C# in Depth

362 APPENDIX LINQ standard query operatorsTable A.3Conversion examples (continued)Expression// Key is first character of wordwords.ToLookup(word => word[0])words.ToDictionary(word => word[0])ResultLookup contents:'z': "zero"'o': "one"'t': "two", "three"'f': "four"Exception: Can only have one entry per key, sofails on 't'I haven’t provided examples for AsEnumerable or AsQueryable because they don’taffect the results in an immediately obvious way. Instead, they affect the manner in whichthe query is executed. Queryable.AsQueryable is an extension method on IEnumerablethat returns an IQueryable (both types being generic or nongeneric, dependingon which overload you pick). If the IEnumerable you call it on is already an IQueryable,it just returns the same reference—otherwise it creates a wrapper around the originalsequence. The wrapper allows you to use all the normal Queryable extension methods,passing in expression trees, but when the query is executed the expression tree is compiledinto normal IL and executed directly, using the LambdaExpression.Compilemethod shown in section 9.3.2.Enumerable.AsEnumerable is an extension method on IEnumerable and has atrivial implementation, simply returning the reference it was called on. No wrappersare involved—it just returns the same reference. This forces the Enumerable extensionmethods to be used in subsequent LINQ operators. Consider the followingquery expressions:// Filter the users in the database with LIKEfrom user in context.Userswhere user.Name.StartsWith("Tim")select user;// Filter the users in memoryfrom user in context.Users.AsEnumerable()where user.Name.StartsWith("Tim")select user;The second query expression forces the compile-time type of the source to beIEnumerable instead of IQueryable, so all the processing is done inmemory instead of at the database. The compiler will use the Enumerable extensionmethods (taking delegate parameters) instead of the Queryable extension methods(taking expression tree parameters). Normally you want to do as much processing aspossible in SQL, but when there are transformations that require “local” code, yousometimes have to force LINQ to use the appropriate Enumerable extension methods.Licensed to Rhona Hadida