Generic property type in intercace and nullable error

Question

Non generic interface has no issue with nullable mark. In generic interface below marking TKey as nullable generates compiler error. Why ? And is it possible to mark generic property type as nullable ?

Error CS0738 'TClass' does not implement interface member 'ITest.Info1'. 'TClass.Info1' cannot implement 'ITest.Info1' because it does not have the matching return type of 'int'.

    public interface ITest<TKey>
{
    public TKey? Info1 { get; set; }
    public BHPGuard.Domain.LocationType.LocationType? Type { get; set; }

}

public class TClass : ITest<int>
{
    public int? Info1 { get; set; }
    public LocationType.LocationType? Type { get; set; }
    // int ITest<int>.Info1 { get; set; }
}

Answer 1

Why ?

The problem here is that TKey is unconstrained generic type, so for value types TKey? is TKey, so to implement the interface you can do:

public class TClass : ITest<int>
{
    public int Info1 { get; set; }
    public LocationType? Type { get; set; }
}

or

public class TClass : ITest<int?>
{
    public int? Info1 { get; set; }
    public LocationType? Type { get; set; }
}

Alternatively you can constrain the generic parameter to be a struct:

public interface ITest<TKey> where TKey : struct
{
    public TKey? Info1 { get; set; }
    // ...
}

is it possible to mark generic property type as nullable ?

Yes, but due to the difference of how nullable reference and nullable value types are treated (the later are actually represented via type Nullable<T> while former are not) it has quite some limitations in terms of how it will behave depending on the constraints defined.

See Nullability and generics in .NET 6 and Why does nullability check not work with value types? for more info.