mxelgin

сжатый ответ:

CreateMap<ExportRetail, ExportRetailDto>()
                .ForMember(dest => dest.ExportRetailNomenklatura, opt => opt.MapFrom((src, dest, _, ctx) =>
                    ExpandHelper.MapWithDepthControl(src, ctx, (source, depth) =>
                        src.ExportRetailNomenklaturaId == null ? null : ExpandHelper.HasExpand(ctx, "nomenklatura", depth)
                            ? ctx.Mapper.Map<NomenklaturaDto>(src.ExportRetailNomenklatura)
                            : new NomenklaturaDto { InternalId = src.ExportRetailNomenklaturaId.Value })))

public enum ExpandMatchMode
    {
        Exact,
        StartsWith,
        EndsWith,
        Contains
    }
    public static class ExpandHelper
    {
        public static bool HasExpand(ResolutionContext ctx, string pattern, int depth = 0)
        {
            return ctx.Items.TryGetValue("expand", out var obj) &&
                obj is string[] expandArray &&
                expandArray.Any(expandItem =>
                {
                    var segments = expandItem.Split('.');
                    return depth >= 0 &&
                           depth < segments.Length &&
                           segments[depth].Equals(pattern, StringComparison.OrdinalIgnoreCase);
                });
        }

        public static TResult MapWithDepthControl<TSource, TResult>(
            TSource source,
            ResolutionContext ctx,
            Func<TSource, int, TResult> mappingFunc)
        {
            int currentDepth = GetOrInitializeDepth(ctx);

            // Увеличиваем глубину для вложенных вызовов
            ctx.Items["TypeDepth"] = currentDepth + 1;

            try
            {
                return mappingFunc(source, currentDepth);
            }
            finally
            {
                // Восстанавливаем оригинальную глубину
                ctx.Items["TypeDepth"] = currentDepth;
            }
        }

        private static int GetOrInitializeDepth(ResolutionContext ctx)
        {
            if (!ctx.Items.TryGetValue("TypeDepth", out var depthObj))
            {
                depthObj = 0;
                ctx.Items["TypeDepth"] = depthObj;
            }
            return (int)depthObj;
        }
    }
}

для ошибок уже есть RFC 7807, в c# уже есть готовый ProblemDetail
https://datatracker.ietf.org/doc/html/rfc7807

не выявленные ошибки также нужно отлавливать в middlware

вот пример

public class ResponseResult<T> : IResponseResult<T>
    {
        public bool IsSuccess { get; private set; }

        public ProblemDetails? Error { get; private set; }

        public T? Data { get; private set; }

        public static ResponseResult<T> Success(T data) => new ResponseResult<T> { IsSuccess = true, Data = data };
        public static ResponseResult<T> Success() => new ResponseResult<T> { IsSuccess = true};
        public static ResponseResult<T> Failure(Exception exception) => new ResponseResult<T>
        {
            IsSuccess = false,
            Error = new ProblemDetails
            {
                Title = GetTitleByException(exception),
                Detail = exception.Message,
                Status = exception switch
                {
                    ArgumentException => StatusCodes.Status400BadRequest,
                    KeyNotFoundException => StatusCodes.Status404NotFound,
                    UnauthorizedAccessException => StatusCodes.Status401Unauthorized,
                    _ => StatusCodes.Status500InternalServerError
                },
                Instance = Guid.NewGuid().ToString()
            }
        };

        private static string GetTitleByException(Exception exception) =>
            exception switch
            {
                ArgumentException => "Invalid argument",
                KeyNotFoundException => "Resource not found",
                UnauthorizedAccessException => "Unauthorized",
                _ => "Internal server error"
            };
    }

Слой бизнес логики

return ResponseResult<bool>.Failure(
                    new KeyNotFoundException(
                        string.Format("User with login '{0}' not found", request.Login))
                    );

Презентационный слой

[HttpGet("verify")]
        public async Task<ActionResult> VerifyUser(
            [FromQuery] string login,
            [FromQuery] string password,
            CancellationToken cancellationToken)
        {
            var result = await mediator.Send(new VerifyQuery() { Login = login, Password = password }, cancellationToken);

            if (result.IsSuccess)
            {
                return Ok(new { verify = result.Data });
            }

            return StatusCode(result.Error?.Status ?? 400, result.Error);
        }