Dim as New与Dim/Set有什么不同 [英] What's the difference between Dim As New vs Dim / Set
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问题描述
在VBA中,我可以通过以下两种方式之一创建对象:
'First way
Dim myCol1 As New Collection
'Second way
Dim myCol2 As Collection
Set myCol2 = New Collection
myCol1.Add "AAA" 'Works
myCol2.Add "BBB" 'Works as well
第二种方式只是第一种方式的更详细版本,还是myCol1和myCol2对象之间实际上存在差异?
推荐答案
有几个主要区别。您绝对应该选择第二种Dim/Set
方法。
原因1-使用As New
,在调用该对象的属性或方法之前不会创建对象,但请看以下示例,其中将对象设置为空,然后调用属性/方法会导致对象重新实例化自身:
Sub ShortcutInstantiation()
Dim x As New Collection
x.Add "FOO", "BAR"
Set x = Nothing
'This line implicitly recreates a new Collection
Debug.Print x.Count
Debug.Print x Is Nothing 'Prints False
End Sub
Sub SafeInstantiation()
Dim x As Collection
Set x = New Collection
x.Add "FOO", "BAR"
Set x = Nothing
'Throws error because x is nothing
Debug.Print x.Count
End Sub
原因2As New
方法速度较慢,因为VBA需要在每个属性或方法调用之前检查是否已实例化对象。
查看此伪代码以了解VBA在幕后执行的操作:
Sub NotSoShortcutInstantiation()
Dim x As New Collection
If x Is Nothing Then Set x = New Collection
x.Add "FOO", "BAR"
If x Is Nothing Then Set x = New Collection
x.Add "FIZZ", "BUZZ"
If x Is Nothing Then Set x = New Collection
x.Add "CAR", "DOOR"
If x Is Nothing Then Set x = New Collection
Debug.Print x.Count
End Sub
原因3如果对象构造函数在预期的之后执行某些操作,而不是在显式实例化它时执行某些操作,则可能存在严重的时序差异:
比较此代码的结果:
Sub InstantiationTiming()
Dim foo As String
Dim x As New Class1
Debug.Print Format(Now(), "hh:mm:ss") & " x should be ready"
foo = x.foo
Dim y As Class1
Set y = New Class1
Debug.Print Format(Now(), "hh:mm:ss") & " y should be ready"
foo = y.foo
End Sub
As New
方法打印:
06:36:57 x should be ready
06:36:57 Class Initialized
Set y = New
方法打印:
06:36:57 Class Initialized
06:36:57 y should be ready
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