rs and private methods of other instances of that type。 …………………………………………………………Page 230…………………………………………………………… 208 CH AP T E R 8 ■ L E A R N IN G AB OU T CO M P O N E N TO R IE N T E D AR C HI TE CT U R E Once the data members of the item have been assigned； the item is integrated into the list； by redirecting the _prev property of the next object （if it is not Nothing）； and then assigning the local _next property to the object to be inserted。 The Remove（） method does the same as Insert（）； but in reverse。 The first step is to redirect the _next and _prev properties of the previous and next objects； assuming that they are not Nothing。 And then the object to be removed has its _next and _prev properties assigned to Nothing。 ■Note The declaration of PrevItem and NextItem is a mon architecture where data members can be read； but to assign them； you need to use methods。 Using read…only properties is one way to make sure that the internal state cannot be corrupted； if you must expose the internal state。 Testing the Linked List BaseLinkedListItem is a core class and serves a utility purpose。 This makes the class eligible to be declared in the kernel or a definitions assembly。 As the class is a core class； it means you need a more exhaustive testing framework to ensure that there will be no problems。 In this section； we will go through one test that demonstrates what you should be testing for and how you should be testing a core class。 BaseLinkedListItem is declared as MustInherit and needs an implementation。 The purpose of the implementation is to give us enough information on the state and context of the object。 In this case； we want to define an object that tests every part of the class BaseLinkedListItem。 If you have ever seen pictures of a test car tire hooked up to dozens of wires； then you will understand what the test class needs to do。 Following is a sample implementation； placed in the TestLightingSystem project。 Remember to include a reference to LibLightingSystem （right…click References in TestLightingSystem and select Add Reference and then Projects LibLightingSystem）。 。 。 。 Imports LibLightingSystem Class LinkedItem Inherits BaseLinkedListItem Private _identifier As String Public Sub New（ByVal identifier As String） _identifier = identifier End Sub Public ReadOnly Property Identifier As String Get Return _identifier End Get End Property …………………………………………………………Page 231…………………………………………………………… C H AP TE R 8 ■ L E AR N IN G AB O U T CO M P O N E N T O R IE N TE D A R CH I TE C TU R E 209 Public Overrides Function ToString（） As String Dim buffer As String buffer = 〃Me（〃 & _identifier & 〃）〃 If NextItem IsNot Nothing Then buffer = buffer & 〃 next（〃 & _ DirectCast（MyBase。NextItem； LinkedItem）。Identifier & 〃）〃 Else buffer = buffer & 〃 next（Nothing）〃 End If If PrevItem IsNot Nothing Then buffer = buffer & 〃 prev（〃 & _ DirectCast（MyBase。PrevItem； LinkedItem）。Identifier & 〃）〃 Else buffer = buffer & 〃 prev（Nothing）〃 End If Return buffer End Function End Class The LinkedItem class has a single data member declaration； _identifier； which is used to identify the instance。 The test code will call the Insert（） and Remove（） methods； and then generate a visual representation of the linked list。 The visual representation is used to under stand what the problem might be if anything goes wrong。 You will not write tests against the visual representation because that would overplicate the testing。 To generate a visual representation of the object； the ToString（） method is overridden。 By default； all objects have a ToString（） implementation； which does absolutely nothing other than spit out the identifier of the object reference。 To make ToString（） do something useful； you need to override it。 In the example； ToString（） will generate a buffer that contains the LinkedItem identifier and the next and previous object identifiers。 These three pieces of infor mation tell you the structure of the linked list。 The next step is to write a test in TestLightingSystem’s Module1。vb that verifies that the Insert（） method works properly； and it is implemented as follows： Module Module1 Sub Main（） TestInsert（） End Sub Public Sub TestInsert（） Console。WriteLine（〃**************〃） Console。WriteLine（〃TestInsert： Start〃） Dim item1 As LinkedItem = New LinkedItem（〃item1〃） Dim item2 As LinkedItem = New LinkedItem（〃item2〃） Dim item3 As LinkedItem = New LinkedItem（〃item3〃） Dim toString As String = item1。ToString（） Console。WriteLine（toString） If （item1。NextItem IsNot Nothing） OrElse _ …………………………………………………………Page 232…………………………………………………………… 210 CH AP T E R 8 ■ L E A R N IN G AB OU T CO M P O N E N TO R IE N T E D AR C HI TE CT U R E （item1。PrevItem IsNot Nothing） Then Throw New Exception（ _ 〃TestInsert： Empty structure is incorrect〃） End If item1。Insert（i