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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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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.

Access to the Developers action will only be allowed to users in the admins or developers roles all other users (authenticated or not) will be issued a 401 response code and, using ASP.NET s forms authentication, will be redirected to the login page. Now that we ve seen a few examples of how AuthorizeAttribute is used, let s talk about how it works.

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.

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.

Figure 1-8. Click the Dashboard icon, and click the Customize Dashboard link to add blocks to the Dashboard.

AuthorizeAttribute checks the IPrincipal associated with the current HttpContext. When Users or Roles is specified, it ensures the IPrincipal s username is in the allowed usernames or is a member of one of the granted roles. The AuthorizeAttribute can be used in a few ways:

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.

It s not a scenario C# attempts to help you with you end up having to construct the expression trees without the compiler s assistance This is not for the fainthearted).

The practice of stitching together strings to form dynamic queries is messy and can be fraught with security issues such as injection attacks. It s occasionally useful if you understand the risks and can mitigate them, but you need to exercise extreme caution.

If AuthorizeAttribute is applied to a controller, it s applied to every action in that controller. If multiple AuthorizeAttributes are applied to an action, all checks occur and the user must be authorized by all of them. AuthorizeAttribute implements a special interface called IAuthorizationFilter. When applied to an action, an IAuthorizationFilter will execute before any other action filters, and before the normal result. Listing 11.4 shows the declaration of IAuthorizationFilter.