Using an Environment Variable in a .NET Core Console App Hosted in Docker

Question

I have a .NET Core Console App. This console app loads a connection string stored in an environment variable. I'm retrieving that connection string using this code:

var connectionString = Environment.GetEnvironmentVariable("connection_string");

The line above works if I run the Console App by just pushing "play" in Visual Studio. However, if I run this Console App in a Docker container, the connectionString is empty. I assume it's because the connection_string environment variable is not in the Docker environment. However, I can't figure out how to add/set an environment variable in a Docker container.

For my last attempt, I added the following in my Dockerfile:

ENV connection_string <my_connection_string_value>

However, that didn't seem to matter. What am I missing?

Is there a reason you aren't using an app settings file for that? — PmanAce
– PmanAce, Commented Aug 10, 2018 at 22:51
Yes. I don't want the connection string floating around in source control, dev machines, etc. That's why I want to pull from an Environment variable. — Some User
– Some User, Commented Aug 11, 2018 at 1:07
Ok if you get it working I'll check it out, if not encrypt it. — PmanAce
– PmanAce, Commented Aug 11, 2018 at 3:25

John · Accepted Answer · 2018-08-11 11:19:16Z

2

You should be able to easily reference environment variables via the configuration builder.

var configuration = new ConfigurationBuilder()
  .AddJsonFile("appsettings.json")
  //...
  .AddEnvironmentVariables()
  .Build();

The reference use IConfiguration via DI.

private readonly IConfiguration _configuration;

public ClassConstructor(IConfiguration configuration)
{
_configuration = configuration ?? throw new ArgumentNullException(nameof(configuration));

var connectionString = _configuration.GetValue("connection_string");
}

answered Aug 11, 2018 at 11:19

John

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Joerg Krause · Accepted Answer · 2020-04-03 14:12:23Z

1

The accepted answer assumes a ASP.NET Core environment using the configuration environment. In a pure console app, however, it's fine to just use Environment.GetEnvironmentVariable.

This seems not to read the environment variables defined in either Dockerfile or provided as parameter -e.

The variables defined that way are handed over in the current process. And this is not the default behavior. Hence, the following access will work:

Environment.GetEnvironmentVariable("VAR_NAME", EnvironmentVariableTarget.Process)

The key is the second parameter EnvironmentVariableTarget.Process.

answered Apr 3, 2020 at 14:12

Joerg Krause

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