Event Lambda with Host.CreateApplicationBuilder

[McDoit]

I would like to know if anyone have any pointers or recommendation, or if its even possible, to create a Top Level style lambda with the Host.CreateApplicationBuilder?

For something like this:

var builder = Host.CreateApplicationBuilder(args);
var host = builder.Build();
host.Run();

//Something to hook in the lambda function invocation

Reason for this would be to get the free stuff from the Host and also ease of DI setup with access to the IHostApplicationBuilder for calls like AddServiceDefaults from Aspire

[GarrettBeatty]

var builder = Host.CreateApplicationBuilder(args);  // Generic host, not web host
builder.Services.AddSingleton<IMessageProcessor, MessageProcessor>();

// Hypothetical — doesn't exist yet
builder.Services.AddAWSLambdaEventHosting<SQSEvent>(async (sqsEvent, context, serviceProvider) =>
{
    var processor = serviceProvider.GetRequiredService<IMessageProcessor>();
    foreach (var record in sqsEvent.Records)
        await processor.ProcessAsync(record.Body);
});

var host = builder.Build();
await host.RunAsync();  // Lambda event loop runs as a hosted service

Hi im assuming you are asking if something like above exists today? Right now i believe the answer is no. I'll have to talk to my team on that.

---

The closest you'll be able to get right now is manual configuration

// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0

// ============================================================================
// This demonstrates the "manual glue" pattern for using Host.CreateApplicationBuilder
// with an event-driven Lambda function (non-HTTP).
//
// The generic host provides: DI, Configuration, Logging, Options pattern, etc.
// The Lambda bootstrap provides: the event polling loop.
//
// Related discussion: https://github.com/aws/aws-lambda-dotnet/discussions/2336
// ============================================================================

using Amazon.Lambda.Core;
using Amazon.Lambda.RuntimeSupport;
using Amazon.Lambda.Serialization.SystemTextJson;
using Amazon.Lambda.SQSEvents;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using TestEventHostBuilder;

// Step 1: Build the generic host — this gives us all the "free stuff"
//   - Configuration (appsettings.json, env vars, etc.)
//   - Logging (ILogger<T>)
//   - DI (IServiceCollection / IServiceProvider)
//   - Options pattern (IOptions<T>)
//   - Environment awareness (IHostEnvironment)
//   - Aspire integration: builder.AddServiceDefaults() could be called here
var builder = Host.CreateApplicationBuilder(args);

// Register services — just like any .NET application
builder.Services.AddSingleton<IMessageProcessor, MessageProcessor>();

var host = builder.Build();

// Step 2: Start the host (but do NOT call Run — that blocks forever waiting for shutdown)
// StartAsync starts any IHostedService instances and initializes the DI container.
await host.StartAsync();

var logger = host.Services.GetRequiredService<ILogger<Program>>();
logger.LogInformation("Host started. Wiring up Lambda bootstrap...");

// Step 3: Wire up the Lambda event loop, resolving services from the host's DI container
var serializer = new DefaultLambdaJsonSerializer();

await LambdaBootstrapBuilder.Create<SQSEvent, SQSBatchResponse>(async (sqsEvent, context) =>
{
    // Services are resolved from the host's DI container
    var processor = host.Services.GetRequiredService<IMessageProcessor>();

    logger.LogInformation("Received SQS event with {Count} records", sqsEvent.Records.Count);

    var batchResponse = new SQSBatchResponse();

    foreach (var record in sqsEvent.Records)
    {
        try
        {
            var result = await processor.ProcessAsync(record.Body);
            context.Logger.LogInformation($"Successfully processed {record.MessageId}: {result}");
        }
        catch (Exception ex)
        {
            context.Logger.LogError($"Failed to process {record.MessageId}: {ex.Message}");
            batchResponse.BatchItemFailures.Add(
                new SQSBatchResponse.BatchItemFailure { ItemIdentifier = record.MessageId });
        }
    }

    return batchResponse;
}, serializer)
.Build()
.RunAsync();

// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0

namespace TestEventHostBuilder;

/// <summary>
/// Simple service interface to demonstrate DI works with the Host + Lambda bootstrap pattern.
/// </summary>
public interface IMessageProcessor
{
    Task<string> ProcessAsync(string messageBody);
}

// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Logging;

namespace TestEventHostBuilder;

/// <summary>
/// Implementation that uses ILogger and IConfiguration from the host to prove
/// that the generic host's "free stuff" is available inside the Lambda handler.
/// </summary>
public class MessageProcessor : IMessageProcessor
{
    private readonly ILogger<MessageProcessor> _logger;
    private readonly string _environment;

    public MessageProcessor(ILogger<MessageProcessor> logger, IConfiguration configuration)
    {
        _logger = logger;
        _environment = configuration["DOTNET_ENVIRONMENT"] ?? "Unknown";
    }

    public Task<string> ProcessAsync(string messageBody)
    {
        _logger.LogInformation("Processing message in environment '{Environment}': {Body}", _environment, messageBody);
        return Task.FromResult($"Processed: {messageBody}");
    }
}

sample request json

{
  "Records": [
    {
      "messageId": "19dd0b57-b21e-4ac1-bd88-01bbb068cb78",
      "receiptHandle": "MessageReceiptHandle",
      "body": "Hello from SQS!",
      "attributes": {
        "ApproximateReceiveCount": "1",
        "SentTimestamp": "1523232000000",
        "SenderId": "123456789012",
        "ApproximateFirstReceiveTimestamp": "1523232000001"
      },
      "messageAttributes": {},
      "md5OfBody": "7b270e59b47ff90a553787216d55d91d",
      "eventSource": "aws:sqs",
      "eventSourceARN": "arn:aws:sqs:us-east-1:123456789012:MyQueue",
      "awsRegion": "us-east-1"
    },
    {
      "messageId": "2e1424d4-f796-459a-8184-9c92662be6da",
      "receiptHandle": "AnotherMessageReceiptHandle",
      "body": "Second message from SQS",
      "attributes": {
        "ApproximateReceiveCount": "1",
        "SentTimestamp": "1523232000002",
        "SenderId": "123456789012",
        "ApproximateFirstReceiveTimestamp": "1523232000003"
      },
      "messageAttributes": {},
      "md5OfBody": "e4e68fb7bd0e697a0ae8f1bb342846b3",
      "eventSource": "aws:sqs",
      "eventSourceARN": "arn:aws:sqs:us-east-1:123456789012:MyQueue",
      "awsRegion": "us-east-1"
    }
  ]
}

output ( i ran using the test tool v2)

$env:AWS_LAMBDA_RUNTIME_API="localhost:5050/TestEventHostBuilder"; dotnet run --project Libraries\test\TestEventHostBuilder\TestEventHostBuilder.csproj --no-build 2>&1
Using launch settings from Libraries\test\TestEventHostBuilder\Properties\launchSettings.json...
info: Microsoft.Hosting.Lifetime[0]                                                                                                                                              
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Production
info: Microsoft.Hosting.Lifetime[0]
      Content root path: C:\dev\repos\aws-lambda-dotnet
info: Program[0]
      Host started. Wiring up Lambda bootstrap...
-----------------------------------
Next invocation returned: 000000000001
info: Program[0]
      Received SQS event with 2 records
info: TestEventHostBuilder.MessageProcessor[0]
      Processing message in environment 'Unknown': Hello from SQS!
2026-04-23T17:25:25.142Z        000000000001    info    Successfully processed 19dd0b57-b21e-4ac1-bd88-01bbb068cb78: Processed: Hello from SQS!
info: TestEventHostBuilder.MessageProcessor[0]
      Processing message in environment 'Unknown': Second message from SQS
2026-04-23T17:25:25.147Z        000000000001    info    Successfully processed 2e1424d4-f796-459a-8184-9c92662be6da: Processed: Second message from SQS
-----------------------------------
Response for request 000000000001
{"batchItemFailures":[]}