How to trigger .NET Lambdas with S3 Events

When a file lands in S3, sometimes you want that to automatically trigger backend processing: image resizing, virus scanning, or sending a notification. AWS makes this possible using S3 event notifications to trigger Lambda functions. In this post, we’ll walk through wiring an S3 upload event to a .NET Lambda function.

Prerequisites

• .NET 6 or later SDK

• AWS CLI (configured with credentials)

• Amazon.Lambda.Tools CLI (dotnet tool install -g Amazon.Lambda.Tools)

• An existing or new S3 bucket

Step 1: Create the Lambda Project

Start with the empty Lambda template:

dotnet new lambda.EmptyFunction --name S3TriggeredLambda
cd S3TriggeredLambda

Step 2: Define the Handler for S3 Events

Update Function.cs to accept an S3Event and log the key of the uploaded file:

using Amazon.Lambda.S3Events;
using Amazon.S3;
using Amazon.S3.Util;
using Amazon.Lambda.Core;

[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

public class Function
{
    private readonly IAmazonS3 _s3Client;

    public Function() : this(new AmazonS3Client()) { }

    public Function(IAmazonS3 s3Client)
    {
        _s3Client = s3Client;
    }

    public async Task FunctionHandler(S3Event evnt, ILambdaContext context)
    {
        foreach (var record in evnt.Records)
        {
            var bucket = record.S3.Bucket.Name;
            var key = record.S3.Object.Key;

            context.Logger.LogLine($"File uploaded to S3: {bucket}/{key}");

            // Optional: read content
            var response = await _s3Client.GetObjectAsync(bucket, key);
            using var reader = new StreamReader(response.ResponseStream);
            var content = await reader.ReadToEndAsync();

            context.Logger.LogLine($"First 100 characters: {content[..Math.Min(100, content.Length)]}");
        }
    }
}

Step 3: Configure Lambda Deployment

Create aws-lambda-tools-defaults.json:

{
  "profile": "default",
  "region": "us-east-1",
  "configuration": "Release",
  "framework": "net6.0",
  "function-runtime": "dotnet6",
  "function-handler": "S3TriggeredLambda::S3TriggeredLambda.Function::FunctionHandler",
  "function-memory-size": 256,
  "function-timeout": 30,
  "function-name": "S3TriggeredLambda",
  "function-role": "arn:aws:iam::123456789012:role/your-lambda-role"
}

Step 4: Deploy the Function

dotnet lambda deploy-function

Take note of the function name or ARN printed after deployment.

Step 5: Configure S3 to Trigger the Lambda

Use the AWS CLI or Console to attach the Lambda as a notification target for your S3 bucket.

Option A: Using AWS Console

• Go to your S3 bucket

• Choose Properties > Event notifications

• Create a new event

  • Event type: PUT

  • Destination: Lambda function

  • Select your Lambda

Option B: Using AWS CLI

aws s3api put-bucket-notification-configuration --bucket your-bucket-name --notification-configuration '{
  "LambdaFunctionConfigurations": [
    {
      "LambdaFunctionArn": "arn:aws:lambda:us-east-1:123456789012:function:S3TriggeredLambda",
      "Events": ["s3:ObjectCreated:*"]
    }
  ]
}'

Make sure your Lambda function has permission to be invoked by S3:

aws lambda add-permission \
  --function-name S3TriggeredLambda \
  --principal s3.amazonaws.com \
  --statement-id s3invoke \
  --action "lambda:InvokeFunction" \
  --source-arn arn:aws:s3:::your-bucket-name

Step 6: Test the Integration

Upload a file to your S3 bucket:

aws s3 cp sample.txt s3://your-bucket-name/

Then check the Lambda logs:

aws logs describe-log-groups
aws logs get-log-events --log-group-name /aws/lambda/S3TriggeredLambda --log-stream-name <latest-stream-name>

You should see log lines indicating that your Lambda processed the S3 object.

Cleanup

To avoid charges, delete the Lambda function and remove the S3 notification:

aws lambda delete-function --function-name S3TriggeredLambda

Summary

You’ve now connected S3 with a .NET Lambda to react to file uploads in real time. This pattern is ideal for file processing pipelines, ingestion workflows, or triggering downstream services without managing infrastructure.

References

• AWS Lambda for .NET

• S3 Event Notification Documentation

• Amazon.Lambda.S3Events Package text

• Lambda Permission Policy for S3

• Working with AWS SDK for .NET