C# in Depth

Translations using IQueryable and IQueryProvider331from user in userswhere user.Name.StartsWith("D")select user.NameQuery expression translationusers.Where(user => user.Name.StartsWith("D").Select(user => user.Name)Overload resolutionIQueryable not implementedIQueryable implementedExtension methods on Enumerableare chosen, which use delegatesas parametersExtension methods on Queryableare chosen, which use expressiontrees as parametersIL to create delegate instances, withcalls to Enumerable.Where andEnumerable.SelectIL to create expression trees, withcalls to Queryable.Where andQueryable.SelectFigure 12.3 A query taking two paths, depending on whether the data source implementsIQueryable or only IEnumerablepoint, the lambda expressions can be converted to either delegate instances or expressiontrees, potentially giving radically different implementations: typically in-memoryfor the left path, and SQL executing against a database in the right path.The second big difference between Enumerable and Queryable is that theEnumerable extension methods do the actual work associated with the correspondingquery operator. There is code in Enumerable.Where to execute the specified filter andonly yield appropriate elements as the result sequence, for example. By contrast, thequery operator “implementations” in Queryable do very little: they just create a newquery based on the parameters or call Execute on the query provider, as described atthe end of section 12.2.1. In other words, they are only used to build up queries andrequest that they be executed—they don’t contain the logic behind the operators. Thismeans they’re suitable for any LINQ provider that uses expression trees.With the Queryable extension methods available and making use of our IQueryableand IQueryProvider implementations, it’s finally time to see what happens when we usea query expression with our custom provider.Licensed to Rhona Hadida