DefaultInterfaceMethod_DiamondOfDeath.cs

  interface IDeveloper
  {
      void LearnNewLanguage(string language)
      {
          Console.Write($"Learning {language} in a default way.");
      }
  }

  interface IBackendDev : IDeveloper
  {
      void LearnNewLanguage(string language)
      {
          Console.Write($"Learning {language} in a backend way.");
      }
  }

  interface IFrontendDev : IDeveloper
  {
      void LearnNewLanguage(string language)
      {
          Console.Write($"Learning {language} in a frontend way.");
      }
  }

  interface IFullStackDev : IBackendDev, IFrontendDev { } // uses default implementation, 
                                                          // of LearnNewLanguage, but which one?
  class Dev : IFullStackDev { }

  class Program
  {
      static void Main(string[] args)
      {
          IFullStackDev dev = new Dev();
          dev.LearnNewLanguage("TypeScript"); // which method is called? The one
                                              // from IBackendDev or IFrontendDev?
      }
  }