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Using Observable Providers

By implementing the IQbservable interface and using the factory extension methods provided by the Qbservable type, you can write a custom LINQ provider to query any type of external data, so that these data are treated as sequences that can be subscribed to. For example, the LINQ to WQL sample in the Rx MSDN Developer Center shows how to build a simple provider for querying WMI Events using WQL. You can use the factory LINQ operators provided by the Qbservable type to abstract a sequence of WMI events and query, filter and compose them. Subscribing to this sequence will trigger the translation of the LINQ query expression into the target language, in this case WQL. 

Using the IQbservable interface to query external data

When we mention that we want to query for data, we are first concerned about what we want to query. This can be a pulled-based IEnumerable collection, or a push-based asynchronous Observable sequence. We also want to know where (under which context) do we want to execute the query. For Observable sequences, that is handled by the IScheduler interface and its various Scheduler implementation types. Finally, we want to know how we do the query. We can represent a query (a lambda expression) in verbatim (compiled into .NET intermediate language (IL) code), in which each operator in the query will be evaluated in a linear fashion. This is the case for the factory operator methods of the Observable type. Or you can represent your query using expression trees, which can be traversed to get the represented algorithm (e.g., predicting whether an item is greater than a value, etc.), then translate the algorithm into some domain-specific code, such as a T-SQL query statement for querying a SQL database, specific HTTP requests for a particular Web service URI, PowerShell commands, DSQLs for cloud notification services, etc. This is the case for the factory operator methods of the Qbservable type. The translated domain-specific code can be executed in a remote target system, or you can use the expression tree representation to do local query optimization.

Just like IObservable/IObserver is a dual of IEnumerable/IEnumerator, IQbservable is the dual of IQueryable and provides an expression tree representation of an IObservable query. You can change between IQbservable and IObservable types by using the AsQbservable and AsObservable methods. Calling AsQbservable produces an expression tree made up of a single node that calls the original IObservable instance. This relationship is important for understanding why a complete IQbservable query has to be defined starting from an IQbservable sequence and cannot be obtained simply by calling AsQbservable on an existing IQbservable query. In the following example, the call to AsQbservable produces a complete query tree only when you build the query by applying IQbservable AsQbservable to the data source.


var source = Observable.Interval(TimeSpan.FromSeconds(1));
var q = source.AsQbservable();
Console.WriteLine(q.ToString());
var sub = q.Subscribe(Console.WriteLine);
Console.ReadKey();

The IQbservable interface is intended for implementation by query providers. It is only supposed to be implemented by providers that also implement IQbservable<T>. If the provider does not also implement IQbservable<T>, the standard query operators cannot be used on the provider's data source. The IQbservable interface inherits the IObservable interface so that if it represents a query, the results of that query can be subscribed to. Subscription and publication causes the expression tree associated with an Qbservable object to be executed. The definition of "executing an expression tree" is specific to a query provider. For example, it may involve translating the expression tree to an appropriate query language for the underlying data source. The Expression property encapsulates the expression tree that is associated with the IQbservable instance, whereas the Provider encapsulates the query provider that is associated with the data source.

The set of methods declared in the Qbservable class provides an implementation of the standard query operators for querying data sources that implement IQbservable. The standard query operators are general purpose methods that follow the LINQ pattern and enable you to express traversal, filter, and projection operations over data in any .NET-based programming language. The majority of the methods in this class are defined as extension methods that extend the IQbservable type. This means they can be called like an instance method on any object that implements IQbservable. These methods that extend IQbservable do not perform any querying directly. Instead, their functionality is to build an Expression object, which is an expression tree that represents the cumulative query. The methods then pass the new expression tree to the CreateQuery method. The actual query execution on the target data is performed by a class that implements IQbservable.

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