Свой источник конфигурации Entity Framework

ConfigValue.cs

public class ConfigValue
{
    public string Key { get; set; }
    public string Value { get; set; }
}

EfConfigurationProvider.cs

public class EfConfigurationProvider : ConfigurationProvider
{
    public Action<DbContextOptionsBuilder> Configurator { get; }

    public EfConfigurationProvider(Action<DbContextOptionsBuilder> Configurator)
    {
        this.Configurator = Configurator;
    }

    public override void Load()
    {
        var builder = new DbContextOptionsBuilder<ExampleContext>();
        
        Configurator(builder);
        
        using var dbContext = new ExampleContext(builder.Options);
        Data = dbContext.Configurations.ToDictionary(x => x.Key, v => v.Value);
    }
}

EfConfigurationSource.cs

public class EfConfigurationSource : IConfigurationSource
{
    private readonly Action<DbContextOptionsBuilder> _Options;
    
    public EfConfigurationSource(Action<DbContextOptionsBuilder> options)
    {
        _Options = options;
    }
    
    public IConfigurationProvider Build(IConfigurationBuilder builder)
    {
        return new EfConfigurationProvider(_Options);
    }
}

EfExtensions.cs

public static class EfExtensions
{
    public static IConfigurationBuilder AddEfConfiguration(this IConfigurationBuilder config, Action<DbContextOptionsBuilder> options)
    {
        return config.Add(new EfConfigurationSource(options));
    }
}

ExampleContext.cs

public class ExampleContext : DbContext
{
    public DbSet<ConfigValue> Configurations { get; set; }
    
    public ExampleContext(DbContextOptions options) : base(options)
    {
    }
}

Использование.cs

WebHost.CreateDefaultBuilder(args)
       .ConfigureAppConfiguration((context, builder) =>
        {
            var config = builder.Build();
            builder.AddEfConfiguration(optionsBuilder =>
            {
                optionsBuilder.UseNpgsql(config.GetConnectionString("DatabaseConnection"));
            });
        })
       .UseStartup<Startup>();