ASP.NET Web PDF Document Viewer/Editor Control Library

Ajax applications traditionally use HTML and its associated technologies such as DHTML and Cascading Style Sheets (CSS) to build user interfaces. Where the interfaces need to change dynamically, they typically call the server using XMLHTTPRequest, and the server delivers new markup language that gets inserted into the DOM and rerendered by the browser. For example, consider the case of implementing a financial portal using Ajax. You will see a full example of this in 11. When you change the company you want to inspect, several areas of the page should update. Figure 2-6 shows the basic application with a basic quote and a couple of charts. Consider the scenario where you want the user to decide whether they want to see more detailed quotation information for the company; if they do, they click a button to get it. You want this information to appear on the same page but don t want to refresh the whole page to get it you just want it to appear (see Figure 2-7). Even if your round-trip is fast, if you don t use an Ajax technique, your entire page will blink as the new data is rendered. The browser will clear and redraw the entire page, even though most of it doesn t need it.

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because the LINQ-based approaches offer the same benefit but are typically easier to use. So DataSets are somewhat out of favor today. The low-level ADO.NET data access interfaces were the main way to access data in .NET right up until .NET 3.5 and Visual Studio 2008 shipped, bringing LINQ.

Listing 1-14 showed you how to use constant iterators, but sometimes it is necessary to be able to modify the list as you iterate over it. Using STL-style iterators, this means skipping the const part of the name. For Java-style iteration, QMutableListIterator is used. Listing 1-15 shows iterating and modifying list contents using Qt classes: Listing 1-15. Modifying lists using iterators QList<int> list; list << 27 << 33 << 61 << 62; QMutableListIterator<int> javaIter( list ); while( javaIter.hasNext() ) { int value = javaIter.next() + 1; javaIter.setValue( value ); qDebug() << value; } QList<int>::Iterator stlIter; for( stlIter = list.begin(); stlIter != list.end(); ++stlIter ) { (*stlIter) = (*stlIter)*2; qDebug() << (*stlIter); } Listing 1-15 shows that the Java-style loop reads the next value using next and then sets the current value using setValue. This means that the loop in the listing increases all the values in the list by one. It also means that setValue should not be used before next has been called as the iterator; it then points at the non-existing value before the actual list.

As we saw in 8, LINQ lets you perform tasks with collections of data including filtering, sorting, and grouping. In that chapter, we were working only with objects, but these are exactly the jobs that databases are good at. Moreover, one of the motivations behind LINQ s design was to make it easier to use databases from code. As you can see in Example 14-2, LINQ blends data access seamlessly into C# code this database example looks very similar to the object examples we saw in the earlier chapter.

Database LINQ providers work very differently from the LINQ to Objects provider, even though queries use the same syntax for both In LINQ to Objects, a where clause does all of its work inside the NET Framework it s similar in action to a loop with an if statement But trying that with a database would be a disaster if your where clause is designed to select a single row out of 20 million, you absolutely don t want C# code to iterate through all 20 million rows! You want the database to do the filtering so that it can use its indexes to locate the row efficiently And it works exactly as you d want the LINQ where clause in Example 14-2 is effectively translated into a SQL WHERE clause.

As you may recall, C# converts a LINQ query expression into a series of method calls, and those method calls just end up building a query object that knows how to return the results LINQ uses deferred execution the query doesn t start returning results until you ask for them LINQ providers for databases do something similar, but instead of working directly with IEnumerable<T>, they use a specialized type that derives from IEnumerable<T>, called IQueryable<T> Since IQueryable<T> derives from IEnumerable<T>, you can still enumerate its contents in the usual ways, but it s only when you do this that it generates a suitable database query; it won t touch the database until you start asking to see elements.

So we still have deferred execution, but crucially, when you finally execute the query the complete chain of processing that your LINQ query represents is turned into a single SQL query so that the database can do all the work In short, whereas LINQ to Objects enumerates all the objects from the source and runs the chain of processing inside your NET application, database LINQ providers push the processing to the database..

Caution When modifying the list by removing or inserting items, the iterators might become invalid. Be

Example 14-2 uses LINQ to Entities a LINQ provider for the Entity Framework. The Entity Framework didn t appear until Service Pack 1 of Visual Studio 2008, and there s an older database LINQ provider called LINQ to SQL that appeared in the first Visual Studio 2008 release.