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 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 adecuado 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

📌 How to trigger .NET Lambdas withEvents (Jun)
🏢 Renato Gonçalves
📍 Jun