You are getting this problem because you have not imported the namespace “System.Globalization”.
once you will import it, you will not get that error.
Not all cultures use the same format for dates and decimal / currency values.
This will matter for you when you are converting input values (read) that are stored as strings to DateTime, float, double or decimal. It will also matter if you try to format the aforementioned data types to strings (write) for display or storage.
In the Windows operating system the user may even customize how numbers and date/times are formatted and may also choose another culture than the culture of his operating system. The formatting used is the choice of the user which is how it should be.
So when you format a value to be displayed to the user using for instance ToString or String.Format or parsed from a string using DateTime.Parse or Decimal.Parse the default is to use the CultureInfo.CurrentCulture. This allows the user to control the formatting.
However, a lot of string formatting and parsing is actually not strings exchanged between the application and the user but between the application and some data format (e.g. an XML or CSV file). In that case you don’t want to use CultureInfo.CurrentCulture because if formatting and parsing is done with different cultures it can break. In that case you want to use CultureInfo.InvariantCulture (which is based on the en-US culture). This ensures that the values can roundtrip without problems