Send email when files are uploaded to Amazon S3 with AWS Lambda and SendGrid using C# and .NET

March 20, 2023
Written by
Néstor Campos
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Send email when files are uploaded to Amazon S3  with AWS Lambda & SendGrid, using C# and .NET

You can store files and data using the Amazon S3 service in all types of projects. Managing these files can be difficult if you don't know they have been uploaded.

In this post, you are going to create a function with AWS Lambda to monitor files that are uploaded to the Amazon S3 service, and this function will send an email with the metadata (name, size, among others) to notify you what has happened in the repository files.

AWS Lambda is a serverless computing service that lets you run code without provisioning or managing servers. Twilio SendGrid is a cloud-based email service that allows you to send and receive emails without maintaining your own email server.


You will need a free Twilio SendGrid account. Sign up for a SendGrid account here and send up to 100 emails per day completely free of charge. You will also need an AWS account.

Finally, you will need the following for your development environment:

You can find the source code of this tutorial in this GitHub repository.

Configure SendGrid

Configure a Single Sender in SendGrid

The information entered in the form that you will create later will be sent by email through Twilio SendGrid, and you need to verify ownership of an email address or domain so that the service can send messages on your behalf. To start quickly, you will use Single Sender Verification for this tutorial. This verifies that you own the email address to which the application will send emails. Single Sender Verification is great for testing purposes, but it is not recommended for production.

Twilio recommends Domain Authentication for production environments. An authenticated domain proves to email providers that you own the domain and removes the "via" text that inbox providers would otherwise append to your from address.

To do this, go to the Sender Authentication option, and then in the Single Sender Verification section, select Get Started.

Page of the "Sender Authentication" submodule, specifically in the "Single Sender Verification" section and with the option to start with the "Get Started" button.

Complete the form, specifying in the From field the email address that you will send the emails from. It must be an email address that you have access to because you will receive a verification email.

Create a Sender form, asking the user for the "From Name", "From Email Address", "Reply To", "company address", "City" and "Country" as required data, with other fields as optional.

After creating the sender, you will receive an email to confirm your ownership of the specified email address.

Page with message "Sender had been created" and a button to "Resend email" to verify the single sender email address.

An email will arrive in a few seconds or minutes with a link that you must access to confirm and verify your ownership of the existing email box. Find the email and click Verify Single Sender.

Email received from SendGrid requesting to verify the ownership of the existing email address. The email has a button to click "Verify Single Sender".

When verifying the email address through the link you received, you will see a confirmation page, and later you will be able to see in the Sender list that your email address is verified and ready to send emails from SendGrid.

Confirmation page after clicking in the verification email, with the message "Sender Verified"

Page displaying the Sender you configured, indicating that it is verified for use.

Create an API Key

You'll need to create an API key to send emails with Twilio SendGrid.

First, access your SendGrid account and navigate to the API Keys page under Settings.

Screen for API Keys option in Settings, with the button "Create API Key" to create a new key for using in your project.

Next, click Create API Key, and in the displayed form, enter a name to distinguish it from other keys for other projects, and give it Restricted Access with only Mail Send permissions.

Create API Key form with a text field for the name and a radio button for the permission. In this project, the name is SendGridFluentEmailSender and the permission radio button is selected for the Restricted Access option

Create API Key form with the Mail Send accordion opened and with the Mail Send is a toggle and is enabled

Scroll to the bottom of the form and click Create & View. On the next screen, you will be presented with the API Key that you must copy (you will use it later for the project).

Page displaying the API Key created in plain text to copy, indicating that it will not be displayed again for security.

Enabling S3

Configure S3 buckets

You are now going to set up buckets in Amazon S3 for file storage. Buckets correspond to a type of container within the S3 service in which you can organize folders and files to be consumed/inserted/updated by other apps.

Next, sign in to your AWS account. Then, in the AWS console, search for "S3" in the search box and click on the S3 option.

If this is your first time in S3, you'll see a page describing the service and what you can do with it.

Click on the "Create bucket" button. Fill out the form using the following value and leave the default values for the remaining fields:

  • Bucket name: Select a unique name for your bucket.
  • AWS region: Select the AWS Region where the files will be stored.

Form for creating a bucket in S3, indicating a unique name for the bucket and the AWS Region where it will be created.

At the bottom of the form, click on "Create bucket".  The screen with the list of buckets will be displayed, where you will see the new one you just created.

Page with list of buckets, displaying in the list the new bucket created in the previous step.

Click on your new bucket. You will see another list (for now empty) where the files that you will upload will be displayed.

New bucket page with options to add, review, or delete files.

This newly created bucket can only be accessed from your own AWS account, since you did not alter the access permissions that change the security of the files contained in this bucket.

Create a Lambda function

Configure the trigger

In this part, you'll create the function in AWS Lambda that monitors incoming files to the bucket you created earlier. To do this, in the search box,  enter "Lambda" and select the Lambda service, which will redirect you to a page with an explanation of the service and the options available at the programming language level.

Now, click on the "Create a function" button and enter the following data into the form:

  • Template: Author from scratch
  • Function name: IncomingFileListener
  • Runtime: .NET 6 (C#/Powershell)

Leave the defaults for the rest of the fields. Then click the "Create function" button.

Once the function is created, you are going to add the trigger to be executed when a new file arrives.  Click on the "Add trigger" button.

Page of the IncomingFileListener function created, with the structure, triggers and destinations that can be set.

On the new page, select S3 as the source, select the bucket you created earlier, and then accept the conditions and click "Add".

Page to add a new trigger to the function created, selecting S3 as source and the previously created bucket.

Both the bucket and the Lambda function must be in the same AWS Region in order to connect to each other.

