# 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](https://dotnet.microsoft.com/download)
    
* 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:

```bash
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:

```csharp
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`:

```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

```bash
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**

```bash
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:

```bash
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:

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

Then check the Lambda logs:

```bash
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:

```bash
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](https://docs.aws.amazon.com/lambda/latest/dg/dotnet.html)
    
* [S3 Event Notification Documentation](https://docs.aws.amazon.com/AmazonS3/latest/userguide/NotificationHowTo.html)
    
* [Amazon.Lambda.S3Events Package](https://www.nuget.org/packages/Amazon.Lambda.S3Events) text
    
* [Lambda Permission Policy for S3](https://docs.aws.amazon.com/lambda/latest/dg/with-s3-example.html)
    
* [Working with AWS SDK for .NET](https://docs.aws.amazon.com/sdk-for-net/v3/developer-guide/)