How to correctly return null values from a resolver?

[eduleite]

I have a resolver that may return a null value, and the corresponding schema definition allows to return null values, but my client receives an error showing the following message: "Object reference not set to an instance of an object". The data field contains the right data, but the error stills comes, even if null is a allowed response. Is there a correct way to setup de server to not throw this error?

[michaelstaib]

can you paste you code?

[eduleite]

I will try to replicate it in a small sample and post here.

[eduleite]

I could not replicate the error in a small sample, but I can paste the stack trace here: The response I receive is this one:

{
  "errors": [
    {
      "message": "Unexpected Execution Error",
      "locations": [
        {
          "line": 10,
          "column": 6
        }
      ],
      "path": [
        "estimativa",
        "cliente"
      ],
      "extensions": {
        "message": "Object reference not set to an instance of an object.",
        "stackTrace": "   at HotChocolate.Resolvers.CodeGeneration.___CompiledResolvers__9163361aef8544be859a761587d56374.<>c.<<-cctor>b__60_21>d.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at HotChocolate.Configuration.ResolverRegistry.<>c__DisplayClass21_0.<<CreateResolverMiddleware>b__0>d.MoveNext() in C:\\hc\\src\\Core\\Types\\Configuration\\ResolverRegistry.cs:line 228\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at HotChocolate.Execution.ExecutionStrategyBase.ExecuteFieldMiddlewareAsync(ResolverTask resolverTask) in C:\\hc\\src\\Core\\Core\\Execution\\ExecutionStrategyBase.Resolver.cs:line 97\r\n   at HotChocolate.Execution.ExecutionStrategyBase.ExecuteMiddlewareAsync(ResolverTask resolverTask, IErrorHandler errorHandler) in C:\\hc\\src\\Core\\Core\\Execution\\ExecutionStrategyBase.Resolver.cs:line 54"
      }
    }
  ],
  "data": {
    "estimativa": {
      "id": "xxxxx",
      "codigo": "xxxxxx",
      "responsavel": {
        "nome": "xxxxxx",
      },
      "cliente": null
    }
  }
}

[eduleite]

Oh, I found the problem :) This is the method:

        public Task<IEmpregado> GetCliente(IEstimativa source, [Service] IForcaTrabalhoService forcaTrabalhoService)
        {
            if (source.Cliente != null)
            {
                return forcaTrabalhoService.ObterPorChaveAsync(source.Cliente);
            }
            return null;
        }

I should be returning Task.FromResult(null), because my method is not declared as async. Lesson learned :) Closing the issue. Unless you think the error message should be better here.

[michaelstaib]

I will look into that since others could fall into that trap.

[michaelstaib]

I might create a custom error for that

[michaelstaib]

A fix for this one will be included into 0.8.0-preview.11

[michaelstaib]

This one is now fixed. Since tasks will in the end be awaited by the query engine we are now returning null if the resolver task is null.