C# in Depth

296 CHAPTER 11 Query expressions and LINQ to Objectsrange variable for both sorting and the projection at the end. Have you spotted the problemyet? We need to use two range variables, but the lambda expression passed to Selectonly takes one parameter! This is where transparent identifiers come on the scene.11.4.2 Transparent identifiersIn listing 11.11, we’ve got two range variables involved in the final projection, but theSelect method only acts on a single sequence. How can we combine the range variables?The answer is to create an anonymous type that contains both variables butapply a clever translation to make it look as if we’ve actually got two parameters for theselect and orderby clauses. Figure 11.5 shows the sequences involved.The let clause achieves its objectives by using another call to Select, creating ananonymous type for the resulting sequence, and effectively creating a new range variablewhose name can never be seen or used in source code. Our query expressionfrom listing 11.11 is translated into something like this:SampleData.AllUsers.Select(user => new { user,length=user.Name.Length }).OrderBy(z => z.length).Select(z => new { Name=z.user.Name,Length=z.length })from user inSampleData.AllUsersUser: { Name="Tim Trotter" ... }User: { Name="Tara Tutu" ... }User: { Name="Dave Denton" ... }...let length = user.Name.Lengthuser=User: { Name="Tim Trotter" ... }, length=11user=User: { Name="Tara Tutu" ... }, length=9user=User: { Name="Dave Denton" ... }, length=11...orderby lengthuser=User: { Name="Tara Tutu" ... }, length=9user=User: { Name="Tim Trotter" ...}, length=11user=User: { Name="Dave Denton" ... }, length=11...select new { Name=user.Name,Length=user.Length }(Result of query)Name="Tara Tutu", Length=9Name="Tim Trotter", Length=11Name="Dave Denton", Length=11...Figure 11.5 Sequences involved in listing11.11, where a let clause introduces thelength range variableLicensed to Rhona Hadida