Add the code

To create your Lambda function, you must install the AWS Lambda Templates to be able to create .NET projects for this technology. To do this, open a command line window and run the installation statement:

dotnet new -i Amazon.Lambda.Templates

Next, create a folder for your new project, go into it, and finally create a C# for Lambda project via the command line:

mkdir SendGridLambda
cd SendGridLambda
dotnet new lambda.EmptyFunction --name IncomingS3FilesMonitor

Go to the project folder and install the SendGrid, Amazon.S3, and Amazon.Lambda.S3Events NuGet packages to use it inside the function:

cd IncomingS3FilesMonitor\src\IncomingS3FilesMonitor\
dotnet add package SendGrid
dotnet add package AWSSDK.S3
dotnet add package Amazon.Lambda.S3Events

Now, in the Function.cs file, replace all existing code with the following:

using Amazon.Lambda.Core;
using Amazon.Lambda.S3Events;
using Amazon.Lambda.Serialization.SystemTextJson;
using SendGrid;
using SendGrid.Helpers.Mail;

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

namespace IncomingS3FilesMonitor;

public class Function
    private string SendGridApiKey = Environment.GetEnvironmentVariable("SendGridApiKey");
    private string FromEmailAddress = Environment.GetEnvironmentVariable("FromEmailAddress");
    private string ToEmailAddress = Environment.GetEnvironmentVariable("ToEmailAddress");

    public async Task FunctionHandler(S3Event s3Event, ILambdaContext context)
        var client = new SendGridClient(SendGridApiKey);
        foreach (var s3EventRecord in s3Event.Records)
            var s3Object = s3EventRecord.S3.Object;
            var objectKey = s3Object.Key;

            var message = MailHelper.CreateSingleEmail(
                from: new EmailAddress(FromEmailAddress), 
                to: new EmailAddress(ToEmailAddress), 
                subject: "New file uploaded to S3", 
                plainTextContent: $"New file uploaded to S3: {objectKey}\n\n" +
                    $"Size of the file (in bytes): {s3Object.Size}", 
                htmlContent: null

            var response = await client.SendEmailAsync(message);
                var errorMessage = "Email failed to sent." +
                    $"Response status code: {response.StatusCode}, " +
                    $"body: {await response.Body.ReadAsStringAsync()}";

The function listen for events on the bucket in S3, receives the data relating to the S3 event, and formats the data in a string that will be sent via email with SendGrid.

Now build the project in Release mode, so you can upload it to AWS Lambda:

dotnet build --configuration Release

Go to the folder where the function has been built, i.e. bin/Release/<target_framework>/

Replace <target_framework> with the version of .NET that you are using for this project. At the time of building the project, the folder will already have the specific version.

Inside the folder are all the generated files (libraries and configuration files) for the function. Compress them into a zip file, as you will be use it later to update the function.

Update the function

The above function uses certain values obtained from environment variables, specifically the source and destination email addresses and the SendGrid API key for sending emails. Add these variables to the function, by accessing the "Configuration" tab and then the "Environment variables" option.

Click “Edit” and add the following environment variables:

  • SendGridApiKey: With the API Key obtained earlier when configuring SendGrid.
  • FromEmailAddress: The email address that will be used to send the emails from, which must maintain the same domain/subdomain configured in SendGrid that is authorized to send this type of email.
  • ToEmailAddress: The recipient of the emails can be, in this case, your personal email address.

Environment variables module with the necessary values added.

Click "Save".

Then go back to the Code section, click "Upload from" and select the ".zip file" option. In this option, browse and select the zip file you created earlier with your function.

Code section on the Lambda function, highlighting the option to upload the function from a zip file.

Modal window to select and upload the zip file, with the Save button that must be clicked to finish the process.

Click "Save" and wait a few seconds for the feature to update with the files you've uploaded.

In order to have your function work correctly, you will need to update the method that will be initially executed when it is triggered. To do this, in the same Code section, in the Runtime settings subsection, click "Edit", and then in the following form, edit the "Handler" property with the value IncomingS3FilesMonitor::IncomingS3FilesMonitor.Function::FunctionHandler and click “Save”.

Runtime settings form, editing the Handler field with the value IncomingS3FilesMonitor::IncomingS3FilesMonitor.Function::FunctionHandler

Now your function is ready to be triggered.

Test the function

The simplest way to test the feature is by uploading a new file to the S3 bucket created in the previous steps. To do that, navigate back to S3, enter the bucket, then click on "Upload", click again on "Add files" and when the modal window on your computer appears to select the files, search for the ones you deem convenient, select and upload them.

File upload form in S3 with the option to select files or folders.

Click the "Upload" button to confirm the new files in the bucket.

File upload form in S3, with 3 selected files and the "Upload" button to confirm the upload to the bucket.

After a couple of seconds or minutes, an email will arrive for each file uploaded to the destination box you specified, similar to the following:

Email received with the result of the Lambda function triggered when uploading files to S3.

With all this work, you already have a function that will monitor and notify you of each new file, which can help you when you are waiting for an important file to work on, regardless of how the file was uploaded.

Additional resources

Check out the following resources for more information on the topics and tools presented in this tutorial:

How to send Emails with C# and .NET 6 using the SendGrid API: In this post on the Twilio blog, you can learn about sending emails using Twilio SendGrid API with C# and .NET framework.

AWS Lambda: Explore AWS Lambda and all the options available for its use.

Building Lambda functions with C#: Review the methods, libraries, and programs for using AWS Lambda with C#.

Source Code to this tutorial on GitHub: You can find the source code for this project at this GitHub repository. Use it to compare solutions if you run into any issues.

Néstor Campos is a software engineer, tech founder, and Microsoft Most Valuable Professional (MVP), working on different types of projects, especially with Web applications.