测试/示例

Question

我找不到将var（Boolean）设置为空/ Null或其他平台然后True / False的方法有没有办法告诉VB这个内存单元不会被使用？

Answer 1

没有。 Variant以外的内在类型不能设置为Null。如果您需要可以为空的值，则需要将其声明为Variant：

Sub Foo()
    Dim x As Variant
    x = Null
    Debug.Print x
End Sub

这种方法的第一个缺点是，您需要在代码中的任何位置测试Null的变量，然后再尝试将其分配给任何需要{{1}的任何过程。 }。否则，您将无法使用Null`运行时错误。

第二个缺点是你现在正在使用一个弱类型变量，这种变量违背了将其声明为Boolean的目的。如果您真的需要表示Boolean，True或False的类型，则可以选择声明您自己的枚举并使用：

Neither

最好以确保您的Private Enum Truthiness IsTrue IsFalse IsNeither End Enum变量实际上代表二元条件的方式重构代码。

Answer 2

一个人可以利用类 default属性和静态模块函数来创建一些支持Bool，{{1 }} （均为单例）， False_个对象以及 True_，Nothing -因此以下会起作用（下面的实现）：

测试/示例

典型用法示例

isTrue()

类似于单元测试

isFalse()

实现

Sub testBool() 'New Bool would throw an exception to ensure object equality for False_ and True_ values Dim bool As Bool: Set bool = True_ 'these 2 work only if you are sure its not 'Nothing'! (otherwise throw error): If bool Then Debug.Print "1: true!" Set bool = False_ If Not bool Then Debug.Print "2: false!" Set bool = Nothing If bool Is Nothing Then Debug.Print "3: Nothing!" 'these 2 work always even if "foo Is Nothing" (unassigned): If isFalse(bool) Then Debug.Print "4: isFalse(Nothing) => true!" If Not isTrue(bool) Then Debug.Print "5: not isTrue(Nothing) => true!" 'support for typical Strings, Integers as well: If isTrue("1") And Not isTrue("0") Then Debug.Print "6: True ~ 1 and False ~ 0" If isTrue("trUe") And Not isTrue("FalsE") Then Debug.Print "7: True ~ UCase('trUe') and False ~ UCase('FalsE')" End Sub

Sub unittestBool() 'usage of object False_ and True_ in boolean evaluation (by default property usage) Debug.Assert False_ = False 'False_.native is the default property which is "False" Debug.Assert True_ = True 'True_.native is the default property whcih is "True" 'test with Bool variable 'test for unknown/nothing/null: Debug.Assert bool Is Nothing 'unassigned and usable as kind of "Null" as in other languages like Java Set bool = False_ 'False_ is a singleton (static) function Debug.Assert bool = False 'bool.native is the default property which is "False" Set bool = True_ 'True_ is a singleton (static) function Debug.Assert bool = True 'bool.native is the default property which is "True" 'test True_(Variant) function: with Bool object Debug.Assert isTrue(bool) Set bool = False_ Debug.Assert Not isTrue(bool) 'test False_(Variant) function: with Bool object Debug.Assert isFalse(bool) Set bool = True_ Debug.Assert Not isFalse(bool) 'test True_(Variant) function: with Integer object Debug.Assert isTrue(1) Debug.Assert Not isTrue(0) 'test True_(Variant) function: with String object Debug.Assert isTrue("true") 'case insensitive Debug.Assert isTrue("True") Debug.Assert isTrue("trUe") Debug.Assert Not isTrue("false") Debug.Assert isTrue("1") Debug.Assert Not isTrue("0") End Sub类（Bool.cls）的实现：
（行Bool很重要，必须从Bool.cls文件（！）中 import it，而不仅仅是创建它！）

Attribute native.VB_UserMemId = 0

Bool.cls

VERSION 1.0 CLASS BEGIN MultiUse = -1 'True END Attribute VB_Name = "Bool" Attribute VB_GlobalNameSpace = False Attribute VB_Creatable = False Attribute VB_PredeclaredId = False Attribute VB_Exposed = False Option Explicit Private val As Boolean Property Get native() As Boolean Attribute native.VB_UserMemId = 0 native = val End Property Property Let native(value As Boolean): val = value: End Property Private Sub Class_Initialize() If Bool_initialized Then err.raise 1, "Bool", "usage of New Bool forbidden. Use assignment of existing True_ and False_ objects only!" End Sub模块（BoolMod.bas）的实现：

BoolMod

在VBA-excel中将Boolean设置为null / Nothing / empty等有没有办法做到这一点？

2 个答案:

测试/示例

典型用法示例

类似于单元测试

实现

`Bool.cls`