vba - Dim As New vs Dim/Set 有什么区别

Question

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？

Answer 1

有几个关键区别。你绝对应该更喜欢第二个 Dim/Set方法。

原因 1 - 与 As New ，在调用该对象的属性或方法之前不会创建该对象，但请查看此示例，其中将对象设置为 Nothing，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 2 As New方法较慢，因为 VBA 需要在每个属性或方法调用之前检查它是否已实例化对象。

看看这个伪代码，看看 VBA 在幕后做了什么:

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:

比较这段代码的结果:

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