MassTransit RespondAsync Cannot Return null Value

I am using Masstransit

The following code returns null and Context.RespondAsync Cannot return null

When City Not Found I Want Return Null And In Controller I Check if response is null then return Not Found

Controller :

 var request = _mediator.CreateRequestClient<TGetQuery>();

 GetCityQuery getQuery = _mapper.Map<GetCityQuery>(id);

 var result = await request.GetResponse<CityViewModel>(getQuery, cancellationToken);
 if (result.Message == null)
 {
      return NotFound();
 }
 return Ok(result.Message);

And Consumer :

public async Task Consume(ConsumeContext<GetCityQuery> context)
    {
        var cancellationToken = context.CancellationToken;
        var request = context.Message;
        var entity = await _cityRepository.TableNoTracking
            .ProjectTo<CityViewModel>(_mapper.ConfigurationProvider)
            .SingleOrDefaultAsync(x => x.Id == request.Id, cancellationToken);

        await context.RespondAsync(entity);
    }

So Masstransit Throw System.ArgumentNullException: Value cannot be null. (Parameter 'message') at MassTransit.Context.BaseConsumeContext.RespondAsync[T](T message)

Is there any way to overcome this problem?

You cannot use null as a message. If you want to use multiple response types for a request client, you can specify the types separately:

var request = _mediator.CreateRequestClient<TGetQuery>();

 GetCityQuery getQuery = _mapper.Map<GetCityQuery>(id);

 var result = await request.GetResponse<CityViewModel, NotFound>(getQuery, cancellationToken);
 if (result.Is(out Response<CityViewModel> response)
 {
     return Ok(response.Message);
 }

return NotFound();

You'd need to define the message type:

public class NotFound
{
}

Then, in your consumer:

public async Task Consume(ConsumeContext<GetCityQuery> context)
{
     var request = context.Message;
     var entity = await _cityRepository.TableNoTracking
          .ProjectTo<CityViewModel>(_mapper.ConfigurationProvider)
          .SingleOrDefaultAsync(x => x.Id == request.Id, context.CancellationToken);
     if(entity == null)
          await context.RespondAsync(new NotFound());
     else
          await context.RespondAsync(entity);
}