https://learn.microsoft.com/fr-fr/dotnet/api/system.globalization.numberformatinfo.nansymbol?view=netframework-4.8.1&viewFallbackFrom=net-6.0
NumberFormatInfo.NaNSymbol Property (System.Globalization) | Microsoft Learn
Gets or sets the string that represents the IEEE NaN (not a number) value.
property systemglobalizationmicrosoftlearn