ConfigurationExtension.cs

using System;
using System.Collections.Specialized;
using System.Configuration;

public static class ConfigurationExtension {
  public static string ReadSetting(this NameValueCollection self, string key) {
    return self[$"{Environment.MachineName}.{key}"] ?? self[key];
  }

   public static string ReadConnectionString(this ConnectionStringSettingsCollection self, string name) {
      return self[$"{Environment.MachineName}.{name}"]?.ConnectionString ?? self[name]?.ConnectionString;
  }
}