How can I perform a mathematical calculation on real numbers stored as strings?

Question

I have a class that contains values that are sometimes monetary values (such as 1.25, 2.50, 10.00, etc.) but can also be the string "N/A"; thus, they are declared as string:

public class PriceVarianceDataExtended
{
    public String ShortName { get; set; }
    public String ItemCode { get; set; }
    public String Description { get; set; }

    public string Week1PriceCraftworks { get; set; }
    public string Week2PriceCraftworks { get; set; }
    public string VarianceCraftworks { get; set; }

    public string Week1PriceChophouse { get; set; }
    public string Week2PriceChophouse { get; set; }
    public string VarianceChophouse { get; set; }

    public string Week1PriceGordonBiersch { get; set; }
    public string Week2PriceGordonBiersch { get; set; }
    public string VarianceGordonBiersch { get; set; }

    public string Week1PriceOldChicago { get; set; }
    public string Week2PriceOldChicago { get; set; }
    public string VarianceOldChicago { get; set; }

    public string Week1PriceOldChiFranchise { get; set; }
    public string Week2PriceOldChiFranchise { get; set; }
    public string VarianceOldChiFranchise { get; set; }

    public string Week1PriceRockBottom { get; set; }
    public string Week2PriceRockBottom { get; set; }
    public string VarianceRockBottom { get; set; }
}

However, this code, meant to store the monetary value (as a string) where that makes sense, and otherwise set the val to "N/A":

private string NOT_APPLICABLE = "N/A";
. . .
if (pvde.Week1PriceCraftworks != NOT_APPLICABLE && 
    pvde.Week2PriceCraftworks != NOT_APPLICABLE)
{
    pvde.VarianceCraftworks = 
        Convert.ToDecimal(pvde.Week2PriceCraftworks) - 
        Convert.ToDecimal(pvde.Week1PriceCraftworks).ToString();
}
else
{
    pvde.VarianceCraftworks = NOT_APPLICABLE;
}

...fails to compile, telling me, "Operator '-' cannot be applied to operands of type 'decimal' and 'string'"

So this is my logic:

If the "if" block is entered, that means Week1PriceCraftworks and Week2PriceCraftworks contains numerical values represented as strings, such as "5.00" and "3.14"

I then convert "5.00" to 5 and "3.14" to 3.14 using the Convert.ToDecimal() calls. That should produce a value of 1.86 which is then converted to string ("1.86") so that the correct data type is assigned to pvde.VarianceCraftworks.

But, reasonable and straightforward as that seems to me, I get that err msg. What is wrong with my approach?

BTW, the "week1" and "week2" values are set by calling a method that ends this way:

return price ?? NOT_APPLICABLE;

...so what is contained in Week1PriceCraftworks and Week2PriceCraftworks are either real values (stored as strings) or "N/A".

Answer 1

The issue is the order of precedence based on your parenthesis. It's evaluating to Decimal - String because of the ToDecimal().ToString().

If you change it to the below it should work

(
  Convert.ToDecimal(pvde.Week2PriceCraftworks) - 
  Convert.ToDecimal(pvde.Week1PriceCraftworks)
).ToString();