Decimal and Thousands separator

Just pass the underlying number instead of a string.  Any formatting should only be for display purposes.

Your code could always force the culture to be that of the server:

System.Globalization.CultureInfo.CurrentCulture = new System.Globalization.CultureInfo("de-DE");

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For a list of available culture names scroll down towards the bottom of this list:
https://msdn.microsoft.com/en-us/library/system.globalization.cultureinfo(v=vs.110).aspx

if you want to handle the separators yourself, you could get the locale settings by WINAPI function GetLocaleInfo

    char szDefDec[4] = {0};
    GetLocaleInfo(LOCALE_USER_DEFAULT, LOCALE_SMONDECIMALSEP, szDefDec, sizeof(szDefDec));
    char szDefGrp[4] = {0};
    GetLocaleInfo(LOCALE_USER_DEFAULT, LOCALE_SMONTHOUSANDSEP, szDefGrp, sizeof(szDefGrp));

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the above is c code which also could be used (with little changes) in vb or c#.

for example you could remove the thousands group separator - if any - from string and replace decimal separator character by a period character. then you should be able to using a toDouble conversion without issues.

note, if you don't want to bother with locale seperators at all and have some information which range the numbers are in and how many digits the decimal fraction is, you could do a reverse loop on the string which might match the following pattern "##'###,##". then you would remove the ' and replace the comma by a period in order to get a valid string number that can be converted to double. you even could use a regular expression for this.

Sara