public interface IMarker {}

​

public class Impl1 : IMarker {}

​

public class Program

{

    //Compiles Ok

    public static T? GetServiceDefaultConstruct<T>() where T : IMarker, new() {

        return new T();

    }

    // Doesn't compile, says that T can be a non-nullable value type.

    public static T? GetServiceNull<T>() where T : IMarker {

        return default(T);

    }

    // Compiles Ok

    public static T? GetServiceNullExplicitConstraint<T>() where T : class,IMarker {

        return null;

    }

    // Doesn't compile: "Can not convert Impl1 to T"

    public static T? GetService<T>() where T : class,IMarker {

        if (typeof(T) == typeof(Impl1))

            return (T) (object) new Impl1();

        return null;

    }

}