Index Out Of Range Exception in C#?

I am trying to convert a program coded in VBA (MS Excel) to C#.
I keep getting an IndexOutOfRange Exception, although I see no probable cause for it.

Here is a snippet of my code in VBA:

Dim gAGA8Xj(21) As Double
    gAGA8Xj(1) = Methane
    gAGA8Xj(2) = Nitrogen
    gAGA8Xj(3) = CarbonDioxide
    gAGA8Xj(4) = Ethane
    gAGA8Xj(5) = Propane
    gAGA8Xj(6) = Water
    gAGA8Xj(7) = HydrogenSulfide
    gAGA8Xj(8) = Hydrogen
    gAGA8Xj(9) = CarbonMonoxide
    gAGA8Xj(10) = Oxygen
    gAGA8Xj(11) = iButane
    gAGA8Xj(12) = nButane
    gAGA8Xj(13) = iPentane
    gAGA8Xj(14) = nPentane
    gAGA8Xj(15) = nHexane
    gAGA8Xj(16) = nHeptane
    gAGA8Xj(17) = nOctane
    gAGA8Xj(18) = nNonane
    gAGA8Xj(19) = nDecane
    gAGA8Xj(20) = Helium
    gAGA8Xj(21) = Argon

and in C#:

double[] gAGA8Xj = new double[21];
gAGA8Xj[1] = Methane;
gAGA8Xj[2] = Nitrogen;
gAGA8Xj[3] = CarbonDioxide;
gAGA8Xj[4] = Ethane;
gAGA8Xj[5] = Propane;
gAGA8Xj[6] = Water;
gAGA8Xj[7] = HydrogenSulfide;
gAGA8Xj[8] = Hydrogen;
gAGA8Xj[9] = CarbonMonoxide;
gAGA8Xj[10] = Oxygen;
gAGA8Xj[11] = iButane;
gAGA8Xj[12] = nButane;
gAGA8Xj[13] = iPentane;
gAGA8Xj[14] = nPentane;
gAGA8Xj[15] = nHexane;
gAGA8Xj[16] = nHeptane;
gAGA8Xj[17] = nOctane;
gAGA8Xj[18] = nNonane;
gAGA8Xj[19] = nDecane;
gAGA8Xj[20] = Helium;
gAGA8Xj[21] = Argon;

I have tried pretty much everything I could think of - changing the base element index to 0, changing the size of the array, adding a null element indexed 0 etc.

Any ideas would be appreciated.
Thanks
Joleyn

Tags: C#