using System;

using System.Threading.Tasks;

using NSubstitute;

var mockB = Substitute.For<IServiceB>();

var _sut = new ServiceA(mockB);

var method = _sut.Method();

Console.WriteLine(method == null);

Console.WriteLine((await method));

​

public class ServiceA(IServiceB service)

{

    public async Task<bool> Method() {

        return await service.GetValue();

    }

}

​

public interface IServiceB

{

    Task<bool> GetValue();

}