How to correctly store connection strings in environment variables for retrieval by production ASP.Net Core MVC applications

There is a typo/wrong value set in your connection variables.

Which can be seen in this output you pasted:

Variable                    Value
ConnectionStrings:TestDb    "Server=TestServer;Database=TestDb;Persist Security Info=True;User ID=TestUser;Password=testpassword;MultipleActiveResultSets=true"

This likely happend while setting the variable via

$env:ConnectionStrings:MyDb = """Server=..."""

the correct way is to set it without the quotation marks.

$env:ConnectionStrings:MyDb = "Server=..."

Old answer (for other users who may search for similar issues)

The convention for connection strings is SQLCONNSTR_, MYSQLCONNSTR_, SQLAZURECONNSTR_ and CUSTOMCONNSTR_ which are used by Azure Web Apps, but should also work for self-hosting, VMs or any other cloud provider.

So if you have an environment variable called CUSTOMCONNSTR_TestDb it will be the same as defining it in appsettings.json in

{
    "connectionStrings": {
        "TestDb": "..."
    }
}

It will also override the value inside it, if AddEnvironmentVariables() is called after .UseJsonFile(...). Last registration wins.

var builder = new ConfigurationBuilder()
    .SetBasePath(env.ContentRootPath)
    .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
    .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
    // This one needs to be last
    .AddEnvironmentVariables();

You can also use other variables to override configuration values. iirc. ASPNETCORE_ is the default prefix (but you can change it in the AddEnvironmentVariables("MY_")).

So a ASPNETCORE_MySettings overrides Configuration["MySettings"] (or Configuration.Get("MySettings")) and ASPNETCORE_My__Settings (use double underscore for level hierarchy on Linux, read where : is used to obtain the config – Linux disallows colon in variable names) overrides Configuration["My:Settings"] so same as

{
    "my": {
        "settings": "..."
    }
}

Unless they changed that recently.

FWIW: Environment variables/configuration key names are case-insensitive as far as I remember.