Debugging C# AWS Lambdas

08 June 2021

While working on a client project, I was distressed by the fact that debugging the Lambda implementations was such a chore.

In order to debug a lambda, we would write a custom test harness around the handler. The test harness would setup the environment variables and the event object. On execution, the lambda would then connect to AWS resources via SSH tunnels.

Why It Sucked

Using the custom test harness wasn’t bad. It was sufficient, but it took some time to refine the test harnesses to a point they were easy to use. The downside came with the fact that we had to create a unique harness for each handler. Each harness was an NUnit test method marked with an Explicit annotation. The annotation allowed the test runner to ignore them on unit test executions.

The aggravating part came with making sure the harness would connect to the external AWS services. In our case, we were leveraging Aurora DB, Document DB, and SQS queues. It was a large project, so we had a dedicated DevOps team managing those resources. But in order to access them, we would also need to ensure that we had all our credentials up to date, and that we had a running VPN tunnel to allow the connection to a bastion host. Any changes to the services, their current state, or the credentials caused a block to our development.

Approach to Debugging

I decided to start from scratch. I created my own example application called TraderNEXT, which mimics an equity order management system.

In starting from scratch, I used the AWS Toolkit for Visual Studio 2019 to setup an example project. The toolkit will create an example lambda, complete with a Dockerfile to be used to generate a container image. The generated solution will also contain a serverless.template file, which is really a YAML file using AWS Serverless configuration to tie your lambdas to an API Gateway or other event source.

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Creating a new Project in Visual Studio

The real benefits from using the toolkit comes in two ways:

  • Launching the lambdas using the Visual Studio Debugger.
  • Using the AWS SAM CLI tool to startup a running API to do integration testing.

Visual Studio Debugger

When you use the debugger in Visual Studio, the toolkit will run a UI called the Mock Lambda Test tool. This allows you to prepare API Gateway payloads to test your lambdas with. Upon invoking the lambda, the Visual Studio debugger will then stop at any breakpoints you’ve defined.

The advantage here is that you can do manual testing across all your lambdas at the same time. There is no longer a need to maintain individual test harnesses for each of the lambdas.

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The Mock Lambda Test Tool

Test Automation

Suppose after some rounds of testing, you’ve accumulated some constantly used example payloads. You’re getting tired of always having to manually copy/paste the payloads into the mock tool. You can scratch that automation itch by taking advantage of the AWS SAM CLI. There is some pre-work that needs to be done, but its entirely possible to automate your tests against your lambdas locally. The setup I had to do was

  1. First, create an AWS Elastic Container Registry (ECR) repository to upload a container image to. Any other container registry (GitHub, DockerHub) will also work.
  2. Build, tag, and push an image of your lambdas to your repository.
  3. Then, in your serverless.template file, you would define the image name using the ImageUri property.

I documented the steps to manually build, tag, and push a container image in a shell script on my GitHub repo. The following is the serverless definition for the GetOrders lambda:

"GetOrders": {
  "Type": "AWS::Serverless::Function",
  "Properties": {
    "PackageType": "Image",
    "ImageConfig": {
      "EntryPoint": [
      "Command": [
    "ImageUri": "",
    "MemorySize": 256,
    "Timeout": 30,
    "Role": null,
    "Policies": [
    "Events": {
      "RootGet": {
        "Type": "Api",
        "Properties": {
          "Path": "/orders",
          "Method": "GET"
  "Metadata": {
    "Dockerfile": "Dockerfile",
    "DockerContext": ".",
    "DockerTag": ""

Once that is done, you can launch your lambdas locally using this command:

sam start local-api

This will start a mock API Gateway instance on your machine, with all the lambdas binded to specific URL paths defined in your serverless template. You can then use any tool you wish to automate your testing. I used Postman to automate this.

When you first invoke a lambda, you will see the SAM CLI download the related image from your repository. This is why setting up the image and uploading it first was necessary.


The one thing that we cannot get away with is that the lambdas still need a dependency on external data sources (databases, messaging queues). I’ve created a docker compose file that will launch two containers for MySQL and MongoDB. Both of these are great replacements for Aurora DB and Document DB. I am using MySQL drivers to connect to Aurora DB anyway. The lambdas would connect to these local resources when debugging or automated testing. You would then need to setup your environment variables in the Mock Lambda tool to ensure your lambdas connect correctly.

One way to circumvent this would be to just debug using your unit tests. Since your dependencies would be mocked, the setup would be a lot simpler. But then we would be moving back into the direction of a test harness!


I believe by leveraging the tools AWS has provided, I’ve created something much easier to debug with. I can debug my lambdas, as well as run automated tests using only locally running resources. I’ve also setup the project to use container images for easy deployment into AWS. I still have lots of work to do on refining this process. I’ll be sure to post more updates on this!

Jose Chavez

Jose is a software engineer that cares about increasing the visiblity of underestimated individuals in technology.