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The user might choose to override some of the values associated with the current culture of Windows through the regional and language options portion of Control Panel. For example, the user might choose to display the date in a different format or to use a currency other than the default for the culture.
If CultureInfo.UseUserOverride is true and the specified culture matches the current culture of Windows, the System.Globalization.CultureInfo uses those overrides, including user settings for the properties of the System.Globalization.DateTimeFormatInfo instance returned by the CultureInfo.DateTimeFormat property, and the properties of the System.Globalization.NumberFormatInfo instance returned by the CultureInfo.NumberFormat property. If the user settings are incompatible with the culture associated with the System.Globalization.CultureInfo, for example, if the selected calendar is not one of the CultureInfo.OptionalCalendars, the results of the methods and the values of the properties are undefined.
Therefore, if CultureInfo.UseUserOverride is true, the value of this property might be different from the default calendar used by the culture.
Your application changes the calendar used by the current System.Globalization.CultureInfo by setting the DateTimeFormatInfo.Calendar property of CultureInfo.DateTimeFormat, which is an instance of the System.Globalization.DateTimeFormatInfo class. The new calendar must be one of the calendars listed in CultureInfo.OptionalCalendars. CultureInfo.DateTimeFormat also includes other properties that customize the date and time formatting associated with that DateTimeFormatInfo.Calendar.