80px" />
<asp:BoundField HeaderText="Date Ordered" DataField="OrderDate" DataFormatString="{0:MM/dd/yyyy}"
ItemStyle-Width="100px" />
<asp:BoundField HeaderText="Date Required" DataField="RequiredDate" DataFormatString="{0:MM/dd/yyyy}"
ItemStyle-Width="110px" />
<asp:BoundField HeaderText="Freight" DataField="Freight" DataFormatString="{0:c}"
ItemStyle-Width="50px" ItemStyle-HorizontalAlign="Right" />
<asp:BoundField HeaderText="Date Shipped" DataField="ShippedDate" DataFormatString="{0:MM/dd/yyyy}"
ItemStyle-Width="100px" />
</Columns>
</asp:GridView>
</div>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</div>
<asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
</form>
and the code behind:
protected void Page_Load(object sender, EventArgs e)
{
}
protected void gvCustomers_RowCreated(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
string custID = ((DataRowView)e.Row.DataItem)["CustomerID"].ToString();
using (DataClassesDataContext dc=new DataClassesDataContext())
{
List<Order> ord = (from o in dc.Orders
where o.CustomerID == custID.Trim()
select o).ToList();
GridView ctrl = e.Row.FindControl("ddd") as GridView;
ctrl.DataSource = ord;
}
}
}
protected void Button1_Click(object sender, EventArgs e)
{
Response.Redirect("Default.aspx");
}
the problem is when I Click on Button1 to redirect to another page gvCustomers_RowCreated raise and I get Null reference error.Why this event raised after postback?
EDIT 1):
I removed SqlDataSource and bind GridView in the code behind like this:
public partial class Default2 : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
using (DataClassesDataContext dc=new DataClassesDataContext())
{
var query = dc.ExecuteQuery<clsRetern>("SELECT [Customers].[CustomerID], [Customers].[CompanyName], COUNT([OrderID]) TotalOrders FROM [Customers] INNER JOIN [Orders] ON [Customers].[CustomerID]=[Orders].[CustomerID] Group By [Customers].[CustomerID], [Customers].[CompanyName]").ToList();
List<clsRetern> ret = new List<clsRetern>();
foreach (var item in query)
{
clsRetern r = new clsRetern();
r.CompanyName = item.CompanyName;
r.CustomerID = item.CustomerID;
r.TotalOrders = item.TotalOrders;
ret.Add(r);
}
gvCustomers.DataSource = ret;
gvCustomers.DataBind();
}
}
}
protected void gvCustomers_RowCreated(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
string custID = ((clsRetern)e.Row.DataItem).CustomerID.Trim();
using (DataClassesDataContext dc=new DataClassesDataContext())
{
List<Order> ord = (from o in dc.Orders
where o.CustomerID == custID.Trim()
select o).ToList();
GridView ctrl = e.Row.FindControl("ddd") as GridView;
ctrl.DataSource = ord;
}
}
}
protected void Button1_Click(object sender, EventArgs e)
{
Response.Redirect("Default.aspx",true);
}
}
public class clsRetern
{
public string CustomerID { get; set; }
public string CompanyName { get; set; }
public int TotalOrders { get; set; }
}
I tried Response.Redirect("Default.aspx"); but the problem still remains.
If you understand ASP.NET page model then this question should not come to you. Every time request goes to ASP.NET page, a new page object is created along with entire control tree and the state is managed using view-state in post-back scenarios. So in your case, whenever post-back happens, a new page and grid-view is created. The data for the gid-view would be persisted in thew view-state and grid would be bound to that data.
The
RowCreatedevent is raised whenever grid row is created regardless of whetherDataBindis explicitly called or not. The intent of the event is so that one can tweak with UI (grid-view cells) – e.g. some controls can be pushed into grid-view cells if required. The same should happen regardless of post-back scenario other wise those dynamic controls will not get created. Typically, these controls will restore their state using view-state and your get your grid-view UI (control tree) back as it was in first page cycle.