Sum value from RDLC report function ?

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C#

VB

RDLC

Hi I am using C# VS2010.With my report I am having function for sum the value by condition.It's working good, but as a starting value it's bringing from the previous period.Hence how to clear this problem?
Thank you

my function code
-----------------
Public MyValue as Decimal = 0 
Public Function MyAddToSum(ByVal mytax as Decimal) as Decimal   
MyValue = mytax + MyValue   
Return MyValue 
End Function 

My text box expression
----------------------
=Code.MyAddToSum(SUM(IIF(Fields!tax.Value="VAT", Fields!cst_vat_amount.Value, 0)))

output
------
date cst_vat_amount
01/01/2016 100.00
code.MyAddToSum() showing the startup value 10100.00, the value10000 coming from
unknown period. Therefore how to clear this problem?

Thanks for the Guidances and helps
Thanks again

What I have tried:

I try to add the common data field cst_vat_amount from the dataset when the field tax="VAT". Field cst_vat_amount contains cst value & vat value. I need to add only vat value. Thanks