Posts Tagged ‘Databind’
DataItem in ASPX
Posted by: admin in Data Binding on June 24th, 2010
The following examples shows how to use the GridViewRow.DataItem property to retrieve a property of the underlying object to which the GridViewRow is bound when using the LinqDataSource control.
I have a GridView whose DataSouce is a LinqDataSource. The LinqDataSource has been set to retrieve all records from the Product entity in the Northwind Linq to SQL data model like so:
<asp:GridView ID=”GridView1″ runat=”server” AutoGenerateColumns=”False” DataSourceID=”LinqDataSource1″
OnRowDataBound=”GridView1_RowDataBound”>
<Columns>
<asp:BoundField DataField=”ProductID” HeaderText=”ProductID” ReadOnly=”True”></asp:BoundField>
<asp:BoundField DataField=”ProductName” HeaderText=”ProductName” ReadOnly=”True”></asp:BoundField>
</Columns>
</asp:GridView>
<asp:LinqDataSource ID=”LinqDataSource1″ runat=”server” ContextTypeName=”NorthwindDataContext”
TableName=”Products”>
</asp:LinqDataSource>
In this case, getting a property of the underlying data object is easy. The LinqDataSource is aware that each object in the result set is of Type Product (as defined in the Linq to SQL class) and creates it accordingly. So all we have to do is cast the DataItem to type Product and then retrieve the productName:
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
Product p = (Product) e.Row.DataItem;
string productName = p.ProductName;
}
}
The LinqDataSource also allows you to shape the data that is retrieved. In the example below, I have specified a declarative select to retrieve only a subset of the values from the Products entity – the ProductID and ProductName.
<asp:GridView ID=”GridView1″ runat=”server” AutoGenerateColumns=”False” DataSourceID=”LinqDataSource1″
OnRowDataBound=”GridView1_RowDataBound”>
<Columns>
<asp:BoundField DataField=”ProductID” HeaderText=”ProductID” ReadOnly=”True”></asp:BoundField>
<asp:BoundField DataField=”ProductName” HeaderText=”ProductName” ReadOnly=”True”></asp:BoundField>
</Columns>
</asp:GridView>
<asp:LinqDataSource ID=”LinqDataSource1″ runat=”server” ContextTypeName=”NorthwindDataContext”
Select=”new (ProductID, ProductName)” TableName=”Products”>
</asp:LinqDataSource>
Since we have defined a custom shape for our result, the LinqDataSouce will create an anonymous type for each object in the resultset like so:
x = {ProductID=17, ProductName=Alice Mutton}
Since we do not know what type to cast the DataItem to, we can use Reflection to retrieve the property. The DataBinder.Eval method already does this so we can retrieve the productName like so:
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
var x = e.Row.DataItem;
string productName = (string) DataBinder.Eval(e.Row.DataItem, “ProductName”);
}
}
How to use Container.DataItem
Reposting here for the benefit of asp.net developers and Google
The databinding expression <%# some expression %> is evaluated in the language of the page (VB, C#, etc.) This can have a big impact on the current syntax, so be very careful when you are looking at docs for the language you are using.
Container.DataItem is a runtime alias for the DataItem for this specific item in the bound list. For a grid which displays 10 rows of data, this is one row from the datasource. The actual type of DataItem is determined by the type of the datasource. For example, if the datasource is a Dataview, the type of DataItem is DataRowView. If the type of the datasource is an array of strings, the type of DataItem is String. If the datasource is a collection of strongly-typed objects (for example “Employees” objects), the type of DataItem is Employees.
Each of these cases requires a slightly different databinding expression, with further differences between VB and C#. In every case, you want the databinding expression to produce a string that can be displayed in the page.
Here are some examples:
Array of Strings:
VB/C# <%# Container.DataItem %>
Field from DataView:
VB <%# Container.DataItem(“EmployeeName”) %>
C# <%# ((DataRowView)Container.DataItem)["EmployeeName"] %>
Property from a collection of objects:
VB <%# Container.DataItem.CustomerName %>
C# <%# ((Customer)Container.DataItem).CustomerName %>
Non-String property from a collection of objects:
VB <%# CStr(Container.DataItem.CustomerID) %>
C# <%# ((Customer)Container.DataItem).CustomerID.ToString() %>
As you can see the syntax is tricky, especially for C#, which requires explicit casting. So we’ve provided aDataBinder.Eval() helper method that figures out the syntax for you and formats the result as a string. It’s really convenient, with a couple of caveats: it’s late bound (uses reflection at runtime to figure out the data types), and it only supports basic data types in the fields: string, int, datetime, etc.
You can use Eval instead of DataBinder.Eval in ASP.net 2.0
DataBinder.Eval takes 2 or 3 arguments. The first arg is the data object to bind to. In the case of DataGrid, DataList and Repeater, Container.DataItem is the single row. The second arg the string name of the field from the data object you which to display. DataBinder.Eval uses these two pieces of information to work out the rest of the expression syntax.
An optional third argument is a .NET formatting string. DataBinder.Eval will handle a single replacement for{0} only. So the example below:
<a href=’<%# “default.aspx?CategoryId=” + Cstr(Databinder.Eval(Container.DataItem, “ID”))%>’>
could be simplified to:
<a href=’<%# Databinder.Eval(Container.DataItem,”ID”,”default.aspx?CategoryId={0}” ) %>’>
Wrapping DataBinder.Eval in CStr() is unnecessary as the compiler will wrap the statement with a Convert.ToString like this:
control1.SetDataBoundString(0, Convert.ToString(DataBinder.Eval(item1.DataItem, “ID”)));
Best of all, the Databinder.Eval syntax is the same for VB and C#.