我在现公司继承了一个庞大的VB6应用程序。我正在工作中学习VB6,但还遇到了许多问题。目前最大的问题是我无法弄清楚如何检查集合对象中是否存在某个键。能否有人提供帮助?
我在现公司继承了一个庞大的VB6应用程序。我正在工作中学习VB6,但还遇到了许多问题。目前最大的问题是我无法弄清楚如何检查集合对象中是否存在某个键。能否有人提供帮助?
我的标准函数非常简单。它可以适用于任何元素类型,因为它不会进行任何赋值,只会执行集合属性获取。
Public Function Exists(ByVal oCol As Collection, ByVal vKey As Variant) As Boolean
On Error Resume Next
oCol.Item vKey
Exists = (Err.Number = 0)
Err.Clear
End Function
@Mark Biek您的keyExists函数与我的标准Exists()函数非常相似。为了使该类对公开的COM集合和检查数字索引更有用,我建议更改sKey和myCollection的类型。如果将该函数用于对象集合,则需要在设置val所在的行上使用“set”。
编辑:我从未注意到基于对象和基于值的Exists()函数有不同的要求,这让我感到困扰。我很少使用非对象集合,但这似乎是一个完美的瓶颈,可以用来找到需要检查存在性时很难追踪的错误。因为如果已经存在错误处理程序,则错误处理将失败,因此需要两个函数来获取新的错误范围。只需要调用Exists()函数即可:
Public Function Exists(col, index) As Boolean
On Error GoTo ExistsTryNonObject
Dim o As Object
Set o = col(index)
Exists = True
Exit Function
ExistsTryNonObject:
Exists = ExistsNonObject(col, index)
End Function
Private Function ExistsNonObject(col, index) As Boolean
On Error GoTo ExistsNonObjectErrorHandler
Dim v As Variant
v = col(index)
ExistsNonObject = True
Exit Function
ExistsNonObjectErrorHandler:
ExistsNonObject = False
End Function
同时进行功能验证:
Public Sub TestExists()
Dim c As New Collection
Dim b As New Class1
c.Add "a string", "a"
c.Add b, "b"
Debug.Print "a", Exists(c, "a") ' True '
Debug.Print "b", Exists(c, "b") ' True '
Debug.Print "c", Exists(c, "c") ' False '
Debug.Print 1, Exists(c, 1) ' True '
Debug.Print 2, Exists(c, 2) ' True '
Debug.Print 3, Exists(c, 3) ' False '
End Sub
我一直使用这样的函数:
public function keyExists(myCollection as collection, sKey as string) as Boolean
on error goto handleerror:
dim val as variant
val = myCollection(sKey)
keyExists = true
exit sub
handleerror:
keyExists = false
end function
正如Thomas所指出的,你需要使用Set而不是Let。以下是我库中适用于值类型和对象类型的通用函数:
Public Function Exists(ByVal key As Variant, ByRef col As Collection) As Boolean
'Returns True if item with key exists in collection
On Error Resume Next
Const ERR_OBJECT_TYPE As Long = 438
Dim item As Variant
'Try reach item by key
item = col.item(key)
'If no error occurred, key exists
If Err.Number = 0 Then
Exists = True
'In cases where error 438 is thrown, it is likely that
'the item does exist, but is an object that cannot be Let
ElseIf Err.Number = ERR_OBJECT_TYPE Then
'Try reach object by key
Set item = col.item(key)
'If an object was found, the key exists
If Not item Is Nothing Then
Exists = True
End If
End If
Err.Clear
End Function
正如Thomas所建议的那样,您可以将Collection类型更改为Object以实现泛化。.Item(key)语法被大多数集合类共享,因此这可能非常有用。
编辑 看起来我被Thomas本人击败了。然而,为了更容易地重复使用,我个人更喜欢一个没有私有依赖关系的单个函数。
使用错误处理程序来捕获集合中不存在键的情况,可以使“在所有错误上中断”选项调试变得非常烦人。为了避免不必要的错误,我经常创建一个类,其中包含Collection中存储的对象和Dictionary中的所有键。Dictionary具有exists(key)函数,因此我可以在尝试从集合中获取对象之前调用它。您只能在字典中存储字符串,因此如果需要存储对象,则仍需要一个集合。
“如果已经有活动的错误处理程序,则错误处理将失败”的说法只是部分正确。
您可以在程序中拥有多个错误处理程序。
因此,您可以通过只使用一个函数来实现相同的功能。
只需像这样重写您的代码:
Public Function Exists(col, index) As Boolean
Dim v As Variant
TryObject:
On Error GoTo ExistsTryObject
Set v = col(index)
Exists = True
Exit Function
TryNonObject:
On Error GoTo ExistsTryNonObject
v = col(index)
Exists = True
Exit Function
ExistsTryObject:
' This will reset your Err Handler
Resume TryNonObject
ExistsTryNonObject:
Exists = False
End Function
然而,如果你只在该例程的TryNonObject部分中加入代码,则会得到相同的信息。
对于对象和非对象都会成功。
但是,对于非对象,这将加速您的代码,因为您只需要执行一个语句来断言该项存在于集合中。
public Function TryGet(key as string, col as collection) as Variant
on error goto errhandler
Set TryGet= col(key)
exit function
errhandler:
Set TryGet = nothing
end function
Option Explicit
'Purpose : Determines if an item already exists in a collection
'Inputs : oCollection The collection to test for the existance of the item
' vIndex The index of the item.
' [vItem] See Outputs
'Outputs : Returns True if the item already exists in the collection.
' [vItem] The value of the item, if it exists, else returns "empty".
'Notes :
'Example :
Function CollectionItemExists(vIndex As Variant, oCollection As Collection, Optional vItem As Variant) As Boolean
On Error GoTo ErrNotExist
'Clear output result
If IsObject(vItem) Then
Set vItem = Nothing
Else
vItem = Empty
End If
If VarType(vIndex) = vbString Then
'Test if item exists
If VarType(oCollection.Item(CStr(vIndex))) = vbObject Then
'Return an object
Set vItem = oCollection.Item(CStr(vIndex))
Else
'Return an standard variable
vItem = oCollection.Item(CStr(vIndex))
End If
Else
'Test if item exists
If VarType(oCollection.Item(Int(vIndex))) = vbObject Then
'Return an object
Set vItem = oCollection.Item(Int(vIndex))
Else
'Return an standard variable
vItem = oCollection.Item(Int(vIndex))
End If
End If
'Return success
CollectionItemExists = True
Exit Function
ErrNotExist:
CollectionItemExists = False
On Error GoTo 0
End Function
'Demonstration routine
Sub Test()
Dim oColl As New Collection, oValue As Variant
oColl.Add "red1", "KEYA"
oColl.Add "red2", "KEYB"
'Return the two items in the collection
Debug.Print CollectionItemExists("KEYA", oColl, oValue)
Debug.Print "Returned: " & oValue
Debug.Print "-----------"
Debug.Print CollectionItemExists(2, oColl, oValue)
Debug.Print "Returned: " & oValue
'Should fail
Debug.Print CollectionItemExists("KEYC", oColl, oValue)
Debug.Print "Returned: " & oValue
Set oColl = Nothing
End Sub
Public Function Exists(ByRef Col As Collection, ByVal Key) As Boolean
On Error GoTo KeyError
If Not Col(Key) Is Nothing Then
Exists = True
Else
Exists = False
End If
Exit Function
KeyError:
Err.Clear
Exists = False
End Function