How to use double.TryParse when the output is allowed to be null?

Question

In my application I have a textbox - txtDiscount where the admin can set a discount percentage for a certain user, or he may not. On the back end I want to save the data so for now I have this:

double? discount = null;
if (!string.IsNullOrEmpty(txtDiscount.Text))
{
  if (!double.TryParse(txtDiscount.Text, out discount)) errors.Add("Discount must be a double.");
}

So I get an error for invalid argument and obviously it's the discount which can not be nullable if I'm gonna use it in TryParse. I saw that many people are making extensions for this type of situations but for now I don't think it's necessary. What I can think of is using another variable like so :

double? discount = null;
private double _discount;
if (!string.IsNullOrEmpty(txtDiscount.Text))
{
  if (!double.TryParse(txtDiscount.Text, out _discount)) 
  {
    errors.Add("Discount must be adouble.");
  }
  else
  {
    discount = _discount;
  }
}

and then use my nullable discount to pass the value to the database. But I actually don't like the code above, it seems to me pretty complicated for such a task but I can't think of something better. So how can I deal with this situation without using extension method?

Answer 1

I know it is an old question, but with the new versions of C# it is possible to use this way:

double? discount;
discount = (double.TryParse(myValueToParse, out double myParsedDouble) ? myParsedDouble : null);

In this way, you try to parse to the local variable myParseddouble. If it is possible, it is assigned as value, if not, it set to null.

Answer 2

You can do parsing without extension method - just use local non-nullable value to pass it to TryParse method:

double? discount = null;

if (!String.IsNullOrEmpty(txtDiscount.Text))
{   
   double value;
   if (Double.TryParse(txtDiscount.Text, out value))       
       discount = value;
   else       
       errors.Add("Discount must be a double."); // discount will have null value
}

But I'd moved all this logic to extension.

Answer 3

You're just going to have to write the ugly code with a local non-nullable type or use another way of defining that there's no discount. I agree that a nullable double is a neat way of representing it, but if the code annoys you so then try something different (a bool, for example: discount_given = true).

Personally, I'd just go with an extension that parses to nullable double:

    public static bool ParseDouble(string s, out double? dd)
    {
        double d;
        bool ret = double.TryParse(s, out d);

        if (ret)
            dd = d;
        else
            dd = null;

        return ret;
    }