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.NET Core Serverless for resize images

  • Self-Paced Lab
  • Version 1.0
  • Duration: 45 minutes

Purpose & Background

In this lab, you will create a simple AWS Lambda project using the Visual Studio to create a lambda function that will resize a jpg file saved in a bucket into a different folder in the same bucket. It shows how to configure and associate an event to a lambda function as well as how to troubleshoot using the CloudWatch logs.

Note: This Lab includes a small amount of sample code illustrating 
a pattern for lazily generate resized images. For deploying a service 
to resize images to production, consider using the Serverless Image Handler, 
which is a robust solution to handle image manipulation.

Lab Exercises

The following exercises should be completed in order for this lab:

  1. Create an AWS Lambda Project
  2. Create an S3 bucket and a role
  3. Deploy to AWS Lambda
  4. Create and Associate the S3 Event to your Lambda
  5. Upload files and check the lambda execution


The following are the prerequisites required in order to complete the lab:

  • Microsoft Visual Studio 2017 or above installed on your computer
  • AWS Toolkit for Visual Studio
  • Internet connection
  • AWS Account

Part 1 – Create an AWS Lambda Project

Follow the steps below to create and customize an ASP.NET Web Project in Visual Studio.

  1. In Visual Studio, use File -> New -> Project to open the New Project dialog.
  2. Under the Web project node, select AWS Lambda and the "AWS Lambda Project with Tests (.Net Core)" template, type Resize-Lambda-Userid as the name for your project, then click the OK button. Picture 1
  3. In the next dialog, select the "API" blueprint, and select "Simple S3 Function", then click the Finish button to generate the project. Picture 1
  4. Right-click on the solution that you have create and select Manage NuGet Packages: Picture 1

    Note: In this Lab we will use an open-source image library to resize the picture as .net core doesn’t have any built-in classes to work with the images.

    • For this Lab we will install an open-source third-party library called ImageSharp that is only available at the NuGet pre-release. Please, search SixLabors selecting the pre-release check-box. Picture 1

    • Select and install the SixLabors.ImageSharp and SixLabors.ImageSharp.Drawing packages.

  5. Replace the Function Handler Method with the code below (code available at link):

    // Copyright <YEAR>, Inc. or its affiliates. All Rights Reserved.
    // SPDX-License-Identifier: MIT-0
    public async Task<string> FunctionHandler(S3Event evnt, ILambdaContext context)
        string[] fileExtentions = new string[] { ".jpg", ".jpeg" };
        var s3Event = evnt.Records?[0].S3;
        if(s3Event == null)
            return null;

            foreach (var record in evnt.Records)
                LambdaLogger.Log("----> File: " + record.S3.Object.Key);
                if (!fileExtentions.Contains(Path.GetExtension(record.S3.Object.Key).ToLower()))
                    LambdaLogger.Log("File Extention is not supported - " + s3Event.Object.Key);
                string suffix = Path.GetExtension(record.S3.Object.Key).ToLower();
                Stream imageStream = new MemoryStream();
                using (var objectResponse = await S3Client.GetObjectAsync(record.S3.Bucket.Name, record.S3.Object.Key))
                using (Stream responseStream = objectResponse.ResponseStream)
                    using (Image<Rgba32> image = Image.Load(responseStream)) 
                        image.Mutate(ctx => ctx.Resize(image.Width / 4, image.Height / 4));
                        image.Save(imageStream, new JpegEncoder());
                        imageStream.Seek(0, SeekOrigin.Begin);
                // Creating a new S3 ObjectKey for the resized objects
                string resizedObjectKey = null;
                int endSlash = record.S3.Object.Key.ToLower().LastIndexOf("/");
                if (endSlash > 0)
                    string S3ObjectName = record.S3.Object.Key.ToLower().Substring(endSlash+1);
                    int beginSlash = 0;
                    if (endSlash > 0)
                        beginSlash = record.S3.Object.Key.ToLower().Substring(0, endSlash - 1).LastIndexOf("/");
                        if (beginSlash > 0)
                            resizedObjectKey = record.S3.Object.Key.ToLower().Substring(0, beginSlash) + "resized-images/" + S3ObjectName;
                            resizedObjectKey = "resized-images/" + S3ObjectName;
                    resizedObjectKey = "resized-images/" + record.S3.Object.Key.ToLower();

                LambdaLogger.Log("----> Resized filed Key: " + resizedObjectKey);

                await S3Client.PutObjectAsync(new PutObjectRequest
                    BucketName = record.S3.Bucket.Name,
                    Key = resizedObjectKey,
                    InputStream = imageStream

            LambdaLogger.Log("Processed " + evnt.Records.Count.ToString());

            return null;
        catch(Exception e)
            context.Logger.LogLine($"Error getting object {s3Event.Object.Key} from bucket {s3Event.Bucket.Name}. Make sure they exist and your bucket is in the same region as this function.");

Part 2 – Create an S3 bucket and the Lambda Role

Follow the steps to create a S3 bucket on the same region that the lambda function will be created using the AWS Management Console.

