Creating a Lambda function in .netCore with Visual Studio and AWS Visual Studio Toolkit

AWS allows .net Core framework also while creating Lambda functions. But the AWS console doesn't have any editor for .Net Core framework. It just has the option to upload the code in a zip file. We need to create the Lambda function in our local machine and upload the package file. We can create the Lambda function easily in Visual Studio with the template given in AWS Toolkit for Visual Studio. AWS Toolkit for Visual Studio have some templates for creating AWS Lambda functions easily using .NET Core. This post will guide you to create a Lambda function in .Net core using Visual Studio

Prerequisites

Following are the prerequisites for creating and deploying Lambda functions in Visual Studio

• Visual Studio 2017 or higher
• AWS Toolkit for Visual Studio

Create a .NET Core Lambda in Visual Studio

Following are the steps to create a Lambda function in Visual Studio

1) Open Visual Studio -> File menu -> New -> Project.
2) In the New Project dialog box, Select "AWS Lambda" under Installed -> Visual C#

     It will show you two types of project.

a) AWS Lambda project: These templates are for creating a project to develop and deploy an                 individual Lambda function.

b) AWS Serverless Application: These templates are for creating Lambda functions with a server-less AWS CloudFormation template. AWS server-less applications enable you to define more than just the function. For example, you can simultaneously create a database, add IAM roles, etc., with server-less deployment. AWS server-less applications also enable you to deploy multiple functions at one time.

3) Select AWS Lambda Project (.NET Core - C#) template.
4) Enter Name and Location for the project.
5) In the next screen, select Blue print for the Lambda you want to develop. For this example, I am selecting Empty Template.

6) It will create the project with the following structure 

Here, we need to examine two files

• Function.cs
• aws-lambda-tools-defaults.json

Function.cs 

Following is the sample code which was created automatically in Function.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

using Amazon.Lambda.Core;

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

namespace LambdaSample
{
    public class Function
    {
        
        /// 

        /// A simple function that takes a string and does a ToUpper
        /// 
        /// 
        /// 
        /// 
        public string FunctionHandler(string input, ILambdaContext context)
        {
            return input?.ToUpper();
        }
    }
}

Here FunctionHandler is the function where we need to write out Lambda function code. The above code will take a string as parameter and will return the same in UPPER Case. Here ILambdaContext object provides properties with information about the invocation, function, and execution environment(Read more about ILambdaContext here). For the time being, I am not writing any logic in the function, but I just added some logs. My FunctionHandler is as below after adding some logs to it

public string FunctionHandler(string input, ILambdaContext context)
{
 context.Logger.LogLine($"Input string:{input}");
 var upperCaseValue = input?.ToUpper();
 context.Logger.LogLine($"Upper case value:{upperCaseValue}");
 return upperCaseValue;
}

aws-lambda-tools-defaults.json

This is the file from where the Lambda function creator reads the default values while deploying the lambda function. You can set the default values like, framework, run time, memory size, timeout, function-handler etc parameters required for Lambda. Here function-handler defines the starting functionof the Lambda. For our sample it is "LambdaSample::LambdaSample.Function::FunctionHandler". In this, first part indicates the NameSpace(LambdaSample), second part indicates the ClassName with namespace(LambdaSample.Function) and third part indicates the Function name in the class(FunctionHandler).

Following is the aws-lambda-tools-defaults.json file created automatically by Visual Studio

{
  "Information" : [
    "This file provides default values for the deployment wizard inside Visual Studio and the AWS Lambda commands added to the .NET Core CLI.",
    "To learn more about the Lambda commands with the .NET Core CLI execute the following command at the command line in the project root directory.",

    "dotnet lambda help",

    "All the command line options for the Lambda command can be specified in this file."
  ],

  "profile":"default",
  "region" : "us-east-1",
  "configuration" : "Release",
  "framework" : "netcoreapp2.1",
  "function-runtime":"dotnetcore2.1",
  "function-memory-size" : 256,
  "function-timeout" : 30,
  "function-handler" : "LambdaSample::LambdaSample.Function::FunctionHandler"
}

Note: If you change the class name or function name, you need to update function-handler in the above file

Publish the Lambda Function

1) In Solution Explorer, right-click the project, and then choose Publish to AWS Lambda.

2) On the Upload Lambda Function page, in Function Name, type a name for the function or select a previously published function to republish -> Choose Next.

3) In the Advanced Function Details page,

• Select Existing role: Select any role assoicated with your role. The role is used to provide credentials for any AWS service calls made by the code in the function. Your account should have IAM:ListPolicies action, or the Role Name list will be empty and you will be unable to continue.
• Change Memory & Timeout values if required. 
• Assign VPC and its Subnets if those are required by your Lambda
• Add any Environment variable that your Lambda function needs

4) Click on Upload

5) This will open Uploading function page as shown below and the screen will automatically closes after Lambda function uploaded to AWS account

6) After function uploaded successfully, it will opens the following screen which allows you to execute the lambda function, change the Configuration,Event Sources and check the logs from Visual Studio. 

Note: You can also open the above screen for any existing Lambda function by Opening "AWS Explorer" -> AWS Lambda -> Double click on the required function.

Invoke the Function from Visual Studio

We can invoke the Lambda function from Visual Studio directly by the screen shown in the above image

1) Select any Example Request to choose any predefined request. But for our sample, it is just accepting a string as input, I have given "gopiportal_test" in the Request box

2) Click on "Invoke" button

3) This will execute the Lambda function automatically and will show the Response and the function logs

That's it...

Happy Coding !😊

Gopikrishna