"DataSeries" variable

I'm trying to read the previous value of a double variable but I can't. Can you help me? What type of variable may I use for do this? In the "OnQuote" I need to read the previous value of VarX variable of type double.

public override void OnQuote()
        {
           double a = 25.5; 
           VarX = Indicators.iMA(CurrentData, 7, PTLRuntime.NETScript.Indicators.MAMode.SMA, 0, PriceType.Close).GetValue()-a + VarX[1]; }