Using a database connection string stored in environment variable during migrations

Question

I have ASP.NET Core project in which I store my PostgreSQL connection string in an environment variable. When I try to do the initial migration the project's build succeeds, but then it throws an exception:

System.ArgumentNullException: Value cannot be null. (Parameter 'connectionString').

So I assume that it does not read the connection string correctly. I provide connection string to my DbContext in ConfigureServices method like this:

services.AddDbContext<GoalsContext>(options =>  
  options.UseNpgsql(Environment.GetEnvironmentVariable("CONNECTION_STRING")));

I also have tried to fix this by creating an implementation of IDesignTimeDbContextFactory like this:

public class GoalsContextFactory : IDesignTimeDbContextFactory<GoalsContext>
{
  public GoalsContext CreateDbContext(string[] args)
  {
    var optionsBuilder = new DbContextOptionsBuilder<GoalsContext>();
    optionsBuilder.UseNpgsql(Environment.GetEnvironmentVariable("CONNECTION_STRING");

    return new GoalsContext(optionsBuilder.Options);
  }
}

Any help would be very appreciated.

Update: I have as suggested in the comments read the official documentation on Environment variables and configuration and I have checked whether the variable exists in the first place by creating a controller with one method, which returns a connection string like this:

[HttpGet]
public string GetConnectionString()
{
  return _config["CONNECTION_STRING"];
}

And correct connection string indeed shows up in the browser when I launch it, but when I tried to add initial migration to database the exception that I have already mentioned is still thrown.

I have also tried to comment out my implementation of IDesignTimeDbContextFactory and to manually add a call to .AddEnvironmentVariables method in my Program.cs (which I read is done automatically, but I still wanted to try), but none of that had any effect - my controller still puts out connection string while Entity Framework Core's migration still does not receive it.

So in short, yes connection string exists and I can even output it, but migration still fails due to the same exception.

Answer 1

As per my comment.

See Command Line Environment Variables

Also, if you want to bypass the setting of the variables, you'll need to look into the OnConfiguring Method overload of the Context and use the IsConfigured flag on the DbOptions parameter passed in to only read from a file when no valid config is passed to the context.

I should caveat that to say, that this also leads to issues when other developers use the solution and also, you shouldn't check the files you read into a public source control repository as this is one way people get credentials for things they shouldn't, look into User Secrets