StringBuilder in VBA

In the process of trying to build a serializable data structure, I found myself building large strings, which gets very slow because VBA copies a string every time concatenation is performed.

To alleviate this, referring to Dynamic Array and Java's StringBuilder interface, I cobbled together a unicode clsStringBuilder class.

This isn't a very big chunk of code, but I'd be interested in any advice about edge cases that I maybe haven't considered, unexpected copying behavior that VBA might be performing "behind my back" which I could avoid, or corrections to coding style (or lack thereof).

Option Compare Database
Option Explicit

'******
'* v2 *
'******


Private Declare Sub CopyMemory Lib "kernel32.dll" Alias "RtlMoveMemory" (ByVal dst As Long, ByVal src As Long, ByVal Length As Long)

Private Const DEFAULT_CAPACITY As Long = &H10
Private m_currLen As Long
Private m_stringBuffer() As Byte

Private Sub Class_Initialize()
    ReDim m_stringBuffer(0 To (DEFAULT_CAPACITY * 2) - 1) 'Each unicode character is 2 bytes
End Sub

Public Function Append(strString As String) As clsStringBuilder
On Error GoTo derp


    If m_currLen + LenB(strString) < UBound(m_stringBuffer) Then
        CopyMemory VarPtr(m_stringBuffer(m_currLen)), StrPtr(strString), LenB(strString)
    Else
        If m_currLen + LenB(strString) < UBound(m_stringBuffer) * 2 Then
            Expand
        Else
            Expand m_currLen + LenB(strString)
        End If
        CopyMemory VarPtr(m_stringBuffer(m_currLen)), StrPtr(strString), LenB(strString)
    End If
    m_currLen = m_currLen + LenB(strString)
    Set Append = Me
    Exit Function

derp:
    Stop
    Resume
End Function

Public Property Get Length() As Long
    Length = m_currLen / 2
End Property

Public Property Get Capacity() As Long
    Capacity = UBound(m_stringBuffer)
End Property

Private Sub Expand(Optional newSize As Long = 0)
    If newSize <> 0 Then
        ReDim Preserve m_stringBuffer(0 To newSize - 1)
    Else
        ReDim Preserve m_stringBuffer(0 To (UBound(m_stringBuffer) * 2) + 1)
    End If
End Sub

Public Function toString() As String
    toString = Mid(m_stringBuffer, 1, m_currLen / 2)
End Function

Here is a test:

Public Sub Main()
    Dim sb As clsStringBuilder
    Set sb = New clsStringBuilder
    Dim strString As String
    Dim i As Long
    Dim StartTime As Double

    'VBA String
    StartTime = MicroTimer()
    For i = 0 To 100000
        strString = strString + "Hello World;"
    Next
    Debug.Print "The VBA String took: " & Round(MicroTimer - StartTime, 3) & " seconds"

    'StringBuilder
    StartTime = MicroTimer()
    For i = 0 To 100000
        sb.Append "Hello World;"
    Next
    Debug.Print "The Stringbuilder took: " & Round(MicroTimer - StartTime, 3) & " seconds"

    'Are the strings the same?
    Debug.Print StrComp(strString, sb.toString, vbBinaryCompare)
End Sub

Here is Microsoft's MicroTimer function, which can be found here:

Private Declare Function getFrequency Lib "kernel32" _
Alias "QueryPerformanceFrequency" (cyFrequency As Currency) As Long
Private Declare Function getTickCount Lib "kernel32" _
Alias "QueryPerformanceCounter" (cyTickCount As Currency) As Long
Function MicroTimer() As Double
'

' Returns seconds.
    Dim cyTicks1 As Currency
    Static cyFrequency As Currency
    '
    MicroTimer = 0

' Get frequency.
    If cyFrequency = 0 Then getFrequency cyFrequency

' Get ticks.
    getTickCount cyTicks1

' Seconds
    If cyFrequency Then MicroTimer = cyTicks1 / cyFrequency
End Function