.NET/ASP.NET/C#/VB.NET PDF Document SDK

Obviously, the first parameter is now an IQueryable<T>, but the second parameter s type is also different rather than accepting a delegate for a predicate function, it now takes an expression. The Expression<T> type is a special type recognized by the C# compiler (and also the VB.NET compiler). When you call methods that expect an Expression<Func<T, TResult>>, you just provide a normal lambda like you can with an ordinary Func<T,TResult>, but the compiler builds a data structure that is essentially an abstract syntax tree for the expression in effect, an object model describing that structure of the code. So while this version of the Where operator looks completely normal when we use it from a LINQ query, it will be passed a description of the expression, rather than a delegate to a method. (An Expression<T> isn t compiled into a real method, whereas an ordinary Func delegate would be. However, expressions offer a Compile method, so you can turn them into code at runtime if necessary.) The database LINQ providers, like the EF and LINQ to SQL, use this description to work out what the database query s WHERE clause should look like.

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Until now, you have been testing only parts of the interfaces of classes that are shipped with Qt. Now you ll create a unit test for the ImageCollection class from 13.

While LINQ to Entities is a very convenient way to build queries, it s just a layer on top of the Entity Framework s underlying query system, which has its own query language.

The Entity Framework defines a query language for making queries against the conceptual model rather than running queries against a database, as you do in normal SQL dialects, you can run queries that work directly against the entities in your model, as the name Entity SQL (or ESQL) suggests.

ESQL can be used for queries against the EDM storage model too. So it can also function as a kind of vendor-neutral SQL. But here we re focusing on queries that target the conceptual model.

Before looking at the unit test class, let s quickly review the ImageCollection class, which is used to keep images and tags. It is possible to add new images, add tags to images, retrieve all tags, retrieve all ids for images matching a set of tags, and get a specific image from an id. The available methods are listed as follows: QImage getImage(int id): Gets an image from a given id. QList<int> getIds(QStringList tags): Retrieves the ids for the images matching any of the tags specified. If no tags are specified, the method returns all ids. QStringList getTags(): Retrieves a list of all tags. addTag(int id, QString tag): Adds a tag to a given image. addImage(QImage image, QStringList tags): Adds an image to the collection with the given tags.

We all know the web browser by now, and it is arguably the most ubiquitous application used on modern computers, displacing solitaire and the word processor for this storied achievement! But the web browser ultimately became much more than just a new means for abstracting the textual nature of the client/server network It became an abstraction on top of the operating system on which applications could be written and executed (see Figure 1-5) This was, and is, important As long as applications are written to the specification defined by that abstraction, they should be able to run anywhere without further intervention or installation on behalf of the application developer Of course, the browser would have to be present on the system, but the value proposition of having a web browser available to the operating system was extremely important and ultimately launched many well-known legal battles.

Why do we need a second way of making queries when we already have LINQ to Entities Well, from a historical perspective that question has things back to front: during the Entity Framework s development, ESQL was around long before LINQ to Entities But since LINQ to Entities made it into the first version of the EF, it s still reasonable to ask why we have both, what ESQL is for, and when it might look like a better choice than LINQ ESQL s main benefit is that it s sometimes useful to be able to represent a query as text In fact, the Entity Data Model itself exploits this there are some advanced scenarios in which ESQL queries can be embedded in the edmx file LINQ wouldn t be an option here because edmx is just XML; to use LINQ requires a language that supports LINQ.

If you wanted to store custom queries in a configuration file, you really wouldn t want to have to run the C# compiler at runtime to interpret the queries And with ESQL you don t need to you can represent a query as a string and the EF can execute that for you at runtime Another feature of a string-based query language is that it s relatively easy to compose queries at runtime With a LINQ query expression, the structure is fixed at compile time and you only really get to tweak individual arguments, much like a fixed SQL query with a few named arguments (Technically, it is actually possible to build LINQ queries dynamically After all, LINQ operators are chained together with simple function calls However, dynamic composition of Expression<T> trees turns out to be surprisingly difficult.