onHeap is allocated and initialized； the value from onStack is copied to the heap and assigned to the instance onHeap。 This is what boxing does； except Visual Basic does it automatically and transparently。 ■Note It is important to remember that when you box and unbox； you are copying values back and forth between the stack and the heap。 Thus； if the variable onStack is changed； the value of onHeap does not change。 Unboxing refers to copying the value from the heap to the stack； which in the case of the example； means transferring the value from the variable onHeap to onStack。 Boxing/unboxing happens automatically； but it has a performance penalty； since memory is allocated and assigned。 Managing a Collection After Visual Basic 2005 The two problems of storing mixed object types and the performance penalty of boxing/unboxing required Microsoft to carefully consider a solution。 After much debate and thinking； Microsoft introduced generics。 In a nutshell； generics solve both collection problems by enforcing a type。 （ generics solve broader problems as well。） Collections are an ideal application of generics because collections are utilitarian。 You don’t use collections to solve the problem of calculating taxes。 You use collections to solve the problem of how to create a collection of ines and a collection of deductions。 Here is an example of how to use generics…based collections （they are in the System。 Collections。Generic namespace； which we also do not need to import）： Dim lst As IList（Of Example） = New List（Of Example）（） lst。Add（New Example（） With ｛ 。Value = 10 ｝） lst。Add（New Example（） With ｛ 。Value = 20 ｝） For Each item As Example In lst Console。WriteLine（〃item （〃 & item。Value & 〃）〃） Next …………………………………………………………Page 257…………………………………………………………… C HA P TE R 9 ■ L E AR N I N G A B O U T L I ST S； DE L E G AT E S ； AN D L A M B D A E X PR E SSI O N S 235 The bolded line represents the generics…based code。 The code used to add an object and the For Each loop are identical to that used in the pre…Visual Basic 2005 example。 Between the brackets in the type declaration and after the Of keyword is an identifier that is the specialization of the general approach。 Whatever is inside the brackets when you declare an IList or List is saying； “I want my collection to contain instances of the type defined inside the brackets。” You cannot add any objects that are not related to the type defined in IList or List； so the following code would not pile。 lst。Add（New Another（）） This is because the generics collection is type…safe and does not allow mixed types。 It allows only objects of type Example。 When you declare a list like this： Dim lst As IList（Of Example） you are saying that the list has a method declared like this： Sub Add（ByVal item As Example） If you are programming with Visual Basic 2008； you should use Visual Basic 2005 and later collection classes。 The non… generics collection classes are； to a large degree； legacy code。 Whenever possible； use generics…based collection classes。 Now that you know how to manage a collection of objects； the next section will present a mon collection…related problem and then solve the problem。 Adding Numbers and Finding Maximum Values Let’s start out with a mon problem： addition of all elements in a collection。 Consider the following code。 Dim elements As IList（Of Integer） = New List（Of Integer）（） elements。Add（1） elements。Add（2） elements。Add（3） Dim runningTotal = 0 For Each value As Integer In elements runningTotal = runningTotal ＋ value Next This code has three parts： initialization of elements； adding of numbers to elements； and iteration of all values in elements that are added to the variable runningTotal。 The code seems acceptable。 But let’s say that you need to write another piece of code where； instead of calcu lating the running total； you want to find the maximum value； like this： …………………………………………………………Page 258…………………………………………………………… 236 CH AP T E R 9 ■ L E A R N IN G AB OU T L I ST S； D E L E G A T E S； A N D L A M B DA E X P R E S SI ON S Dim elements As IList（Of Integer） = New List（Of Integer）（） elements。Add（1） elements。Add（2） elements。Add（3） Dim maxValue As Integer = Integer。MinValue For Each value As Integer In elements If value 》 maxValue Then maxValue = value End If Next The difference between the two code pieces is the bolded code。 The inner loop is different； which is a potential problem。 In separate code files； the repetition is not apparent； but what if you wanted to bine the code bases？ The following code adds all elements and finds the maximum value。 Dim elements As IList（Of Integer） = New List（Of Integer）（） elements。Add（1） elements。Add（2） elements。Add（3） Dim runningTotal As Integer = 0 For Each value As Integer In elements runningTotal = runningTotal ＋ value Next Console。WriteLine（〃RunningTotal （〃 & runningTotal & 〃）〃） Dim maxValue As Integer = Integer。MinValue For Each value As Integer In elements If value 》 maxValue Then maxValue = value End If Next Console。Wr