c# - changing Datatype of DataTable using decimal -

i comparing 2 datatables , built new table want sort values in new table since has -ve values(if not converted decimal -ve sign not considered) want convert decimal type string , return table sorting. getting error input string not in correct format how solve this?and sort -ve values in asc order

private static datatable comparetwodatatable(datatable table1, datatable table2) {     datatable table3 = new datatable();     datarow dr = null;     string filterexp = string.empty;     (int = 0; < table1.rows.count; i++)     {          string col = table1.rows[i]["parameter name"].tostring();         if (table2.columns.contains(col))         {             if (!table3.columns.contains(col))             {                 table3.columns.add(col, typeof(string));                 filterexp = filterexp + col + " asc ,";             }              (int j = 0; j < table2.rows.count; j++)             {                 if (table3.rows.count != table2.rows.count)                 {                     dr = table3.newrow();                     table3.rows.add(dr);                 }                 table3.rows[j][col] = (table2.rows[j][col].tostring());             }           }       }     dataview dv = new dataview(table3);     filterexp = filterexp.trimend(',');     dv.sort = filterexp;     table3 = dv.totable();      return table3; } 

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    private static datatable comparetwodatatable(datatable table1, datatable table2)         {          datatable table3 = new datatable();         datarow dr = null;         string filterexp = string.empty;         (int = 0; < table1.rows.count; i++)         {              string col = table1.rows[i]["par name"].tostring();             if (table2.columns.contains(col))             {                 if (!table3.columns.contains(col))                 {                     table3.columns.add(col, typeof(double));                     filterexp = filterexp + col + " asc ,";                 }                  (int j = 0; j < table2.rows.count; j++)                 {                     if (table3.rows.count != table2.rows.count)                     {                         dr = table3.newrow();                         table3.rows.add(dr);                     }                     table3.rows[j][col] = table2.rows[j][col];                 }               }           } 

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