C# in Depth

Beyond the basics75parameters. You don’t have to use it, but it can make your code a lot easier to readwhen used appropriately. We’ll see in part 3 that the C# compiler is gradually beingallowed to infer a lot more information from your code, while still keeping the languagesafe and statically typed.The last part of this section deals with obtaining the default value of a type parameterand what comparisons are available when you’re writing generic code. We’ll wrapup with an example demonstrating most of the features we’ve covered, as well as beinga useful class in itself.Although this section delves a bit deeper into generics, there’s nothing really hardabout it. There’s plenty to remember, but all the features serve a purpose, and you’llbe grateful for them when you need them. Let’s get started.3.3.1 Type constraintsSo far, all the type parameters we’ve seen can be applied to any type at all—they areunconstrained. We can have a List, a Dictionary, anything.That’s fine when we’re dealing with collections that don’t have to interact with whatthey store—but not all uses of generics are like that. Often you want to call methodson instances of the type parameter, or create new instances, or make sure you onlyaccept reference types (or only accept value types). In other words, you want to specifyrules to say which type arguments are considered valid for your generic type ormethod. In C# 2, you do this with constraints.Four kinds of constraints are available, and the general syntax is the same for all ofthem. Constraints come at the end of the declaration of a generic method or type,and are introduced by the contextual keyword where. They can be combined togetherin sensible ways, as we’ll see later. First, however, we’ll explore each kind of constraintin turn.REFERENCE TYPE CONSTRAINTSThe first kind of constraint (which is expressed as T : class and must be the first constraintspecified for that type parameter) simply ensures that the type argument usedis a reference type. This can be any class, interface, array, or delegate—or anothertype parameter that is already known to be a reference type. For example, considerthe following declaration:struct RefSample where T : classValid closed types include■■■RefSampleRefSampleRefSampleInvalid closed types include■■RefSampleRefSampleLicensed to Rhona Hadida