Creating a AWS Cognito PreSignup Lambda in DotNet

user_4843 0
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Using a .Net Core 1.0 Lambda I want to be able to create a Lambda function which handles the PreSignUp trigger from an AWS Cognito User pool.

using Amazon.Lambda.Core;

[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.Json.JsonSerializer))]

public class PreSignUp_SignUp
{
  public string userPoolId { get; set; }
  public const string EmailKey = "email";
  public const string PhoneNumber = "phone_number";
  public Dictionary<string,string> userAttributes { get; set; }
  public Dictionary<string, string> validationData { get; set; }
}

public class PreSignup_SignUpResponse
{
  public bool autoConfirmUser { get; set; }
}

public class Function
{
  public PreSignup_SignUpResponse FunctionHandler(PreSignUp_SignUp input, ILambdaContext context)
  {
      return new PreSignup_SignUpResponse { autoConfirmUser = true };
  }
}

Though the request succeeds and returns a response when invoking the Lambda with an example request of:

{
  "datasetName": "datasetName",
  "eventType": "SyncTrigger",
  "region": "us-east-1",
  "identityId": "identityId",
  "datasetRecords": {
    "SampleKey2": {
      "newValue": "newValue2",
      "oldValue": "oldValue2",
      "op": "replace"
    },
    "SampleKey1": {
      "newValue": "newValue1",
      "oldValue": "oldValue1",
      "op": "replace"
    }
  },
  "identityPoolId": "identityPoolId",
  "version": 2
}

When performing an actual SignUp via the .Net AmazonCognitoIdentityProviderClient I get back an error:

Amazon.CognitoIdentityProvider.Model.InvalidLambdaResponseException : Unrecognizable lambda output

Which I'm guessing means I have not got the shape of the response (and possibly even request) correct.

Does anyone have an example of a .Net Lambda function that works for the PreSignUp trigger in AWS Cognito?

aws-lambda amazon-cognito .net-core amazon-web-services c#
3 Answers

The previous two responses are now inaccurate unless you still use the old, less performant Amazon.Lambda.Serialization.Json.JsonSerializer. This old serializer uses Newtonsoft.Json while the new Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer implements the recent System.Text.Json.

As a result, a JObject parameter is no longer appropriate and should instead be replaced with JsonElement. If you try to use JObject with this new serializer, you will get an error since the serializer doesn't know how to deal with this object.

You should read this to gain a better understanding of how it all works, but you access properties of the JsonElement using GetProperty("[insert property name here]").

For example:

public async Task<JsonElement> FunctionHandler(JsonElement input, ILambdaContext context)
{
    var request = input.GetProperty("request");
    var userAttributes = request.GetProperty("userAttributes");
    string email = userAttributes.GetProperty("email").GetString();

    return input;
}

This way, you don't need to construct entire classes to accommodate the required request and response parameters, just get and set the properties you need.

user_137719250

There is already another great answer in here. However I'm not a expert .NET developer so this solution makes more sense to me.

class AutoVerifyEmail
{
    public AutoVerifyEmail() { }

    public JObject AutoVerifyEmailPreSignup(JObject input, ILambdaContext context)
    {
        //Console.Write(input); //Print Input

        input["response"]["autoVerifyEmail"] = true;
        input["response"]["autoConfirmUser"] = true;

        return input;
    }
}
user_50385570
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