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Painting QR in Visual C#.NET Part IV

Part IV
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Infrastructure of the Application
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Caching the Output of User Controls
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User controls are not only good at modularizing your user interface, they re also great at caching portions of Web pages. User controls, therefore, fully support the @OutputCache directive, although they do so with some differences with ASP.NET pages, as outlined in Table 18-8. A page that contains some dynamic sections cannot be cached entirely. What if the page also contains sections that are both heavy to compute and seldom updated In this case, you move static contents to one or more user controls and use the user control s @OutputCache directive to set up output caching. To make a user control cacheable, you declare the @OutputCache attribute using almost the same set of attributes we discussed earlier for pages. For example, the following code snippet caches the output of the control that embeds it for one minute:
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<% @OutputCache Duration="60" VaryByParam="None" %>
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The Location attribute is not supported because all controls in the page share the same location. So if you need to specify the cache location, do that at the page level and it will work for all embedded controls. The same holds true for the VaryByHeader attribute. The output of a user control can vary by custom strings and form parameters. More often, though, you ll want to vary the output of a control by property values. In this case, use the new VaryByControl attribute. Note A user control is made cacheable in either of two ways: by using the @OutputCache
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directive, or by defining the PartialCaching attribute on the user control s class declaration in the code-behind file, as follows:
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[PartialCaching(60)] public partial class CustomersGrid : UserControl { ... }
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The PartialCaching attribute allows you to specify the duration and values for the VaryByParam, VaryByControl, and VaryByCustom parameters.
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Vary by Controls
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The VaryByControl attribute allows you to vary the cache for each specified control property. For user controls, the property is mandatory unless the VaryByParam attribute has been specified. You can indicate both VaryByParam and VaryByControl, but at least one of them is required.
18
ASP.NET Caching
The following user control displays a grid with all the customers in a given country/region. The country/region is specified by the user control s Country property.
<%@ Control Language="C#" CodeFile="CustomersGrid.ascx.cs" Inherits="CustomersGridByCtl" %> <%@ OutputCache Duration="30" VaryByControl="Country" %> <h3><%= DateTime.Now.ToString() %></h3> <asp:ObjectDataSource ID="ObjectDataSource1" runat="server" TypeName="Core35.DAL.Customers" SelectMethod="LoadByCountry"> </asp:ObjectDataSource> <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false"> <Columns> <asp:BoundField DataField="ID" HeaderText="ID" /> <asp:BoundField DataField="CompanyName" HeaderText="Company" /> <asp:BoundField DataField="ContactName" HeaderText="Contact" /> <asp:BoundField DataField="Country" HeaderText="Country" /> </Columns> </asp:GridView>
Here is the code file of the user control:
public partial class CustomersGridByCtl : System.Web.UI.UserControl { public String Country; protected void Page_Load(Object sender, EventArgs e) { if (!String.IsNullOrEmpty(Country)) { ObjectDataSource1.SelectParameters.Add("country", Country); GridView1.DataSourceID = "ObjectDataSource1"; } } }
The @OutputCache directive caches a different copy of the user control output based on the different values of the Country property. Figure 18-9 shows it in action.
Part IV
Infrastructure of the Application
FIGURE 18-9 Two pages created at different moments use the same user control output, as you can see from the creation time of the grid.
The strings you assign to VaryByControl can be properties of the user controls as well as ID property values for contained controls. In this case, you ll get a distinct cached copy for each distinct combination of property values on the specified control.
The Shared Attribute
In Figure 18-9, you see two instances of the same page sharing the cached output of a user control. Try the following simple experiment. Make a plain copy of the page (say, page1.aspx), and give it another name (say, page2.aspx). You should have two distinct pages that generate identical output. In particular, both pages contain an instance of the same cacheable user control. Let s say that the cache duration of the user control is 30 seconds. As the next step of the experiment, you open both pages at different times while the cache is still valid. Let s say you open the second page ten seconds later than the first. Interestingly enough, the two pages no longer share the same copy of user control output, as Figure 18-10 documents.
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