  1. Sign in to the AWS Management Console and open the S3 service at
  2. Please, name you bucket as dotnet-immersionday-lambda-userid using the region that you have been using during this lab following the steps described on the To create an S3 bucket at the URL:
  3. To create the Lambda role, open the Access Management Access Service by accessing
  4. In the navigation pane of the IAM console, choose Roles, and then choose Create role.
  5. For Select type of trusted entity, choose AWS service, select Lambda, and select Next: Permissions. Picture 1
  6. On the filter policies search, type “LambdaBasic” and select the AWSLambdaBasicExecutionRole policy. Also, type “S3Full” at the search field and select the AmazonS3FullAccess policy. Click on Next: Review button.
  7. On the Review page name your role as “LambdaBasicAndS3FullRole”, make sure you seeing the two policies added on the step above and click on the Create Role button.

Part 3 – Deploy to AWS Lambda

Follow the steps below to deploy the Lambda function.

  1. Right-click your project in Solution Explorer, and select "Publish to AWS Lambda" to launch the publishing wizard. See the figure below. Picture 1
  2. Ensure the "Account profile to use" drop-down and "Region" drop-down are set to the profile and region you are using for today's labs. Name your Lambda Function as Resize-Lambda-Userid, and click Next. Picture 1
  3. Select the Role Name that you have created at the Part 2 of this LAB and click on Upload. Picture 1

Part 4 – Create and Associate the S3 Event to your Lambda.

For this lab, we’ll access the AWS Management Console to associate a S3 event to your lambda function.

  1. Sign in to the AWS Management Console and open the Lambda service at
  2. Click on the lambda function name you have published
  3. Add a S3 trigger to the lambda function by clicking on S3 on the left panel. Picture 1
  4. Configure the Trigger, which you can reach by rolling down the screen; select the bucket you have created on the Part 2 on the bucket drop-down, select the PUT as event type, and enter images/ as the prefix the lambda will be watching, and press Add. Picture 1

    Note: The lambda function will save the resized image in a new folder called resized-images within the same bucket. If you don’t define a prefix you lambda function will enter in a recursive loop as every time you save an image in the bucket the lambda function will be triggered

  5. Click on Save on the top-right corner of the lambda console.

Part 5 – Uploading files and check the lambda function execution.

Now it is time to upload a jpg file to the S3 bucket to test its execution.

  1. Sign in to the AWS Management Console and open the S3 service at
  2. Click on the S3 bucket you have created.
  3. Create a folder called images, but selecting the Create Folder button, with no encryption, at the top right of the S3 console. Once you create the folder, select the folder.
  4. Upload a test image into your bucket for testing. The blue marble - is a great sample image for testing. Please, download the image to your local computer.
  5. Click on the Upload button at the S3 console. In the dialog box, click on Add files, select the file above and click Upload at the dialog. Picture 1
  6. After a few seconds, you will notice that a new folder got created in the bucket called resized-images. Go to this folder and download the file that was resized. It should have gone from 6.2 MB to 93.2KB.

    Picture 1

  7. To check the lambda function execution, open the Lambda service at

  8. Click on the lambda function that you have created for resizing the image
  9. Select the Monitoring tab and click on the View logs in CloudWatch Picture 1
  10. Select the first log stream listed in the Log Group and you will see the lambda output and logs; similar to the picture below: Picture 1

Congratulations! You have deployed a lambda function that is trigger by an S3 event, which performs a task defined into your lambda function. Now go build on AWS.

Part 6 – Removing all the resources.

In this part we will remove all the resources that we created in this LAB.

  1. Sign in to the AWS Management Console and open the S3 service at
  2. Click on the row of the dotnet-immersionday-lambda-userid S3 bucket you have created, not on the S3 bucket name.
  3. Select Delete on the top right of the S3 Console
  4. Type the bucket name on the Delete dialog and confirm.
  5. Open the Lambda service at
  6. Click on the Resize-Lambda-Userid Lambda function name you have published
  7. On the Action Dropdown Button select Delete
  8. Confirm that you want to delete the Function
  9. Open the IAM service at
  10. Click on roles on the left panel
  11. Check the LambdaBasicAndS3FullRole
  12. Click on Delete role button on the top left