按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
data member because it belongs to the
custom type declaration
Figure 4…4。 Declaring custom types
When declaring user…defined types； the data members and methods are declared inside
the block of code。 You can think of the declaration as the writing on the outside of the box； and
anything in the block of code as the contents of the box。 The custom types （MyValueType and
MyReferenceType） as declared in Figure 4…4 do not have a scope identifier。 Think of a scope identifier
as defining who has access to your pockets and wallet。 In the case of the types in the example；
the scope is like saying that your spouse is allowed to peek into your wallet； but strangers
cannot。 In other words； the types cannot be used outside their own namespace。
If the Public keyword had been in front of the type identifier； then the user…defined type
would be exposed like a wallet that is allowed to be peeked into by the general public。 This means
the types can be used by other code。 In the case of the wallet； it’s a bad idea to have unfettered
public scope； but sometimes public scope is desirable when you are able to control its access。
And you do this every time you pay for something by handing a credit card to the cashier。 In
that case； you are publicly exposing parts of your wallet under your supervision。 In code terms；
this means making your types public； but not necessarily making the data members public。
Next； consider the code in Figure 4…5。 It instantiates MyValueType and assigns it to another
variable of the same type。
The example in Figure 4…5 illustrates what happens to two variables when one is instanti
ated and assigned to the other； and then the other is modified。 You want to understand how
each data type is modified when an interaction with another type occurs。
…………………………………………………………Page 105……………………………………………………………
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 83
To create a valid instance
Variable var is declared； associated with the
but a declaration does not variable var； you need to
Another way to associate a
imply a valid instance allocate the object using valid instance with a variable is
the New keyword
to assign the variable
（e。g。； copiedVar） from another
variable that contains a valid
Dim var As MyValueType = New MyValueType（）
Dim copiedVar As MyValueType = var instance （e。g。； var）
Console。WriteLine（〃var value=〃 & var。value & _
〃 copiedVar value=〃 & copiedVar。value） When an instance is allocated； the data
var。value = 10 members are assigned their appropriate
Console。WriteLine（〃var value=〃 & var。value & _ default value； which is 0 for Integer and most
〃 copiedVar value=〃 & copiedVar。value）
numbers。 For strings； it is an empty buffer。
These lines display the empty values。
The value of var。value is
modified from 0 to 10 The contents of var and copiedVar
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：
…………………………………………………………Page 106……………………………………………………………
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 me