Serverless Functions in C#: AWS Lambda from A to Z — Complete Guide

Introduction: Why Serverless and C# Are a Perfect Pair

Serverless architecture (or serverless computing) is a model where you write code without worrying about managing servers. AWS Lambda is one of the most popular services for running such code. Combined with C# (.NET), you get a powerful, type-safe, and high-performance tool for creating microservices, API handlers, and background tasks.

In this article, we will cover the entire journey: from environment setup to deployment and monitoring of your first C# function on AWS Lambda. The material is suitable for both beginners and experienced developers looking to systematize their knowledge.

1. Environment Setup and Project Creation

1.1 Installing Tools

To work with C# and AWS Lambda, you will need:

\n
• .NET SDK (8.0 or 6.0) — download from dotnet.microsoft.com;
\n
• AWS CLI — for managing resources from the terminal;
\n
• Amazon.Lambda.Tools — a global .NET tool for deployment.
\n

Install Lambda Tools with a single command:

dotnet tool install -g Amazon.Lambda.Tools

1.2 Creating a Function Template

Use the built-in template for a quick start:

dotnet new lambda.EmptyFunction --name MyFirstLambda --profile default --region us-east-1

This command will create a project with the Function.cs file and aws-lambda-tools-defaults.json for deployment settings.

2. Writing Serverless Function Code in C#

2.1 Basic Lambda Function Structure

Each Lambda function is a method that accepts an event object and an execution context. Let's look at a simple example:

using Amazon.Lambda.Core;\nusing Amazon.Lambda.SQSEvents;\n\n[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]\n\npublic class Function\n{\n    public async Task<string> FunctionHandler(SQSEvent evnt, ILambdaContext context)\n    {\n        context.Logger.LogInformation("Received messages: " + evnt.Records.Count);\n        \n        foreach (var record in evnt.Records)\n        {\n            await ProcessMessageAsync(record.Body, context);\n        }\n        \n        return "Processed " + evnt.Records.Count + " messages";\n    }\n\n    private async Task ProcessMessageAsync(string messageBody, ILambdaContext context)\n    {\n        context.Logger.LogInformation("Processing: " + messageBody);\n        await Task.Delay(100); // Simulating work\n    }\n}

Note the LambdaSerializer attribute — it specifies how to serialize/deserialize JSON. For C#, SystemTextJson is recommended.

2.2 Working with Different Triggers

AWS Lambda supports many event sources. Here's an example for an HTTP request via API Gateway:

public async Task<APIGatewayProxyResponse> FunctionHandler(APIGatewayProxyRequest request, ILambdaContext context)\n{\n    var name = request.QueryStringParameters?.ContainsKey("name") == true \n        ? request.QueryStringParameters["name"] \n        : "world";\n    \n    var responseBody = $"Hello, {name}!";\n    \n    return new APIGatewayProxyResponse\n    {\n        StatusCode = 200,\n        Body = responseBody,\n        Headers = new Dictionary<string, string> { { "Content-Type", "text/plain" } }\n    };\n}

To work with HTTP, you will need to install the Amazon.Lambda.APIGatewayEvents package.

3. Deploying and Testing the Function

3.1 Local Testing

Before deployment, test your code locally using dotnet lambda invoke:

dotnet lambda invoke --payload "{\\"Records\