Pass Json string to AWS Lambda in Step Function - JsonReaderException error

Question

I am attempting to use a AWS Lambda function in a Step Function. The Lambda Function works correctly when the it is tested individually and the json input is escaped. However when the input is passed to the lambda function through a step function, I am getting a JsonReaderException error. What am I doing wrong? Would the community know of a workaround for this issue?

lambda function:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Runtime.Serialization.Formatters.Binary;
using Amazon.Lambda.Core;
using Newtonsoft.Json.Linq;

// Assembly attribute to enable the Lambda function's JSON input to be converted into a .NET class.
[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.Json.JsonSerializer))]
namespace AWSLambda1
{
    public class Function
    {
        public void PostsBasedOnOddOrEven(string input, ILambdaContext context)
        {
            var details = JObject.Parse(input);
            var postId = (int) details["id"];
            var oddOrEvenResult = (int) details["OddOrEvenPostsResult"];
        }
    }
}

Input to Lambda function from Step Function:

{
  "id": "1",
  "OddOrEvenPostsResult": 2
}

Input to Lambda function (which works through the Visual Studio Invoke):

"{ \"id\": \"1\", \"OddOrEvenPostsResult\": 2}"

Exception Stack Trace:

{
  "errorType": "JsonReaderException",
  "errorMessage": "Unexpected character encountered while parsing value: {. Path '', line 1, position 1.",
  "stackTrace": [
    "at Newtonsoft.Json.JsonTextReader.ReadStringValue(ReadType readType)",
    "at Newtonsoft.Json.JsonTextReader.ReadAsString()",
    "at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.ReadForType(JsonReader reader, JsonContract contract, Boolean hasConverter)",
    "at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)",
    "at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)",
    "at Newtonsoft.Json.JsonSerializer.Deserialize[T](JsonReader reader)",
    "at Amazon.Lambda.Serialization.Json.JsonSerializer.Deserialize[T](Stream requestStream)",
    "at lambda_method(Closure , Stream , Stream , LambdaContextInternal )"
  ]
}

Lambda Function not working when it is part of Step Function

Lambda Function working when tested individually

Answer 1

Since your lambda function is expecting input to be a string, the framework tries to parse the input as though it's a string.

However, the input is actually a JSON object, not a string.

Therefore the parser will fail with an "unexpected character" error. The parser is expecting a " character which would indicate the start of a string.

---

So, here's how you can fix it:

1. Declare a c# class which represents the input

   public class FunctionInput
   {
       public int id { get; set; }
       public int OddOrEvenPostsResult { get; set; }
   }
   

2. Change your function signature to expect input of type FunctionInput

   public class Function
   {    
       public void PostsBasedOnOddOrEven(FunctionInput input, ILambdaContext context)
       {
           var postId = input.id;
           var oddOrEvenResult = input.OddOrEvenPostsResult;
       }
   }
   

Note: you don't need to parse the input yourself.