C# in Depth

Translations using IQueryable and IQueryProvider327Figure 12.2Class diagram based on the interfaces involved in IQueryablewords, calling the GetEnumerator method) or by a call to the Execute method on anIQueryProvider, passing in an expression tree.So, with at least some grasp of what IQueryable is for, what is IQueryProvider?Well, we can do more than execute a query—we can also build a bigger query from it,which is the purpose of the standard query operators in LINQ. 2 To build up a query,we need to use the CreateQuery method on the relevant IQueryProvider. 3Think of a data source as a simple query (SELECT * FROM SomeTable in SQL, forinstance)—calling Where, Select, OrderBy, and similar methods results in a differentquery, based on the first one. Given any IQueryable query, you can create a new queryby performing the following steps:1 Ask the existing query for its query expression tree (using the Expression property).2 Build a new expression tree that contains the original expression and the extrafunctionality you want (a filter, projection, or ordering, for instance).2Well, the ones that keep deferring execution, such as Where and Join. We’ll see what happens with the aggregationssuch as Count in a little while.3Both Execute and CreateQuery have generic and nongeneric overloads. The nongeneric versions make iteasier to create queries dynamically in code. Compile-time query expressions use the generic version.Licensed to Rhona Hadida