1、VBA数组底层结构：

VBA的数组在底层是SafeArray：

'https://docs.microsoft.com/zh-cn/windows/win32/api/oaidl/ns-oaidl-safearraybound?redirectedfrom=MSDNType SafeArrayBound    cElements As Long '// 该维的长度    lLbound   As Long ' // 该维的数组存取的下限，一般为0End Type'https://docs.microsoft.com/zh-cn/windows/win32/api/oaidl/ns-oaidl-safearray?redirectedfrom=MSDNType SafeArray    cDims      As Integer ' // 数组的维度    fFeatures  As Integer '    cbElements As Long ' // 数组元素的字节大小    cLocksas   As Long '    pvDataas   As Long ' // 数组的数据指针    rgsabound(0) As SafeArrayBoundEnd Type

如果要取数组的地址，需要用到API函数：

Public Declare Function VarPtrArray Lib "msvbvm60.dll" Alias "VarPtr" (ByRef Var() As Any) As Long

VarPtrArray 返回的并不是SafeArray的地址，我们可以进行测试：

Sub TestArray()    Dim Arr() As Byte        ReDim Arr(3) As Byte        Dim sa As SafeArray    CopyMemory VarPtr(sa), VarPtrArray(Arr), Len(sa)        Printf "sa.cDims = %d, sa.cbElements = %d", sa.cDims, sa.cbElementsEnd Sub输出：sa.cDims = -27936, sa.cbElements = 3994212

从输出可以看出，我们预计的sa.cDims应该=1和sa.cbElements=1并没有出现，显然VarPtrArray(Arr)获取到的还不是Arr的地址。

像c语言这样的语言，是有指针的，VBA数组的底层实现应该是使用了一个指针来引用SafeArray结构，而VarPtrArray(Arr)获取到的应该是指针的地址。

Sub TestArray()    Dim Arr() As Byte    ReDim Arr(3) As Byte    Dim ptr As Long '保存[Arr指针]的地址        CopyMemory VarPtr(ptr), VarPtrArray(Arr), 4        Dim sa As SafeArray    CopyMemory VarPtr(sa), ptr, Len(sa)        Printf "sa.cDims = %d, sa.cbElements = %d", sa.cDims, sa.cbElementsEnd Sub输出sa.cDims = 1, sa.cbElements = 1

从输出可以看出，预计的sa.cDims应该=1和sa.cbElements=1出现了。

进一步测试，从pvDataas 中提取数据，看看获取的数据是否能正确：

Sub TestArray2()    Dim Arr() As Byte    ReDim Arr(3) As Byte    Dim ptr As Long '保存[Arr指针]的地址        Arr(0) = &H88    Arr(1) = &H21    Arr(2) = &H27    Arr(3) = &H99        CopyMemory VarPtr(ptr), VarPtrArray(Arr), 4        Dim sa As SafeArray    CopyMemory VarPtr(sa), ptr, Len(sa)        Dim b(3) As Byte    Dim i As Long    For i = 0 To 3        CopyMemory VarPtr(b(i)), sa.pvDataas + sa.cbElements * i, sa.cbElements        Printf "b(%d) = 0x%x ", i, b(i)    NextEnd Sub输出：b(0) = 0x88 b(1) = 0x21 b(2) = 0x27 b(3) = 0x99

完全正确。

2、改变SafeArray的pvDataas地址会有什么情况：

既然知道了数组的内存结构，那我们就尝试把pvDataas改变看看会怎么样：

Sub TestArray2()    Dim Arr() As Byte    ReDim Arr(0) As Byte    Dim ptr As Long '保存[Arr指针]的地址        CopyMemory VarPtr(ptr), VarPtrArray(Arr), 4        Dim lValue As Long    lValue = &HABCDEF99        Dim plValue As Long    plValue = VarPtr(lValue)        '修改pvDataas指向lValue    CopyMemory ptr + 12, VarPtr(plValue), 4    '修改Arr的SafeArrayBound为4    Dim cElements As Long    cElements = 4    CopyMemory ptr + 16, VarPtr(cElements), 4        Dim sa As SafeArray    CopyMemory VarPtr(sa), ptr, Len(sa)        Printf "sa.pvDataas = 0x%x, plValue = 0x%x", sa.pvDataas, plValue        Dim i As Long    For i = 0 To 3        Printf "Arr(%d) = 0x%x ", i, Arr(i)    NextEnd Sub输出：sa.pvDataas = 0x36ecbc, plValue = 0x36ecbcArr(0) = 0x99 Arr(1) = 0xef Arr(2) = 0xcd Arr(3) = 0xab 

