ExtensionEverything.cs

extension ExtendedEntity extends Entity
{
  private string _additionalField = "AdditionalField";
  
  public string AdditionalField
  {
    get 
    { 
      return _additionalField; 
    }
    set
    {
      _additionalField = value;
    }
  }
  
  public static void PrintSomething() => Console.Write("Someting");
  
  public void PrintAdditionalInfo() => Console.Wirte(_additionalInfo);
}

There's a typo on line 19. Console.Wirte should be Console.Write.

Line 5 also contains a typo. AditionalField should be AdditionalField.

Fixed it, thanks for pointing it out!