Cast object to decimal? (nullable decimal)

Question

If have this in the setter of a property:

decimal? temp = value as decimal?;

value = "90"

But after the cast, temp is null...

What is the proper way to do this cast?

Answer 1

Unboxing only works if the type is identical! You can't unbox an object that does not contain the target value. What you need is something along the lines of

decimal tmpvalue;
decimal? result = decimal.TryParse((string)value, out tmpvalue) ?
                  tmpvalue : (decimal?)null;

This looks whether the value is parsable as a decimal. If yes, then assign it to result; else assign null. The following code does approximately the same and might be easier to understand for people not familiar with the conditional operator ?::

decimal tmpvalue;
decimal? result = null;
if (decimal.TryParse((string)value, out tmpvalue))
    result = tmvalue;

Answer 2

and if you use decimal? temp = (decimal?)value;

Answer 3

you should parse the decimal. But if you want your decimal to be null when the string is not correct, use TryParse :

decimal parsedValue;
decimal? temp = decimal.TryParse(value, out parsedValue)
                ? value
                : (decimal?)null;

This way you will avoid exceptions while parsing ill formated strings.

Almost all primitive types provide a Parse and TryParse methods to convert from string.

Is is also recommended to pass a culture for the provider argument to the method to avoid problems with the decimal separator. If you're reading from another system, CultureInfo.InvariantCulture is probably the way to go (but it's not the default).

bool TryParse(string s, NumberStyles style,
  IFormatProvider provider, out decimal result)