in dot net core model some data types contain ? at the end....what is the purpose of question mark

Question

[DisplayName("Early Grace Minutes")]
        public long? Grace_Early { get; set; }

        [DisplayName("Minimum Minutes For Present")]
        public long? Min_Present { get; set; }

        [DisplayName("Minimum Minutes For Full Day")]
        public long? Min_FullDay { get; set; }

what is mean by this question mark

what is the purpose of question mark.... i removed that question mark but no issue found

what is the purpose of question mark...

Answer 1

in dot net core model some data types contain ? at the end....what is the purpose of question mark link

Structs are similar to classes in that they represent data structures that can contain data members and function members. However, unlike classes, structs are value types and do not require heap allocation. A variable of a struct type directly contains the data of the struct, whereas a variable of a class type contains a reference to the data, the latter known as an object