public IConfigurationRoot Configuration { get; }

public Startup(IHostingEnvironment env)
{
    var builder = new ConfigurationBuilder()
        .SetBasePath(env.ContentRootPath)
        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
        .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
        .AddEnvironmentVariables();

    if (env.IsDevelopment())
    {
        // This will push telemetry data through Application Insights pipeline faster, allowing you to view results immediately.
        builder.AddApplicationInsightsSettings(developerMode: true);
    }

    Configuration = builder.Build();

    Environment = env;
}

services.AddSingleton<IConfiguration>(Configuration);

private readonly IConfiguration _configuration;
private readonly string _connectionString;

public SuaClasseRepository(IConfiguration configuration)
{
    _configuration = configuration;
    _connectionString = configuration.GetConnectionString("DefaultConnection");
}

"ConnectionStrings": {
    "DefaultConnection": "myDatabaseConnectionString"
  }