Get count of credit cards that are expired
If a class has one or more private constructors and no public constructors, other classes (except nested classes) cannot create instances of this class.


https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/private-constructors

Make your constructor public.

There is an obvious bug in GetCountOfExpiredCreditCards  A card that expires in 01/19(January, 2019) will show up as expired if checked today.  Check for both year and month.