C# in Depth

Value types and reference types49Once you “get it,” the difference between value types and reference types is simple.It can take a while to reach that stage, and you may well have been able to write a lot ofcorrect code without really understanding it. It’s worth persevering, though: for onething, a lot of seemingly complicated situations are much easier to understand whenyou’re aware of what’s really going on.This section is not a complete breakdown of how types are handled, marshalingbetween application domains, interoperability with native code, and the like. Instead,it’s a fairly brief look at the absolute basics of the topic (as applied to C# 1) that are crucialto understand in order to come to grips with C# 2 and 3.We’ll start off by seeing how the fundamental differences between value types andreference types appear naturally in the real world as well as in .NET.2.3.1 Values and references in the real worldSuppose you’re reading something really fantastic, and want a friend to read it too.Let’s further suppose that it’s a document in the public domain, just to avoid any accusationsof supporting copyright violation. What do you need to give your friend so thathe can read it too? It entirely depends on just what you’re reading.First we’ll deal with the case where you’ve got real paper in your hands. To giveyour friend a copy, you’d need to photocopy all the pages and then give it to him. Atthat point, he has his own complete copy of the document. In this situation, we aredealing with value type behavior. All the information is directly in your hands—youdon’t need to go anywhere else to get it. Your copy of the information is also independentof your friend’s after you’ve made the copy. You could add some notes to yourpages, and his pages wouldn’t be changed at all.Compare that with the situation where you’re actually reading a web page. Thistime, all you have to give your friend is the URL of the web page. This is reference typebehavior, with the URL taking the place of the reference. In order to actually read thedocument, you have to navigate the reference by putting the URL in your browser andasking it to load the page. On the other hand, if the web page changes for some reason(imagine it’s a wiki page and you’ve added your notes to the page) both you andyour friend will see that change the next time each of you loads the page.The differences we’ve seen in the real world form the heart of the distinctionbetween value types and reference types in C# and .NET. Most types in .NET are referencetypes, and you’re likely to create far more reference than value types. Aside fromthe special cases that follow, classes (declared using class) are reference types, andstructures (declared using struct) are value types. The other cases are as follows:■ Array types are reference types, even if the element type is a value type (soint[] is still a reference type, even though int is a value type).■■■Enumerations (declared using enum) are value types.Delegate types (declared using delegate) are reference types.Interface types (declared using interface) are reference types, but they can beimplemented by value types.Licensed to Rhona Hadida