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C# float to string separator

I am new to C # and I am currently having problems with the following. In C #, I have a Pi floating point number and I want to convert it to a string using the ToString() method. But the conversion gives a string result with a comma "3,1415". On another machine, the same gives the string result with the dot "3.1415". What is the reason for this and what should I do to get a dotted string result?
EDIT: The problem is, I can't change the code, but I can install and uninstall .Net frameworks, change my OS settings, etc.

Edit: if you can't change the code. Change the language/localization of the system to one which uses dot as decimal separator. In Control Panel or Settings.
You should look at internationalization and localization in the System.Globalization namespace.
The advice here is to use one CultureInfo specific for parsing numbers or writing numbers to string.
var flt = 232.23f;
var str = flt.ToString(CultureInfo.InvariantCulture); //for example, you can use CultureInfo.CurrentCulture
This allows you to keep the ThreadCulture without change it.
But take a look at this link https://learn.microsoft.com/en-us/dotnet/api/system.globalization.cultureinfo?view=net-5.0 .Take your time, is dense.

I would just set the current culture at the entry point of your program.
Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture("en-GB");
However I would also check the regional settings and so forth on the machine where the comma separator appears.

It is related with current culture info. You can specify the culture info in ToString method as a parameter like;
var convertedFloat = floatVariable.ToString(new CultureInfo("en-GB"));

Thanks to GSerg, for comment about Windows regional settings. That solves my problem. In the Windows Control panel enter Region and Language. In the Formats tab click Additional Settings and in the Decimal symbol field specify what decimal separator must be used when converting a floating point number to a string.

Get Culture by currency

Well... i search the web and found many solutions for the other way, but none for these.
I have a application which gets different currencys by the user. I dont know the currencys in before, it could be everything (russian rubels, usd, €, Yen...)
I need to convert the amount into a decimal, for that i need the current culture. My current solution is very bad 8and incomplete, cause i cant cover all cultures that way), it just checks the currency sign.
if (currency.Contains("zł"))
{
cult = CultureInfo.GetCultureInfo("PL-pl")
}
else if (currency.Contains("$"))
{
//blah blah blah
}
Is there a possiblility to get Culture base on the currency sign. Another maybe difficult thing is, that i dont know if the currency symbol is before or beyond the amount (varys by culture i.E: $45.00 <-> 45.00€)

Create a lookup once and use it for fast access. Notice that a particular currency symbol may be used by multiple cultures:
ILookup<string, CultureInfo> cultureByCurrency =
CultureInfo.GetCultures(CultureTypes.AllCultures)
.ToLookup(_ => _.NumberFormat.CurrencySymbol);
Then to lookup $ for example:
IEnumerable<CultureInfo> culturesThatUseDollar = cultureByCurrency["$"];

There is no exact mapping from a currency code or symbol to a culture. Consider basic examples like EUR (€), which is used as the official currency in 18 countries. There are many issues arising from this mere fact, like whether the symbol is placed before or after the value etc. You should ask the user about the specific formatting to use instead of trying to deduce it from the currency symbol.
Also, a single currency symbol is used for many currencies. Consider that $ can denote both USD, CAD, AUD and other currencies that call themselves as 'dollars'. You should use currency codes if you want an exact specification of a currency.

It is not possible.
EUR for example would map to de-DE, fr-FR, nl-NL and other countries.
There is no mapping from Currency to culture, because multiple countries share currencies

In your else if block, which culture would you assign after finding the $? en-US? fr-CA?
I would suggest a different approach that would remove any sort of ambiguity. Have the user specify their nationality before entering this chunk of code. Consider having the culture information given to you instead of attempting to guess it.

return CultureInfo.GetCultures(CultureTypes.AllCultures).Where(c => c.NumberFormat.CurrencySymbol.Equals("$"));

Make Convert.ToDouble work with different CultureInfos

Whats the best way to make the conversion work when you have:
string a = "10.0123";
string b = "10,0123";
And the cultureinfo is either swedish or english, it needs to work with both.
I tried:
double aSwe = Convert.ToDouble(a, CultureInfo.GetCultureInfo("sv-SE"));
double bSwe = Convert.ToDouble(b, CultureInfo.GetCultureInfo("sv-SE"));
double aInv = Convert.ToDouble(a, CultureInfo.InvariantCulture);
double bInv = Convert.ToDouble(b, CultureInfo.InvariantCulture);
Since '.' is not a valid separator in Swe and ',' is not valid in Eng I dont know how to make it work with both using the same code.
Only solution I come up with is to replace the comma or dot before converting but it feels like there should be better solution?

