Building a REST API in .NET With API Gateway and Lambda Proxy Integration

Last updated: October 13, 2026

Spinning up a serverless REST API in .NET with API Gateway and Lambda proxy integration is straightforward once you understand the moving pieces. The data path is short enough that one developer can build it in an afternoon—and test most of it locally before touching the cloud.

This walkthrough covers creating the Lambda function in Visual Studio, wiring it to DynamoDB, deploying via the AWS Toolkit, and validating requests end-to-end with the Mock Lambda Test Tool.

At a glance

• Amazon API Gateway manages APIs at scale in two flavors: REST and HTTP.
• REST APIs support more advanced features; HTTP APIs trade depth for simplicity.
• AWS Lambda proxy integration passes the raw HTTP request straight to the function.
• The example uses .NET Core with the Amazon.Lambda.APIGatewayEvents NuGet package.
• The Mock Lambda Test Tool simulates API Gateway calls on your local machine.

What the architecture looks like

API Gateway sits at the edge, accepting HTTP traffic and routing it to Lambda. In proxy mode, the gateway doesn't unpack the request—it hands the whole thing to your function as an APIGatewayProxyRequest, and your function returns an APIGatewayProxyResponse. You decide how to parse query strings, headers, and body.

The Lambda function in the example reads a userId from the query string, validates it as a GUID, and uses the DynamoDB context to load a User record. A valid request returns a 200 with the serialized object; malformed input short-circuits with a 400 before touching the database. No JSON marshaling by hand, no custom request mapping templates.

Background: how proxy integration works

Proxy integration removes a whole layer of configuration that used to be mandatory. Without it, you'd define request and response mapping templates in API Gateway for every endpoint. With proxy, your function owns the contract.

For .NET developers, the Amazon.Lambda.APIGatewayEvents package ships the request and response types that match the gateway's wire format. That eliminates manual deserialization of raw JSON streams and keeps the function code readable.

Our analysis

Proxy integration is the pragmatic choice for most teams, but it shifts responsibility. Your Lambda function now handles input validation, error formatting, and data contract decisions that the gateway would otherwise manage. That trade-off is worth it for flexibility, but it means your function code has to be solid—there's no gateway-level guardrail catching bad input before it reaches your business logic.

The DynamoDB context's class-name-by-convention table lookup keeps code clean, but it creates a silent failure mode: rename your entity class and lookups break at runtime. Lock the data model before you add traffic.

For teams already on .NET, this is the path of least resistance to a serverless API. The tooling (Visual Studio + AWS Toolkit) handles credentials, deployment, and IAM role creation in a few clicks. For teams on other runtimes, the same pattern applies—Node.js, Python, and Java all have equivalent event types.

What to watch next

• Move deployment into a CloudFormation or CDK pipeline rather than publishing from Visual Studio.
• Scope IAM policies to the minimum required table and actions—don't let the Lambda function touch anything else.
• Add structured logging for cold start diagnosis once traffic picks up.

Who should care

Bottom line: proxy integration in .NET with API Gateway is the cleanest serverless API pattern available today. Use the Mock Test Tool to catch input parsing bugs locally, keep validation in the function, and automate the deploy before you scale.