定义了一个容量为1的byte数组，但是我把pvDataas的值修改成了1个Long变量的地址，同时也改变了他的SafeArrayBound为4，从输出可以看到，这个数组已经变成了一个容量为4的byte数组。

但是End Sub后，我的电脑测试Excel崩溃，这个和前面讲到的String类型里的情况差不多，到底是什么原因？

3、ReDim Preserve做了什么

我们经常会用ReDim Preserve来改变数组的容量，当然一般都是扩大。扩大的话要更多的内存空间来保存数据，所以应该是要重新分配内存，测试：

Sub TestArray()    Dim Arr() As Byte    ReDim Arr(3) As Byte    Dim ptr As Long '保存[Arr指针]的地址    Dim sa As SafeArray        CopyMemory VarPtr(ptr), VarPtrArray(Arr), 4    CopyMemory VarPtr(sa), ptr, Len(sa)    Printf "ptr = 0x%x, sa.pvDataas= 0x%x, sa.cDims = %d, sa.cbElements = %d, sa.rgsabound(0).cElements = %d", ptr, sa.pvDataas, sa.cDims, sa.cbElements, sa.rgsabound(0).cElements    ReDim Preserve Arr(4) As Byte    CopyMemory VarPtr(ptr), VarPtrArray(Arr), 4    CopyMemory VarPtr(sa), ptr, Len(sa)    Printf "ptr = 0x%x, sa.pvDataas= 0x%x, sa.cDims = %d, sa.cbElements = %d, sa.rgsabound(0).cElements = %d", ptr, sa.pvDataas, sa.cDims, sa.cbElements, sa.rgsabound(0).cElementsEnd Sub输出：ptr = 0x16597108, sa.pvDataas= 0x167c3a40, sa.cDims = 1, sa.cbElements = 1, sa.rgsabound(0).cElements = 4ptr = 0x16597108, sa.pvDataas= 0x167c3790, sa.cDims = 1, sa.cbElements = 1, sa.rgsabound(0).cElements = 5

pvDataas已经改变，重新分配了内存空间。

但是，如果是减小呢？

我本来猜测只要修改sa.rgsabound(0).cElements的值就可以达到减小容量的目的，可测试却同样重新分配了内存：

Sub TestArray()    Dim Arr() As Byte    ReDim Arr(3) As Byte    Dim ptr As Long '保存[Arr指针]的地址    Dim sa As SafeArray        CopyMemory VarPtr(ptr), VarPtrArray(Arr), 4    CopyMemory VarPtr(sa), ptr, Len(sa)    Printf "ptr = 0x%x, sa.pvDataas= 0x%x, sa.cDims = %d, sa.cbElements = %d, sa.rgsabound(0).cElements = %d", ptr, sa.pvDataas, sa.cDims, sa.cbElements, sa.rgsabound(0).cElements    ReDim Preserve Arr(1) As Byte    CopyMemory VarPtr(ptr), VarPtrArray(Arr), 4    CopyMemory VarPtr(sa), ptr, Len(sa)    Printf "ptr = 0x%x, sa.pvDataas= 0x%x, sa.cDims = %d, sa.cbElements = %d, sa.rgsabound(0).cElements = %d", ptr, sa.pvDataas, sa.cDims, sa.cbElements, sa.rgsabound(0).cElementsEnd Sub输出：ptr = 0x167d5068, sa.pvDataas= 0x167c3a50, sa.cDims = 1, sa.cbElements = 1, sa.rgsabound(0).cElements = 4ptr = 0x167d5068, sa.pvDataas= 0x167c37f0, sa.cDims = 1, sa.cbElements = 1, sa.rgsabound(0).cElements = 2

本文使用 文章同步助手 同步  ，关注xyjvba，查看更多