Chapter64:IQueryableinterface

Section64.1:TranslatingaLINQquerytoaSQLquery

The IQueryableand IQueryable<T>interfaces allows developers to translate a LINQ query (a ‘language-integrated’ query) to a specific datasource, for example a relational database. Take this LINQ query written in C#:

varquery=frombookinbooks

wherebook.Author==”StephenKing”selectboo

k;

IfthevariablebooksisofatypethatimplementsIQueryable<Book>thenthequeryabovegetspassedtothe provider (set on the IQueryable.Providerproperty) in the form of an expression tree, a data structure that reflects the structure of the code.

Theprovidercaninspecttheexpressiontreeatruntimetodetermine:

• that there is a predicate for the Authorproperty of the Bookclass;
• that the comparison method used is ‘equals’ (==);
• thatthevalueitshouldequalis”StephenKing”.

WiththisinformationtheprovidercantranslatetheC#querytoaSQLqueryatruntimeandpassthatquerytoa relationaldatabasetofetchonlythosebooksthatmatchthepredicate:

SELECT*

FROMBooks

WHEREAuthor=’StephenKing’

Theprovidergetscalledwhenthequeryvariableisiteratedover(IQueryableimplementsIEnumerable).

(Theproviderusedinthisexamplewouldrequiresomeextrametadatatoknowwhichtabletoqueryandtoknow howtomatchpropertiesoftheC#classtocolumnsofthetable,butsuchmetadataisoutsideofthescopeofthe IQueryableinterface.)