Granting access by using an IAM role

NuGet packages:

Programming elements:

using System;
using System.Collections.Generic;
using System.IO;
using System.Threading.Tasks;
using Amazon.S3;
using Amazon.S3.Model;

namespace S3GetTextItem
{
  // = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
  // Class to retrieve a text file from an S3 bucket and write it to a local file
  class Program
  {
    static async Task Main(string[] args)
    {
      // Parse the command line and show help if necessary
      var parsedArgs = CommandLine.Parse(args);
      if(parsedArgs.Count == 0)
      {
        PrintHelp();
        return;
      }

      // Get the application arguments from the parsed list
      string bucket =
        CommandLine.GetArgument(parsedArgs, null, "-b", "--bucket-name");
      string item =
        CommandLine.GetArgument(parsedArgs, null, "-t", "--text-object");
      string outFile =
        CommandLine.GetArgument(parsedArgs, null, "-o", "--output-filename");
      if(   string.IsNullOrEmpty(bucket)
         || string.IsNullOrEmpty(item)
         || string.IsNullOrEmpty(outFile))
        CommandLine.ErrorExit(
          "\nOne or more of the required arguments is missing or incorrect." +
          "\nRun the command with no arguments to see help.");

      // Create the S3 client object and get the file object from the bucket.
      var response = await GetObject(new AmazonS3Client(), bucket, item);

      // Write the contents of the file object to the given output file.
      var reader = new StreamReader(response.ResponseStream);
      string contents = reader.ReadToEnd();
      using (var s = new FileStream(outFile, FileMode.Create))
      using (var writer = new StreamWriter(s))
        writer.WriteLine(contents);
    }


    //
    // Method to get an object from an S3 bucket.
    private static async Task<GetObjectResponse> GetObject(
      IAmazonS3 s3Client, string bucket, string item)
    {
        Console.WriteLine($"Retrieving {item} from bucket {bucket}.");
        return await s3Client.GetObjectAsync(bucket, item);
    }


    //
    // Command-line help
    private static void PrintHelp()
    {
      Console.WriteLine(
        "\nUsage: S3GetTextItem -b <bucket-name> -t <text-object> -o <output-filename>" +
        "\n  -b, --bucket-name: The name of the S3 bucket." +
        "\n  -t, --text-object: The name of the text object in the bucket." +
        "\n  -o, --output-filename: The name of the file to write the text to.");
    }
  }


  // = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
  // Class that represents a command line on the console or terminal.
  // (This is the same for all examples. When you have seen it once, you can ignore it.)
  static class CommandLine
  {
    //
    // Method to parse a command line of the form: "--key value" or "-k value".
    //
    // Parameters:
    // - args: The command-line arguments passed into the application by the system.
    //
    // Returns:
    // A Dictionary with string Keys and Values.
    //
    // If a key is found without a matching value, Dictionary.Value is set to the key
    //  (including the dashes).
    // If a value is found without a matching key, Dictionary.Key is set to "--NoKeyN",
    //  where "N" represents sequential numbers.
    public static Dictionary<string,string> Parse(string[] args)
    {
      var parsedArgs = new Dictionary<string,string>();
      int i = 0, n = 0;
      while(i < args.Length)
      {
        // If the first argument in this iteration starts with a dash it's an option.
        if(args[i].StartsWith("-"))
        {
          var key = args[i++];
          var value = key;

          // Check to see if there's a value that goes with this option?
          if((i < args.Length) && (!args[i].StartsWith("-"))) value = args[i++];
          parsedArgs.Add(key, value);
        }

        // If the first argument in this iteration doesn't start with a dash, it's a value
        else
        {
          parsedArgs.Add("--NoKey" + n.ToString(), args[i++]);
          n++;
        }
      }

      return parsedArgs;
    }

    //
    // Method to get an argument from the parsed command-line arguments
    //
    // Parameters:
    // - parsedArgs: The Dictionary object returned from the Parse() method (shown above).
    // - defaultValue: The default string to return if the specified key isn't in parsedArgs.
    // - keys: An array of keys to look for in parsedArgs.
    public static string GetArgument(
      Dictionary<string,string> parsedArgs, string defaultReturn, params string[] keys)
    {
      string retval = null;
      foreach(var key in keys)
        if(parsedArgs.TryGetValue(key, out retval)) break;
      return retval ?? defaultReturn;
    }

    //
    // Method to exit the application with an error.
    public static void ErrorExit(string msg, int code=1)
    {
      Console.WriteLine("\nError");
      Console.WriteLine(msg);
      Environment.Exit(code);
    }
  }

}