Environment Variables Provider

The environment variable provider reads environment variables and converts them to nested JSON.

rule.For<AppSettings>().FromEnvironment("APP_")

How It Works

1. Reads all environment variables at evaluation time
2. Filters by prefix (if specified)
3. Strips the prefix from matching keys
4. Converts flat key-value pairs to nested JSON

This provider is static — it reads environment variables once per recompute and does not watch for changes. Environment variables typically don't change during a process lifetime.

Prefix Filtering

The prefix filters which variables are included. Matching is case-insensitive:

// Only variables starting with "APP_"
rule.For<AppSettings>().FromEnvironment("APP_")

// All environment variables (no filtering)
rule.For<AppSettings>().FromEnvironment()

With prefix APP_, the variable APP_MaxRetries=10 becomes { "MaxRetries": 10 }.

Nesting Convention

Use __ (double underscore) or : to create nested JSON structures. A single _ is treated as a literal character:

# Double underscore = nesting
APP_Database__Host=localhost
APP_Database__Port=5432

# Produces:
# { "Database": { "Host": "localhost", "Port": 5432 } }

# Single underscore = literal
APP_App_Name=MyApp

# Produces:
# { "App_Name": "MyApp" }

# Colon also works
APP_Database:Host=localhost

# Produces:
# { "Database": { "Host": "localhost" } }

Common Pattern

Environment variables are typically the last rule, overriding everything else:

rule => [
    rule.For<AppSettings>().FromFile("appsettings.json").Required(),
    rule.For<AppSettings>().FromFile($"appsettings.{env}.json"),
    rule.For<AppSettings>().FromEnvironment("APP_"),  // Final override
]

This lets you override any config property via environment variables without touching files — useful for containers, CI/CD, and local development.

Dynamic Prefix ADV

Use the factory overload to derive the prefix from earlier config:

rule.For<TenantConfig>().FromEnvironment(accessor =>
{
    var tenant = accessor.GetConfig<TenantSettings>();
    return new EnvironmentVariableRuleOptions($"TENANT_{tenant.TenantId}_");
})