How should I parse Infinity with Double?

Question

If I try

Double.Parse("Infinity")

I get

Double.Parse("Infinity") threw an exception of type 'System.FormatException'

Why? And what should I do if I want to parse it anyway and get a Double with the value Infinity?

Answer 1

I just found this out:

Decimal.Parse("Infinity", System.Globalization.CultureInfo.InvariantCulture);

will work and return a double with the value +Infinity.

The reason it did not work is that I am not, I think, automatically in the InvariantCulture but perhaps in the de-DE culture which does not handle the exact string "Infinity". (Perhaps it would handle some other string.)

Answer 2

All the below parsing are valid, since your system settings are different is causing the issue. As dontomaso had answered above just need to add the Invariant Culture.

Double.Parse("NaN", System.Globalization.CultureInfo.InvariantCulture)
Double.Parse("-Infinity", System.Globalization.CultureInfo.InvariantCulture)
Double.Parse("Infinity", System.Globalization.CultureInfo.InvariantCulture)

Answer 3

Maybe try this:

if(yourstring.ToLower() == "infinity")
{
   yourdouble = double.PositiveInfinity;
}

Answer 4

you can use double.PositiveInfinity .The value of this constant is the result of dividing a positive number by zero.

double infinity = double.PositiveInfinity;

Answer 5

.NET has inconsistent behavior for parsing infinity and negative infinity depending on culture.

If you are using the CultureInfo.InvariantCulture, the strings to use are "Infinity" and "-Infinity".

var infinity = double.Parse("Infinity", CultureInfo.InvariantCulture);
var negativeInfinity = double.Parse("-Infinity", CultureInfo.InvariantCulture);

However, if you are in a specific culture, the strings to use are "∞" and "-∞".

var infinity = double.Parse("∞", new CultureInfo("en-US"));
var negativeInfinity = double.Parse("-∞", new CultureInfo("en-US"));

Unfortunately, it doesn't seem that there are strings that will parse to infinity and negative infinity that work in every culture by default, including CultureInfo.InvariantCulture. If you try to parse "∞" or "-∞" using CultureInfo.InvariantCulture, it will throw a FormatException.

Workaround

If you don't mind adding an additional helper method, you could make this work in any culture by changing the NumberFormatInfo.PostiveInfinitySymbol and NumberFormatInfo.NegativeInfinitySymbol to make every culture parse the strings you want.

public static IFormatProvider NormalizeInfinity(
    string positiveInfinitySymbol = "Infinity",
    string negativeInfinitySymbol = "-Infinity")
{
    return NormalizeInfinity(CultureInfo.CurrentCulture, positiveInfinitySymbol, negativeInfinitySymbol);
}

public static IFormatProvider NormalizeInfinity(
    IFormatProvider provider,
    string positiveInfinitySymbol = "Infinity",
    string negativeInfinitySymbol = "-Infinity")
{
    NumberFormatInfo numberFormat;
    if (provider is null)
    {
        numberFormat = (NumberFormatInfo)CultureInfo.InvariantCulture.NumberFormat.Clone();
    }
    else
    {
        numberFormat = (NumberFormatInfo)((NumberFormatInfo)provider.GetFormat(typeof(NumberFormatInfo))).Clone();
    }
    numberFormat.PositiveInfinitySymbol = positiveInfinitySymbol;
    numberFormat.NegativeInfinitySymbol = negativeInfinitySymbol;
    return numberFormat;
}

Usage

double infinity = double.Parse("Infinity", NormalizeInfinity());
double negativeInfinity = double.Parse("-Infinity", NormalizeInfinity());

double aDouble = double.Parse("1.23456", NormalizeInfinity());

double notANumber = double.Parse("NaN", NormalizeInfinity());

Note the above will work with the same symbols for infinity and negative infinity regardless if you set the CultureInfo.CurrentCulture to CultureInfo.InvariantCulture or not.

You can also pass through any culture and it will change the infinity and negative infinity settings, but leave all of the others intact.

double infinity = double.Parse("Infinity", NormalizeInfinity(new CultureInfo("de")));