TryParse failing with negative numbers

Question

I'm having a problem getting TryParse to work correctly for me. I have a list of values that I am almost assured are valid (as they come from another component in our system) but I would like to make sure there is proper error handling in place.

Here is an example list of my values:

20.00
20.00
-150.00

And here is the method I originally wrote:

 private decimal CalculateValue(IEnumerable<XElement> summaryValues)
        {
            decimal totalValue = 0;

            foreach (XElement xElement in summaryValues)
            {
                decimal successful;
                Decimal.TryParse(xElement.Value, out successful);
                if (successful > 0)
                    totalValue += Decimal.Parse(xElement.Value);
            }
            return totalValue;
        }

The variable 'successful' was returning false for -150.00, so I added NumberStyles:

private decimal CalculateValue(IEnumerable<XElement> summaryValues)
        {
            decimal totalValue = 0;

            foreach (XElement xElement in summaryValues)
            {
                decimal successful;
                Decimal.TryParse(xElement.Value, NumberStyles.AllowLeadingSign, null, out successful);
                if (successful > 0)
                    totalValue += Decimal.Parse(xElement.Value, NumberStyles.AllowLeadingSign);
            }
            return totalValue;
        }

However, now that I have the NumberStyles in there, none of the numbers will parse! I feel good about having IFormatProvider set to null as this is all within our system. Does anyone see what I may be doing wrong?

Answer 1

This is not how you are supposed to use TryParse.

TryParse returns a boolean (true/false), so your code above should be:

private decimal CalculateValue(IEnumerable<XElement> summaryValues)
        {
            decimal totalValue = 0;

            foreach (XElement xElement in summaryValues)
            {
                decimal valueReturned;
                bool successful = Decimal.TryParse(xElement.Value, out valueReturned);
                if (successful)
                    totalValue += valueReturned;
            }
            return totalValue;
        }

or more succinctly,

private decimal CalculateValue(IEnumerable<XElement> summaryValues)
        {
            decimal totalValue = 0;

            foreach (XElement xElement in summaryValues)
            {
                decimal valueReturned;
                if (Decimal.TryParse(xElement.Value, out valueReturned))
                    totalValue += valueReturned;
            }
            return totalValue;
        }

Answer 2

Others are explaining how to do it right, but not really explaining what you're doing wrong.

Where you're using "successful" above isn't the success value, it's the actual number that is being parsed. So if you're parsing "-150.00" of course successful will be negative. The out value of TryParse is the actual parsed value and the boolean indicating whether the process was successful or not is the returned value. Using what you have to help understand would be something like:

string inputValue = "-150.00";
decimal numericValue;
bool isSucessful = Decimal.TryParse(inputValue , out numericValue);

In this case, isSuccessful will be TRUE, numericValue will be -150. When you're using user-provided values instead of the hardcoded one I used above you'll want to check:

if(isSuccessful)
{
    // Do something with numericValue since we know it to be a valid decimal
}
else
{
    // Inform User, throw exception, etc... as appropriate, Don't use numericValue because we know it's wrong.
}