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are displayed again to show
which variable has been modified
Figure 4…5。 Using a custom value type
As a parison； the same operations can be carried out using a reference type； where the
reference code is as follows：
Dim val As MyReferenceType = New MyReferenceType（）
Dim copiedVal As MyReferenceType = val
Console。WriteLine（〃val value=〃 & val。value & _
〃 copiedVal value=〃 & copiedVal。value）
val。value = 10
Console。WriteLine（〃val value=〃 & val。value & _
〃 copiedVal value=〃 & copiedVal。value）
So； if two code pieces are functionally identical； with the single difference being their type
（value type versus reference type）； will they generate the same results？
Running both code pieces results in the following output：
var value=0 copiedVar value=0
var value=10 copiedVar value=0
val value=0 copiedVal value=0
val value=10 copiedVal value=10
Looking closely at the results； you see that two functionally identical code pieces that differ
only in the variable type they use generate pletely different results：
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84 CH AP T E R 4 ■ L E A R N IN G AB OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S
o When you assign and modify a value type； only the contents of the modified variable
change。
o When you assign and modify a reference type； the original object changes； so the original
variable and assigned variable change because they both point to the same original object。
This example demonstrates that when defining user…defined types； you need to be careful
how you treat value and reference types。
As you learned in Chapter 2； a value type is stored on the stack。 Thus； declaring a user
defined value type means the full contents of the user…defined type are stored on the stack； and
when you assign one value type variable to another value type variable； you are copying the
plete contents of the value type。 This copying was obvious in our example when we used
simple numeric number types （such as Double）； but when you copy plete structures with
contents； the side effects might not be what you expect。
Value Types That Contain Reference Types
Value types copy the contents when you assign variables； but there is caveat in that this rule
does not apply if the value type contains a reference type。 Consider the following declaration。
Structure MyValueTypeWithReferenceType
Public value As Integer
Public reference As MyReferenceType
End Structure
The first line in the value type declaration contains a single value type （Integer）。 The second
line is the data member declaration that is a reference type。 The declaration implies that the
value type is stored on the stack； but the reference type is on the heap。
The value type that contains a reference type is manipulated using the following test code。
Dim var As MyValueTypeWithReferenceType = New MyValueTypeWithReferenceType（）
var。reference = New MyReferenceType（）
Dim copiedVar As MyValueTypeWithReferenceType = var
Console。WriteLine（〃var value=〃 & var。reference。value & _
〃 copiedVar value=〃 & copiedVar。reference。value）
var。reference。value = 10
Console。WriteLine（〃var value=〃 & var。reference。value & _
〃 copiedVar value=〃 & copiedVar。reference。value）
It is important to realize that allocating MyValueTypeWithReferenceType does not imply
an allocation of the embedded custom type。 In the test code； the allocation of the
MyValueTypeWithReferenceType is the same as the previous code examples； but a second allocation
of MyReferenceType is required because MyReferenceType is a reference type。 Had MyReferenceType
been a value type； the allocation would not have been necessary。 But if you allocate a value
type like a reference type； the piler will ignore the directive。
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CH AP T E R 4 ■ L E A R N I N G A B OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S 85
Running the test code results in the following：
var value=0 copiedVar value=0
var value=10 copiedVar value=10
When you assign and modify an embedded reference type； the reference type instance is
modified for both variables。 In this case； when we assigned the value type； the contents were
copied； including the pointer to the reference type。
Table 4…1 summarizes the behavior of types when the allocated variable is assigned to
another variable and the data member from the original is modified。 For example； if the code
is custom2 = custom1 custom1。member = ＇new value＇； what’s the value of custom2。member？
Table 4…1。 Behavior When the Allocated Variable Is Assigned to Another Variable and the Data
Member from the Original Is Modified
Type Behavior
Value type Assigned data member is not modified
Reference type Assigned data member is modified
Value type embedding value type Assigned embedded data member is not modified
Value type embedding reference type Assigned embedded data member is modified
Reference type embedding value type Assigned embedded data member is modified
Reference type embedding Assigned embedded data member is modified
reference type
Value Types and Parameters
Another value type constraint relates to how variables are stored and manipulated when they
are passed via a method。 Suppose you create a method that has as parameters a value type and
a reference type。 If in the method the parameters are modified； what modifications does the
caller of the method see？ Consider the following code：
Sub Method（ByVal value As MyValueType； ByVal reference As MyReferenceType）
value。value = 10
reference。value = 10
End Sub
The caller can pass in instances of a value type and a reference type that are manipulated
in the context of the method。 Now let’s call Method（） with the following code：
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86 CH AP T E R 4 ■ L E A R N IN G AB OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S
Dim value As MyValueType = New MyValueType（）
Dim reference As MyReferenceType = New MyReferenceType（）
Method（value； reference）
Console。WriteLine（〃value value=〃 & value。value & _
〃 reference value=〃 & reference。value）
The calling code instantiates the types MyValueType and MyReferenceType； calls the method
Method（）； and then inspects the value of the data member value from the value and reference
types。
Running the code will generate output similar to the following：
value value=0 reference value=10
The executed code shows that the value type （MyValueType） data member was not altered；
whereas the reference type （MyReferenceType） data member was altered。 This is correct； and it
demonstrates that when you call a method； you are assigning the parameters of the method to
variables in the