LINQ Programming: Letting a Sub-List

	Letting a Sub-List

Introduction

We saw that you can get the result of a LINQ statement from the Select section. In reality, the Select statement simply indicates that the result is ready and it hands it to the other parts of the program. Instead of getting the result directly from the Select statement, you can first store it in a local LINQ variable. This allows you to treat the result as a variable that you can then manipulate before getting the final result.

To create a local variable in the LINQ statement, you can use the Let operator. You must use it before the Select statement to hold the result. Here is an example:

Imports System.Linq
Imports System.Collections.Generic

Public Class Employee
    Public EmployeeNumber As Integer
    Public FirstName As String
    Public LastName As String
    Public HourlySalary As Double

    Public Sub New(Optional ByVal Number As Integer = 0,
                   Optional ByVal FName As String = "John",
                   Optional ByVal LName As String = "Doe",
                   Optional ByVal salary As Double = 0D)
        EmployeeNumber = Number
        FirstName = FName
        LastName = LName
        HourlySalary = salary
    End Sub
End Class

Module Exercise
    Public Function Main() As Integer
        Dim Employees() As Employee =
        {
            New Employee(971974, "Patricia", "Katts", 24.68),
            New Employee(208411, "Raymond", "Kouma", 20.15),
            New Employee(279374, "Hélčne", "Mukoko", 15.55),
            New Employee(707912, "Bertrand", "Yamaguchi", 24.68),
            New Employee(294800, "Peter", "Mukoko", 18.85),
            New Employee(971394, "Gertrude", "Monay", 20.55)
        }

        Dim FullNames = From Empls
                        In Employees
                        Let FullName = Empls.LastName + ", " + Empls.FirstName
                        Select FullName

        For Each Empl In FullNames
            Console.WriteLine(Empl)
        Next

        Console.WriteLine()
        Return 0
    End Function
End Module

This would produce:

Employees

If you need a Where condition but your Let variable would be used only to hold the final result, you can declare that Let variable after the Where statement. Here is an example:

Dim FullNames = From Empls
                In Employees
                Where Empls.LastName = "Mukoko"
                Let FullName = Empls.LastName & ", " & Empls.FirstName
                Select FullName

This would produce:

Employees

You can create the Let variable before the Where statement and you would get the same result:

Dim FullNames = From Empls
                In Employees
                Let FullName = Empls.LastName & ", " & Empls.FirstName
                Where Empls.LastName = "Mukoko"
                Select FullName