How do I set & fetch Environment variable in AWS Lambda Project in C#

It's pretty close but after some digging around I found out how to actually set these for local runs using the Mock Lambda Test Tool. It is, in-fact, inside the launchSettings.json file. You want to drop the settings inside the Mock Lambda Test Tool section of the profiles node, not outside it.

{
 "profiles": {
  "Mock Lambda Test Tool": {
   "commandName": "Executable",
   "commandLineArgs": "--port 5050",
   "workingDirectory": ".\\bin\\Debug\\netcoreapp2.1",
   "executablePath": "C:\\Users\\%USERNAME%\\.dotnet\\tools\\dotnet-lambda-test-tool-2.1.exe",
   "environmentVariables": {
     "environment": "test"
   }
  }
 }
}

1. Setting Environment variables docs used

There are two places where you’ll need to set environment variables: development-time and deployment-time. To do this, open the launchSettings.json file from under the Properties folder in the Solution Explorer. Then add the following JSON property:

    "environmentVariables": {
      "my-api": "something"
    }

To set environment variables at deployment-time, you can add these to the aws-lambda-tools-defaults.json file. (Just remember to escape the double quote marks.)

    environment-variables, its format is: "<key1>=<value1>;<key2>=<value2>;".

In your case you should have

    "environment-variables" : "\"my-api\"=\"http://myapihost.com/api/attendance-backfill\";"

2. Consuming/fetching the environment variables

   Consuming the environment variables as part of the Lambda function’s logic is done intuitively in the C# code, by using the System library aws blog:

   System.Environment.GetEnvironmentVariable(<key>);
   

   In your case you can use the following;

   var apiUrl = System.Environment.GetEnvironmentVariable("my-api");
   

In this document it is suggested that your approach for fetching environment variable is correct.

    var variableValue = Environment.GetEnvironmentVariable("nameOfVariable"); 

These solutions worked. Need to set environment variables in two places 1.) deployment purpose and 2.) test locally using mock tool

serverless.template file

launchSettings.json file