Secrets provider interface

ConfigSecretsProvider.cs

public class ConfigSecretsProvider : ISecretsProvider
{
    private readonly NameValueCollection _secretCollection;

    public ConfigSecretsProvider() {
        _secretCollection = ConfigurationManager.GetSection("appSecrets") as NameValueCollection;
    }

    public string GetSecret(string key) {
        return Environment.GetEnvironmentVariable(key) ?? _secretCollection[key];
    }

    public Task<string> GetSecretAsync(string key)
    {
        return Task.FromResult(GetSecret(key));
    }
}

ISecretsProvider.cs

public interface ISecretsProvider
{
    /// <summary>
    /// Retrieves a secret by key
    /// </summary>
    /// <param name="key"></param>
    /// <returns></returns>
    string GetSecret(string key);

    /// <summary>
    /// Retrieves a secret by key (async)
    /// </summary>
    /// <param name="key"></param>
    /// <returns></returns>
    Task<string> GetSecretAsync(string key);
}

web.config

<!-- Example -->
<configuration>
    <configSections>
        <section name="appSecrets" type="System.Configuration.NameValueSectionHandler" />
    </configSections>
    <appSettings>
        <!-- No longer has secrets! -->
    </appSettings>
    <appSecrets>
        <add key="FooSecret" value="SuperSecret"/>
    </appSecrets>
</configuration>