Problem to compare double with lenght over 15 digits in visual basic 6

I face a problem to compare double with lenght over 15 digits in visual basic 6.
For example:
Dim a  as double, b as double
a = "100000000000000000"
b = "100000000000000001"
when compare a = b .. this will return True. Why true ?
when a = a + 1.. it shows 1E + 17.. how to make it show 100000000000000001 ? Pls help. Thanks.
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alankwanAsked:
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JackOfPHCommented:
The only possible I know is to declare the variable variant.
Dim a, b
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JackOfPHCommented:
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TreyHCommented:
Working with that many significant digits (In VB6) would require a 3rd party library. I've used one called xlPrecision. There's a free edition you can download to try it out. It's mainly designed for Excel but I've used it in VB with no problem.

http://precisioncalc.com/xlprecision.html
http://precisioncalc.com/UseInVBA.html

Example Code for comparing your two numbers.

Dim oXLP As xlPrecision.cls_xlPrecision
Set oXLP = New xlPrecision.cls_xlPrecision
If oXLP.xlpISEQUAL("100000000000000000", "100000000000000001") Then
    MsgBox "Numbers are equal"
Else
    MsgBox "Numbers are not equal"
End If
Set oXLP = Nothing
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TreyHCommented:
Your call Dan, anything fine by me...
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JackOfPHCommented:
Yeah, its your call...
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