Excel/Expression/Math operation on DataTable

want to put Minimum Value of (cpKL,cpKR) into cpK How to write the expression for cpK


sigma.Expression = "(Convert(point90,'System.Double')-Convert(point10,'System.Double'))/2.56";

cpKL.Expression = "(Convert(point50,'System.Double')- Convert(LSL,'System.Double'))/(3.0*sigma)";

cpKR.Expression = "(Convert(USL,'System.Double') - Convert(point50,'System.Double'))/(3.0*sigma)";

// The expression contains undefined function call Math.Min().
//cpK.Expression = "Math.Min((Convert(cpKL,'System.Double'),(Convert(cpKR,'System.Double'))";

Consider only this .Here  I have to inser new column Sigma and cpk and find the values

using System;
using System.Data;
using Microsoft.VisualBasic.FileIO;


namespace ReadDataFromCSVFile
{
    static class Program
    {
        static void Main()
        {
            string csv_file_path = @"C:\Matlab\limit_sheet.csv";
            DataTable table1 = GetDataTabletFromCSVFile(csv_file_path);
           

       
        }


        private static DataTable GetDataTabletFromCSVFile(string csv_file_path)
        {
            DataTable table1 = new DataTable();
              using (TextFieldParser csvReader = new TextFieldParser(csv_file_path))
                {
                    csvReader.SetDelimiters(new string[] { "," });
                    csvReader.HasFieldsEnclosedInQuotes = true;
                    string[] colFields = csvReader.ReadFields();
                    foreach (string column in colFields)
                    {
                        DataColumn datecolumn = new DataColumn(column);
                        datecolumn.AllowDBNull = true;
                        table1.Columns.Add(datecolumn);
                    }
                    while (!csvReader.EndOfData)
                    {
                        string[] fieldData = csvReader.ReadFields();
                        //Making empty value as null
                        for (int i = 0; i < fieldData.Length; i++)
                        {
                            if (fieldData[i] == "")
                            {
                                fieldData[i] = null;
                            }
                        }
                       


                        table1.Rows.Add(fieldData);
                    }
                }    
           /*   DataColumn sigma = table1.Columns.Add("Sigma", typeof(double));
              DataColumn cpK = table1.Columns.Add("CpK", typeof(double));


           // set their expression properties
              sigma.Expression = "point90 - point10 /2.56";


              cpK.Expression = "IIF(([point50] - LSL) < (USL - [point50]), [point50] - LSL, USL - [point50])/3/Sigma"; */


       


              return table1;
        }
    }
}

I was expecting all the columns except the first to be doubles.

But, if COL is a string, then replace it in the Expression property with:

Convert(COL, 'System.Double')
 

cpK.Expression = "IIF(([point50] - LSL) < (USL - [point50]), [point50] - LSL, USL - [point50])/3/Sigma";
Cannot perform '-' operation on System.Double and System.String.

sigma.Expression = "point90 - point10 /2.56"; Error cannot perform / on System.String