how to aplly formulas on range in excel using c#

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MS-Excel

Hello,
I have a VB code like the one given below.
I'm trying to achieve the same functionality in c#. But could not find the Replace function as in VB.
VB Code

Index = 2;
formula = "8.09331951577158-3.45596769548775E-02*X^1+1.79729745455284E-05*X^2-3.31696353974908E-09*X^3+2.77566533348736E-13*X^4-8.40169194251658E-18*X^5";

Set xRange = vSheet.Range(vSheet.Cells(1, (Index * 4) - 1), vSheet.Cells(500, (Index * 4) - 1))
        Set yRange = vSheet.Range(vSheet.Cells(1, (Index * 4)), vSheet.Cells(500, (Index * 4)))

        xMinVal = 390
        
        xMaxVal = 13400
    
        step = (xMaxVal - xMinVal) / (500- 1)
        
        Xvalue(1, 1) = xMinVal
        For i = 2 To 500 - 1
            Xvalue(i, 1) = Xvalue(i - 1, 1) + step
        Next i
        Xvalue(500, 1) = xMaxVal
        
        xRange.value = Xvalue
        vSheet.Names.Add name:="X" & Index & "P", RefersTo:=xRange
        yRange = "=" & Replace(Formula, "X", "X" & Index & "P")

c# Code

xRange = (Excel.Range)newWorksheet.get_Range((Excel.Range)newWorksheet.Cells[1, (2 * 4) - 1], (Excel.Range)newWorksheet.Cells[NB_POINTS_POLYNOME, (2 * 4) - 1]);
           yRange = (Excel.Range)newWorksheet.get_Range((Excel.Range)newWorksheet.Cells[1, (2 * 4)], (Excel.Range)newWorksheet.Cells[NB_POINTS_POLYNOME, (2 * 4)]);

           step = (xmax - xmin) / (NB_POINTS_POLYNOME - 1);
           xValue[1,0] = xmin;

           for (int i = 2; i < 500; i++)
           {
               xValue[i,0] = xValue[(i - 1),0] + step;
           }
           xValue[NB_POINTS_POLYNOME-1,0] = xmax;
           xRange.Value2 = xValue;
          
    newWorksheet.Names.Add("X", xRange);
yRange = (Excel.Range)newWorksheet.Range.Replace(formula, "X", "X", 2, Type.Missing, Type.Missing, Type.Missing, Type.Missing);

I am unable to retrieve Y range values. I am not getting how to replace the formula as in VB so that Yrange is retrieved correctly.

Thanks in advance.
Jyoti