How To Store Values In Byte Array

I'm trying to convert old code to VB in VS2005. The code I'm working on is used to store values in a string, which will then be handed to a proprietary scramble function. This function is a given in this case. I've already switched from a string to an array of bytes in my .NET code.

The old code (VBA) was using a string value to store the data, example function for Double value:
Private Function DoubleToStr(ByVal value As Double) As String
Dim Bytes(7) As Byte ' 0-7 Double is 8 bytes
Dim n As Long
CopyMemory(Bytes(0), value, 8)
DoubleToStr = ""
For n = 0 To UBound(Bytes)
DoubleToStr = DoubleToStr & Chr(Bytes(n))
Next n
End Function

The code basically copies the memory contents (using API call CopyMemory) of the memory (held by the double variable) to the memory held by a byte array. It then reads the byte array and converts it into a string. Workarounds could be to convert a bool or long to a string, though that would be expensive in terms of memory/storage eg. 1234564787 would be 10 bytes in a string where as a long is only 4 bytes (factor 2.5 increase).

How to retain the original precision? Converting it to a string would result in endless numbers. Another problem (at least I think it could be) is that in VBA the variables are rather simple, but in .NET they are all objects, so how to retrieve the actual memory location of the value? let alone garbage collection moving stuff around. How to get those variables stored in my byte array?