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How would you write a custom configuration provider to load settings from an unsupported source, like a remote database?

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Answer

You implement IConfigurationSource (returning an IConfigurationProvider instance) and a corresponding ConfigurationProvider subclass overriding Load() to populate the provider's internal Data dictionary from your custom source, then register it via an extension method calling configurationBuilder.Add(new MyCustomSource()) -- this integrates seamlessly with the rest of the configuration system, including layering with other providers and (if you implement change detection) triggering hot-reload.

public class DatabaseConfigurationProvider : ConfigurationProvider {
    public override void Load() {
        using var connection = new SqlConnection(_connectionString);
        var settings = connection.Query("SELECT [Key], [Value] FROM AppSettings");
        foreach (var s in settings) Data[s.Key] = s.Value;
    }
}

public class DatabaseConfigurationSource : IConfigurationSource {
    public IConfigurationProvider Build(IConfigurationBuilder builder) => new DatabaseConfigurationProvider();
}

// Usage: builder.Configuration.Add(new DatabaseConfigurationSource());

Real-world example A multi-tenant SaaS platform builds a custom configuration provider that loads tenant-specific feature flags from a database table at startup, integrating seamlessly alongside the standard appsettings.json and environment variable providers.

Common follow-ups: How would you add live change detection (polling or push notifications) to this custom provider?;How does provider registration order affect precedence for a custom source?

Entity Framework Core & Data Access;Multiple Inheritance & MRO