public Visit Get(long id, bool loadGraph = false) {
	const string sqlVisit = @"SELECT * FROM Visit WHERE Id = @VisitId;";
	const string sqlCC = @"SELECT * FROM ChiefComplaint WHERE VisitId = @VisitId;";
	var sqlParam = new { VisitId = id };

	if (loadGraph) {
		using (var extractor = base.ExecuteQueryMultiple(sqlVisit + sqlCC, sqlParam)) {
			/*fails with exception*/var v = extractor.Extract<Visit>().FirstOrDefault();
			var cc = extractor.Extract<ChiefComplaint>().AsList();
			return new Visit(v.Id, v.DateOfService, cc);
		}                
	}
	else {
		using (var extractor = base.ExecuteQueryMultiple(sqlVisit, sqlParam)) {
			return extractor.Extract<Visit>().FirstOrDefault();
		}
	}
}

public class Visit : BaseEntity {

	private List<ChiefComplaint> _chiefComplaints = new List<ChiefComplaint>();
	public IReadOnlyList<ChiefComplaint> ChiefComplaints => _chiefComplaints.ToList();
	public DateTime DateOfService { get; }

	public Visit(long id, DateTime dateOfService, IEnumerable<ChiefComplaint> chiefComplaints) : base(id) {
		this.DateOfService = dateOfService;
		_chiefComplaints.AddRange(chiefComplaints);
	}
}

public abstract class BaseEntity {
	public long Id { get; }

	protected BaseEntity(long id) {
		this.Id = id;
	}

	public override bool Equals(object obj) {
		if (!(obj is BaseEntity other))
			return false;

		if (ReferenceEquals(this, other))
			return true;

		if (this.GetType() != other.GetType())
			return false;

		if (this.Id == 0 || other.Id == 0)
			return false;

		return this.Id == other.Id;
	}

	public static bool operator ==(BaseEntity a, BaseEntity b) {
		if (a is null && b is null)
			return true;

		if (a is null || b is null)
			return false;

		return a.Equals(b);
	}

	public static bool operator !=(BaseEntity a, BaseEntity b) {
		return !(a == b);
	}

	public override int GetHashCode() {
		return (this.GetType().ToString() + this.Id).GetHashCode();
	}

}

CREATE TABLE [dbo].[Visit] (
    [Id]            BIGINT        IDENTITY (1, 1) NOT NULL,
    [DateOfService] DATETIME2 (7) NOT NULL
);

CREATE TABLE [dbo].[ChiefComplaint] (
    [Id]          BIGINT         IDENTITY (1, 1) NOT NULL,
    [Description] NVARCHAR (MAX) NULL,
    [HpiId]       BIGINT         NULL,
    [VisitId]     BIGINT         NULL
);

 HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=RepoDb
  StackTrace:
   at RepoDb.Reflection.Compiler.<>c__DisplayClass62_0`1.<GetClassPropertyParameterInfos>b__1(ParameterInfo parameterInfo)
   at System.Collections.Generic.List`1.ForEach(Action`1 action)
   at RepoDb.Reflection.Compiler.GetClassPropertyParameterInfos[TResult](IEnumerable`1 readerFieldsName, IDbSetting dbSetting)
   at RepoDb.Reflection.Compiler.GetMemberBindingsForDataEntity[TResult](ParameterExpression readerParameterExpression, IEnumerable`1 readerFields, IDbSetting dbSetting)
   at RepoDb.Reflection.Compiler.CompileDataReaderToDataEntity[TResult](DbDataReader reader, IEnumerable`1 dbFields, IDbSetting dbSetting)
   at RepoDb.Reflection.Compiler.CompileDataReaderToType[TResult](DbDataReader reader, IEnumerable`1 dbFields, IDbSetting dbSetting)
   at RepoDb.Reflection.FunctionFactory.CompileDataReaderToType[TResult](DbDataReader reader, IEnumerable`1 dbFields, IDbSetting dbSetting)
   at RepoDb.FunctionCache.DataReaderToTypeCache`1.Get(DbDataReader reader, IEnumerable`1 dbFields, IDbSetting dbSetting)
   at RepoDb.FunctionCache.GetDataReaderToTypeCompiledFunction[TResult](DbDataReader reader, IEnumerable`1 dbFields, IDbSetting dbSetting)
   at RepoDb.Reflection.DataReader.<ToEnumerable>d__0`1.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at RepoDb.Extensions.EnumerableExtension.AsList[T](IEnumerable`1 value)
   at RepoDb.QueryMultipleExtractor.Extract[TEntity](Boolean isMoveToNextResult)
   at Repositories.VisitRepository.Get(Int64 id, Boolean loadGraph) 

public class ChiefComplaintPropertyHandler : IPropertyHandler<string, ChiefComplaint> {
	public ChiefComplaint Get(string input, ClassProperty property) {
		return JsonConvert.DeserializeObject<ChiefComplaint>(input);
	}
	public string Set(ChiefComplaint input, ClassProperty property) {
		return JsonConvert.SerializeObject(input);
	}
}

public static class DependencyInjection {
	public static IServiceCollection AddDataCoreServices(this IServiceCollection services, IConfigurationSection dbConfigSection) {
		SqlServerBootstrap.Initialize();
		FluentMapper
			.Entity<Visit>()
			.PropertyHandler<ChiefComplaintPropertyHandler>(v => v.ChiefComplaints, true);

		services.Configure<AppSetting>(dbConfigSection);
		services.AddTransient<IUnitOfWork, UnitOfWork>();
		services.AddTransient<IVisitRepository, VisitRepository>();

		return services;
	}
}

public void ConfigureServices(IServiceCollection services) {
	services.AddControllers();
	services.AddDataCoreServices(Configuration.GetSection("AppSettings"));
}