C# in Depth

Translations using IQueryable and IQueryProvider329}}return Enumerable.Empty().GetEnumerator();IEnumerator IEnumerable.GetEnumerator(){Logger.Log(this, Expression);return Enumerable.Empty().GetEnumerator();}public override string ToString(){return "FakeQuery";Overrides for}sake of loggingEThe property members of IQueryable are implemented in FakeQuery with automaticproperties B, which are set by the constructors. There are two constructors: a parameterlessone that is used by our main program to create a plain “source” for the query,and one that is called by FakeQueryProvider with the current query expression.The use of Expression.Constant(this) as the initial source expression C is just away of showing that the query initially represents the original object. (Imagine animplementation representing a table, for example—until you apply any query operators,the query would just return the whole table.) When the constant expression islogged, it uses the overridden ToString method, which is why we’ve given a short,constant description E. This makes the final expression much cleaner than it wouldhave been without the override. When we are asked to iterate over the results of thequery, we always just return an empty sequence D to make life easy. Production implementationswould parse the expression here, or (more likely) call Execute on theirquery provider and just return the result.As you can see, there’s not a lot going on in FakeQuery, and listing 12.7 shows thatFakeQueryProvider is equally simple.DReturns emptyresult sequenceListing 12.7An implementation of IQueryProvider that uses FakeQueryclass FakeQueryProvider : IQueryProvider{public IQueryable CreateQuery(Expression expression){Logger.Log(this, expression);return new FakeQuery(this, expression);}public IQueryable CreateQuery(Expression expression){Logger.Log(this, expression);return new FakeQuery(this, expression);}public T Execute(Expression expression){Logger.Log(this, expression);return default(T);Licensed to Rhona Hadida