C# in Depth

Partial types189Listing 7.2A partial method called from a constructor// Generated.csusing System;partial class PartialMethodDemo{public PartialMethodDemo(){OnConstructorStart();Console.WriteLine("Generated constructor");OnConstructorEnd();}}partial void OnConstructorStart();partial void OnConstructorEnd();// Handwritten.csusing System;partial class PartialMethodDemo{partial void OnConstructorEnd(){Console.WriteLine("Manual code");}}As shown in listing 7.2, partial methods are declared just like abstract methods: by providingthe signature without any implementation but using the partial modifier.Similarly, the actual implementations just have the partial modifier but are otherwiselike normal methods.Calling the parameterless constructor of PartialMethodDemo would result in “Generatedconstructor” and then “Manual code” being printed out. Examining the IL forthe constructor, you wouldn’t see a call to OnConstructorStart because it no longerexists—there’s no trace of it anywhere in the compiled type.Because the method may not exist, partial methods must have a return type ofvoid and can’t take out parameters. They have to be private, but they can be staticand/or generic. If the method isn’t implemented in one of the files, the whole statementcalling it is removed, including any argument evaluations. If evaluating any of thearguments has a side effect that you want to occur whether or not the partial methodis implemented, you should perform the evaluation separately. For instance, supposeyou have the following code:LogEntity(LoadAndCache(id));Here LogEntity is a partial method, and LoadAndCache loads an entity from the databaseand inserts it into the cache. You might want to use this instead:MyEntity entity = LoadAndCache(id);LogEntity(entity);That way, the entity is loaded and cached regardless of whether an implementationhas been provided for LogEntity. Of course, if the entity can be loaded equallyLicensed to Rhona Hadida