MS Access - VBA - String manipulation - ms-access

Question

I developed some code for an Access Database that manipulates a string with a statement like:
myString = Left(myString, somePosition) & Right(myString, someOtherPosition)
the above is part of a loop that has thousands of iterations and the variable myString is thousand of characters long.
I know the above code is bad practice in Java and a StringBuffer should be used instead of a string.
My code is taking a lot of time to run (about 7 minutes) and I suspect the problem might be related to the heavy string manipulation that is going on. Can you please confirm if there is anything similar to StringBuffer in VBA that could improve the efficiency of my code?
Update: full code with StringBuilder
Function SelectColumns2(str As String, columns As String, separator As String) As String
'column_number is the number of the column we are reading when we loop through a line
'z is the counter of the field (a portion of str between two separators)
'i is the counter of the str (the position of the modified string)
Dim column_number As Integer, i As Double, z As Integer, leftPosition As Double
'stringbuilder that stores the string that will represent the final file
Dim sb As StringBuilder, leftStr As StringBuilder, rightStr As StringBuilder
Set sb = New StringBuilder
Set leftStr = New StringBuilder
Set rightStr = New StringBuilder
sb.Append str
column_number = 1
i = 1 ' full str
z = 0 ' full field
While sb.Length >= i
z = z + 1
If Mid(sb.Text, i, 1) = separator Then
If InStr(1, columns, "/" & column_number & "/") = 0 Then
leftStr.Append left(sb.Text, i - z)
rightStr.Append right(sb.Text, sb.Length - i)
sb.Clear
sb.Append leftStr.Text
sb.Append rightStr.Text
leftStr.Clear
rightStr.Clear
i = i - z
End If
column_number = column_number + 1
z = 0
ElseIf Mid(sb.Text, i, 1) = Chr(10) Then
If InStr(1, columns, "/" & column_number & "/") = 0 Then
leftPosition = max((i - z - 1), 0)
If leftPosition = 0 Then
leftStr.Append left(sb.Text, leftPosition)
rightStr.Append right(sb.Text, sb.Length - i)
sb.Clear
sb.Append leftStr.Text
sb.Append rightStr.Text
Else
leftStr.Append left(sb.Text, leftPosition)
rightStr.Append right(sb.Text, sb.Length - i + 1)
sb.Clear
sb.Append leftStr.Text
sb.Append rightStr.Text
End If
leftStr.Clear
rightStr.Clear
i = i - z
End If
column_number = 1
z = 0
End If
i = i + 1
Wend
SelectColumns2 = left(sb.Text, sb.Length - 1)
End Function

Answer 1

You can use CreateObject to create the .Net stringbuilder class. Note that you will have to have the relevant .Net library installed, and VBA does not support overloading, so it will handle a little differently than in VB.Net.
Sample code:
Public Sub TestSB()
Dim sb As Object
Set sb = CreateObject("System.Text.StringBuilder")
sb.Append_3 "Hello"
sb.Append_3 " "
sb.Append_3 "World"
sb.Append_3 "!"
Debug.Print sb.ToString
End Sub
Alternatively, you can build your own stringbuilder. This answer provides a stringbuilder class, and this question also shows some sample code.

Answer 2

You can - for an extremely simple implementation - use Mid.
For example, this code runs in about 0.1 ms for the quite large strings entered:
Public Function ChopString() As String
Dim Source As String
Dim LeftPart As Long
Dim RightPart As Long
Dim Result As String
Source = String(100000, "x")
LeftPart = 30000
RightPart = 40000
Result = Space(LeftPart + RightPart)
Mid(Result, 1) = Left(Source, LeftPart)
Mid(Result, 1 + LeftPart) = Right(Source, RightPart)
ChopString = Result
End Function
For smaller strings of a few K, it runs way faster.