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In the example； _networkIdentifier will not be written or read from the data stream。
Separating Data Objects from Action Objects
Another approach is to develop a number of data objects whose only role is to be used in seri
alization and data referencing。 Such an approach is useful when using binary serialization；
because you are then able to more effectively manage the version problem。 The following is an
example of how such an architecture would be realized。
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Class MyObject2
End Class
Class Doer
Private _object As MyObject2
Private _networkIdentifier As Integer
End Class
The class Doer has no serialization attribute and will not be serialized； but it references
MyObject2。 The network identifier data member has been moved from MyObject2 to Doer。 The
result is that MyObject2 contains nothing that is transient。
pleting Custom Types
When writing custom types； two methods are often implemented； especially where objects are
to be pared a lot： Equals（） and GetHashCode（）。 These two methods are used by the
library API when paring and manipulating instances in a list or a collection。 It just happened
in this chapter’s example that a list of string types did the right thing。 If TextProcessor had used
the Ticket type； then the Equals（） method of the list used to find date duplicates would not have
worked。 The default implementations of Equals（） and GetHashCode（） are not implemented
properly。 This is not the API’s fault。 Rather； it is a recognition that Microsoft cannot know
the structure of an object and what is considered as making a type unique。
Implementing GetHashCode（）
The MSDN documentation for Object。GetHashCode defines the GetHashCode（） method as follows
（http：//msdn2。microsoft。/en…us/library/system。object。gethashcode（VS。90）。aspx）：
The GetHashCode method can be overridden by a derived type。 Value types must over
ride this method to provide a hash function that is appropriate for that type and to
provide a useful distribution in a hash table。 For best results； the hash code must be
based on the value of an instance field or property instead of a static field or property。
Objects used as a key in a Hashtable object must also override the GetHashCode method
because those objects must generate their own hash code。 If an object used as a key does
not provide a useful implementation of GetHashCode； you can specify a hash code
provider when the Hashtable object is constructed。
But what does this actually mean？ The purpose of GetHashCode（） is to identify the object in
a collection of other objects using a hash code。 Imagine creating a table of same…type instances。
The situation occurs when you create a collection and store a bunch of instances in the collec
tion。 With the GetHashCode（） method； you can generally separate each instance from each other。
I say “generally” because GetHashCode（） is an approximation。 To verify if one object instance
equals another； the Equals（） method needs to be implemented。
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280 CH AP T E R 1 0 ■ L E A R N I N G A B OU T P E R S IS TE N CE
Implementing a hash code is best delegated to a helper class that does the heavy lifting。
The book Effective Java Programming Language Guide by Joshua Bloch （Prentice…Hall； 2001）
outlines a robust technique。 Basically； you store some constant nonzero value； such as 17； in
a variable。 Then； for each data member of the type； perform a mathematical operation that
results in Integer values that are successively multiplied and added； where the operation is
specific to the type and defined as follows：
o Boolean： If true return 0； otherwise return 1。
o Byte； Char； Short； or Integer： Return the value of the type。
o Long： Return CInt（value Xor （value 》》 &H20））。 &H20 is the hexadecimal for 32。
o Single： Return Convert。ToInt32 of the value。
o Object： Return the value generated by calling Object。GetHashCode（）。
o Array ： Iterate and treat each element individually。
The rules are implemented in a class called HashCodeAutomater。 The following source code
is an implementation in an abbreviated form：
Public Class HashCodeAutomater
Private Readonly _constant As Integer
Private _runningTotal As Integer
Public Sub New（）
_constant = &H25
_runningTotal = &H11
End Sub
Public Function AppendSuper（ByVal superHashCode As Integer） As HashCodeAutomater
_runningTotal = （（_runningTotal * _constant） ＋ superHashCode）
Return Me
End Function
Public Function Append（ByVal obj As Object） As HashCodeAutomater
If （obj Is Nothing） Then
Me。_runningTotal = （Me。_runningTotal * Me。_constant）
ElseIf Not obj。GetType。IsArray Then
Me。_runningTotal = （（Me。_runningTotal * Me。_constant） ＋ obj。GetHashCode）
ElseIf TypeOf obj Is Long（） Then
Me。Append（DirectCast（obj； Long（）））
ElseIf TypeOf obj Is Integer（） Then
Me。Append（DirectCast（obj； Integer（）））
ElseIf TypeOf obj Is Short（） Then
Me。Append（DirectCast（obj； Short（）））
ElseIf TypeOf obj Is Char（） Then
Me。Append（DirectCast（obj； Char（）））
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ElseIf TypeOf obj Is Byte（） Then
Me。Append（DirectCast（obj； Byte（）））
ElseIf TypeOf obj Is Double（） Then
Me。Append（DirectCast（obj； Double（）））
ElseIf TypeOf obj Is Single（） Then
Me。Append（DirectCast（obj； Single（）））
ElseIf TypeOf obj Is Boolean（） Then
Me。Append（DirectCast（obj； Boolean（）））
Else
Me。Append（DirectCast（obj； Object（）））
End If
Return Me
End Function
Public Function Append（ByVal array As Long（）） As HashCodeAutomater
If （array Is Nothing） Then
Me。_runningTotal = （Me。_runningTotal * Me。_constant）
Else
Dim i As Integer
For i = 0 To array。Length 1
Me。Append（array（i））
Next i
End If
Return Me
End Function
Public Function Append（ByVal value As Long） As HashCodeAutomater
Me。_runningTotal = _
（（Me。_runningTotal * Me。_constant） ＋ CInt（（value Xor （value 》》 &H20））））
Return Me
End Function
Public Function ToHashCode（） As Integer
Return _runningTotal
End Function
End Class
The different implementations of the Append（） method belong to a single grouping for a
single data type； Long。 For example； there is an Append（） method that accepts a Long and a Long
array。 The full implementation of HashCodeAutomater would have an Append（） method for Short
and Short array； and all the other data types。 There is no specific group implementation for the
String type； because it is treated like an object that has its own hash…code calculation
implementation。
Notice in the implementations of the Append（） methods how a calculation is performed
and then added to the data member _runningTotal。 The return value is a Me reference； so that
the methods can be chained together。 This allows a client to use the HashCodeAutomater class；
as the following GetHashCode（） implementation demonstrates：
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282 CH AP T E R 1 0 ■ L E A R N I N G A B OU T P E R S IS TE N CE
Class HashcodeExample
Public value As Integer
Public buffer As String
Public Sub New （ByVal val As Integer； ByVal buf As String）
value = val
buffer = buf
End Sub
Public Overrides Function GetHashCode（） As Integer
Return New HashCodeAutomater（）
。Append（value）
。Append（buffer）。toHashCode（）
End Function
End Class
The implementation of HashcodeExample has two data members： value and buffer。 The
two data members make up the class’s state。 Not all data members are used when calculating
a class instance’s hash…code value。 For example； if HashcodeExample had a data member that
referenced a database connection； it should not be used when calculating the hash code； because
the database connection is the type used to get the state and does not influence the state—it is
a means to an end。
Implementing Equals（）
Once the GetHashCode（） method has been implemented； the Equals（） method can be
implemented：
Public Overrides Function Equals（ByVal obj As Object） As Boolean
If TypeOf（obj） Is HashCodeExample