You shouldn't try to make it work with both without any extra context.
It's like trying to parse "06/05/2010" as a date without any cultural information: it could mean the May 6th or June 5th.
Likewise "1,234" is either a value a bit more than a thousand, or a bit more than one: you need to know the cultural information in order to interpret it unambiguously.
So instead of trying to solve the problem of interpreting something without enough information, I suggest you focus on the problem of getting all the information you need (or changing the way you get your text data to always be in one particular format).

How to prohibit comma as decimal separator

I'm writing code with german culture settings
Nevertheless I would like to force the user to use the point as a decimal separator.
My piece of test code outputs the wrong value.
How do I detect the "wrong" comma ?(throw an exception)
string nok_str = "14,9";
string ok_str = "14.9";
double nok_Var1 = double.Parse(nok_str, CultureInfo.InvariantCulture.NumberFormat); // outputs 149.0
double nok_Var2 =Convert.ToDouble(nok_str, CultureInfo.InvariantCulture.NumberFormat); // outputs 149.0

First off, and please forgive me, I’d like to question your design decision:
How is this enhancing the user experience? The application should rather try to accept all unambiguous user input, not reject theoretically sound input.
That said, a number such as “19,2” will be interpreted, with an invariant culture, as having a thousands separator (which is simply discarded). This is why your code silently produces bad values. If you want to explicitly forbid this input, the easiest way to achieve this is an explicit test:
if (nok_str.Contains(","))
throw new FormatException(…);
As an alternative, you can try modifying the NumberFormatInfo.NumberGroupSeparator property of a custom NumberFormatInfo object that you pass to the Parse method.

Basically the default is to include AllowThousands in the number style. If you specify the number style you want, you can prohibit this:
using System;
using System.Globalization;
class Test
{
static void Main(string[] args)
{
string text = "19,2";
double value;
bool valid = double.TryParse(text, NumberStyles.Float,
CultureInfo.InvariantCulture,
out value);
Console.WriteLine(valid); // Prints false
}
}
Note that NumberStyles.Float is a composite style for AllowLeadingWhite, AllowTrailingWhite, AllowLeadingSign, AllowDecimalPoint, and AllowExponent - but not AllowThousands.

I am not sure what is the source of your input.
If it comes from user it also depends... If it is GUI application, you may think of restricting the input to certain possible keys, excluding comma. If it is a console app, you can try regular expressions to pre-validate input strings.
If it comes from various sources (i.e. web service) maybe simply brute-force string replace will do the trick?
Last, but not least: there are reasons for parsing to be culture-sensitive and if I were you, I would encourage users to enter valid regional number format instead forcing them to provide incorrect one.

How to make doubles always contain a . character?

I noticed my doubles contain different precision characters depending on regional settings.
For example:
3,47 or 3.45
How can I enforce the double should always contain a . precision character?
Problem is once I serialize the class containing this double to XML, it gets shipped to other systems expecting a standard result.
So after reading your responses (and thanks), do you guys recommend changing the property to a string, (making the replacements in a string), so that it serializes with the string value (not the double)?

You need to put the double to string using the Invariant Culture.
double d = 3.47;
Console.WriteLine(d.ToString(CultureInfo.InvariantCulture));

You will need to format with the InvariantCulture.
Note that the "." and "," formatting characters are interpreted according to the culture settings.

This little tutorial will be the answer you need, I expect:
http://www.java2s.com/Tutorial/CSharp/0440__I18N-Internationalization/Parsingnumberswithcultureinvariantparsing.htm
double.Parse(numberString, CultureInfo.InvariantCulture);

The double itself doesn't include a "." or a ",", only the print out of the representation does. You can read up on custom formats here.
[Update according to OP]
I don't know exactly what your design looks like, but it would probably be smart to create a string property on your DTO, which would output the formatted string of your double, and then mark your double property as not serializable.

When you need to do this for all numbers in your current applicaton, you can use the following to set it application-wide (new threads will inherit this setting):
// use this statement to force the default:
Application.CurrentCulture = CultureInfo.InvariantCulture;
string s = myNumber.ToString();
// for one number you have to remember to use:
string s = myNumber.ToString(CultureInfo.InvariantCulture);
Note: by default, your application, whether ASP.NET or WinForms, will use the culture settings of the system it is running on (in ASP.NET, you can set the culture globally in the web.config).