* Entry.cs: virtual method call demonstration

* Main.cs:
* Entry.cs:
* BaseClass.cs:
* DerivedClass.cs:
* csharp-experiments.sln:
* deilycode-sources.csproj:
* csharp-experiments.csproj:
* Program.cs:
* Entry.cs:
* BaseClass.cs:
* ExternalService.cs:
* DerivedClass.cs: