Intermediate 25 min read Module 4 of 8

Lambda - Serverless Computing

Lambda is AWS's serverless computing service. You write C# code, AWS runs it. No servers to manage, no infrastructure to maintain. Just C# code that runs when you need it.

What is Lambda?

Lambda lets you run C# code without provisioning or managing servers. You upload your .NET code, and AWS handles everything else:

Scaling
Automatic

Pay Per Use
Milliseconds

Languages
C#, .NET 6/8

1 million requests FREE per month Always free tier .NET 6/8 Support

Setting Up .NET for Lambda

  1. Install the AWS .NET SDK:
    dotnet add package AWSSDK.Lambda
  2. Install the Lambda .NET Core Global Tool:
    dotnet tool install -g Amazon.Lambda.Tools
  3. Create a new Lambda project:
    dotnet new lambda.EmptyFunction -n MyCSharpLambda
  4. Deploy your function:
    dotnet lambda deploy-function MyCSharpLambda

Your Third Mission: Create a C# Lambda Function

  1. Go to Lambda: Type Lambda in the search bar and click it.
  2. Click "Create function".
  3. Name it: HelloWorldCSharp.
  4. Runtime: Choose .NET 6 or .NET 8.
  5. Architecture: Choose x86_64 or arm64.
  6. Click "Create function" (the orange button).
  7. Scroll down to the code editor. You will see default C# code.
  8. Click "Test" (the orange button right above the code).
  9. Event name: Type MyTest, ignore everything else, click "Save".
  10. Click "Test" again.
  11. BOOM! You will see a green box at the top saying "Execution succeeded".
  12. AWS just woke up, ran your C# code for 100 milliseconds, gave you a response, and went back to sleep. You just did "Serverless" computing with C#!

C# Lambda Code Examples

1. Basic C# Lambda Function
// Using Amazon.Lambda.Core
using Amazon.Lambda.Core;

// Assembly attribute to enable the Lambda function's JSON input to be converted into a .NET class.
[LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

public class Function
{
    /// <summary>
    /// A simple function that takes a string and returns a greeting
    /// </summary>
    public string FunctionHandler(string name, ILambdaContext context)
    {
        return $"Hello, {name ?? "World"}!";
    }
}
2. Lambda with Input Parameters
using Amazon.Lambda.Core;
using System.Text.Json;

[LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

public class Function
{
    public class Request
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }

    public class Response
    {
        public string Message { get; set; }
        public bool IsAdult { get; set; }
    }

    public Response FunctionHandler(Request request, ILambdaContext context)
    {
        return new Response
        {
            Message = $"Hello, {request.Name}!",
            IsAdult = request.Age >= 18
        };
    }
}
3. Lambda with S3 Integration
using Amazon.Lambda.Core;
using Amazon.S3;
using Amazon.S3.Model;
using System.Text.Json;

[LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

public class Function
{
    private readonly IAmazonS3 _s3Client;

    public Function()
    {
        _s3Client = new AmazonS3Client();
    }

    public async Task<string> FunctionHandler(S3Event s3Event, ILambdaContext context)
    {
        foreach (var record in s3Event.Records)
        {
            var bucketName = record.S3.Bucket.Name;
            var objectKey = record.S3.Object.Key;

            // Get the object from S3
            var response = await _s3Client.GetObjectAsync(bucketName, objectKey);

            using (var reader = new StreamReader(response.ResponseStream))
            {
                var content = await reader.ReadToEndAsync();
                context.Logger.LogInformation($"File content: {content}");
            }
        }

        return "S3 event processed successfully!";
    }
}
4. Lambda with API Gateway (HTTP API)
using Amazon.Lambda.Core;
using Amazon.Lambda.APIGateway.Events;
using System.Text.Json;

[LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

public class Function
{
    public async Task<APIGatewayHttpApiV2ProxyResponse> FunctionHandler(
        APIGatewayHttpApiV2ProxyRequest request, 
        ILambdaContext context)
    {
        // Get the HTTP method
        var httpMethod = request.RequestContext.Http.Method;
        
        // Get query parameters
        var name = request.QueryStringParameters?.GetValueOrDefault("name") ?? "World";

        // Create response body
        var responseBody = new
        {
            Message = $"Hello, {name}!",
            Method = httpMethod,
            Timestamp = DateTime.UtcNow
        };

        return new APIGatewayHttpApiV2ProxyResponse
        {
            StatusCode = 200,
            Headers = new Dictionary<string, string>
            {
                { "Content-Type", "application/json" },
                { "Access-Control-Allow-Origin", "*" }
            },
            Body = JsonSerializer.Serialize(responseBody)
        };
    }
}

Key Lambda Concepts

Concept Description C# Implementation
Function Your C# code that runs when triggered public class Function
Handler The method that gets called FunctionHandler()
Trigger What causes your function to run API Gateway, S3, SNS, etc.
LambdaSerializer Serializes JSON input/output [LambdaSerializer] attribute
ILambdaContext Info about execution environment ILambdaContext context
Timeout Max time your function can run 15 minutes max, 5 minutes default
Memory RAM allocated to your function 128 MB to 10 GB

Exercise: Create a C# Weather API

Task: Create a C# Lambda function that returns weather data.

  1. Create a new .NET 6 Lambda function named "WeatherAPI".
  2. Write C# code that takes a city name as input.
  3. Return mock weather data (temperature, conditions, humidity).
  4. Test the function with different city names.
  5. Add API Gateway trigger to access it via HTTP.
  6. Test the API URL in your browser.
Show Solution 
using Amazon.Lambda.Core;
using Amazon.Lambda.APIGateway.Events;
using System.Text.Json;

[LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

public class Function
{
    private static readonly Random _random = new Random();
    private static readonly string[] _conditions = { "Sunny", "Cloudy", "Rainy", "Snowy", "Windy" };

    public APIGatewayHttpApiV2ProxyResponse FunctionHandler(
        APIGatewayHttpApiV2ProxyRequest request, 
        ILambdaContext context)
    {
        // Get city from query string
        var city = request.QueryStringParameters?.GetValueOrDefault("city") ?? "Unknown";

        // Generate mock weather data
        var weather = new
        {
            City = city,
            Temperature = _random.Next(-10, 41),
            Conditions = _conditions[_random.Next(_conditions.Length)],
            Humidity = _random.Next(20, 91),
            WindSpeed = _random.Next(0, 51),
            Timestamp = DateTime.UtcNow
        };

        return new APIGatewayHttpApiV2ProxyResponse
        {
            StatusCode = 200,
            Headers = new Dictionary<string, string>
            {
                { "Content-Type", "application/json" },
                { "Access-Control-Allow-Origin", "*" }
            },
            Body = JsonSerializer.Serialize(weather)
        };
    }
}
Key Takeaway

Lambda is the easiest way to run C# code in the cloud. No servers to manage. Just write C# code, upload it, and AWS handles the rest. You get 1 million requests for FREE every month, forever! Use .NET 6 or 8 for the best performance.

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