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XLinq Part 2: Using Standard Query Operators : Page 2

XLinq's query power goes way beyond the now-common methods of retrieving XML and querying it with XPath.


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Using the "where" Operator
Beyond the basic query operators you just saw, XLinq has operators such as the important where operator that lets you filter query results based on supplied criteria. To demonstrate this, add a text box named txtValue and a button named btnGetProductNameForID to the page. As the name of the button suggests, clicking this button retrieves the product name for a specific product ID. The Click event code for the new button looks like this:

 
Figure 2. Displaying a Product Name for a Specific Product ID: In this example, when a users enters a product ID and clicks the button, the application returns the corresponding product name.

void btnGetProductNameForID_Click (object sender, EventArgs e) { XElement products = XElement.Load (Server.MapPath ("~/App_Data/Products.xml")); var productNames = from prod in products.Elements("Product") where (string)prod.Attribute("ID") == txtValue.Text select (string) prod.Element("Name"); gridResult.DataSource = productNames; gridResult.DataBind(); }

In this example, when a user enters a product ID into the txtValue text field, and clicks button, the "where" operator selects the product name that matches the supplied product ID. Figure 2 shows the example in action.

Using the "Take" Operator
Similar to the where operator, the Take operator provides you with a way to select all the elements before some supplied position. For example, the following code snippet shows how to select the first five product names from the Products XML file.

Author's Note: Although the syntax for the "where" operator is different from the "Take" operator—which is obviously a method—the documentation refers to both as "operators," so I have followed that lead in the terminology used for this article.

void btnGetTop5ProductNames_Click (object sender, EventArgs e) {

 
Figure 3. Filtering the First Five Product Names: Here's the page in a browser, displaying the first five product names from the products list.
XElement products = XElement.Load (Server.MapPath ("~/App_Data/Products.xml")); var productNames = (from prod in products.Elements("Product") orderby (string) prod.Element("Name") select (string) prod.Element("Name")).Take(5); gridResult.DataSource = productNames; gridResult.DataBind(); }
I won't go through the full description of how to set up an example page, but you can easily see what's involved by looking at Figure 3.

Note how adding the Take operator at the end of the query simply changes the output returned by the query. This just shows the power of LINQ as a query language.

Similarly, you can also get the unique list of product names using the Distinct operator.



var productNames = (from prod in products.Elements("Product") orderby (string) prod.Element("Name") select (string)prod.Element ("Name")).Distinct();

The Distinct operator eliminates duplicate product names from the resultant output. In addition to the Where, Take, and Distinct operators shown so far, LINQ exposes a number of other operators, the most important of which are shown in Table 1.

Table 1. Standard Query Operators: The table shows common query operators used for querying objects that are based on IEnumerable.
Operator Description
select Specifies the return value for the query.
where Allows you to filter the output.
First Retrieves the first member from the resultant list.
ElementAt Allows you to access the specific member at specified position.
Take Accesses the members before the specified position.
Skip Accesses the members after the specified position.
ToDictionary Allows you to return the results in a key pair value list.
orderby Allows sorting of the resultset in ascending order.
orderbyDescending Allows sorting of the resultset in descending order.
Reverse Allows you to reverse the order of sequence.
Distinct Allows you to filter duplicate members.
ToList Allows you to return the results as a list represented by List<T>.
Except Allows you to filter elements that are members of a specific set.



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