C# in Depth

Beyond the basics79combined together. Obviously no type can be both a reference type and a value type, sothat combination is forbidden, and as every value type has a parameterless constructor,specifying the construction constraint when you’ve already got a value type constraintis also not allowed (but you can still use new T() within methods if T is constrained tobe a value type). Different type parameters can have different constraints, and they’reeach introduced with a separate where.Let’s see some valid and invalid examples:Valid:class Sample where T : class, Stream, new()class Sample where T : struct, IDisposableclass Sample where T : class where U : struct, Tclass Sample where T : Stream where U : IDisposableInvalid:class Sample where T : class, structclass Sample where T : Stream, classclass Sample where T : new(), Streamclass Sample where T : struct where U : class, Tclass Sample where T : Stream, U : IDisposableI included the last example on each list because it’s so easy to try the invalid oneinstead of the valid version, and the compiler error is not at all helpful. Just rememberthat each list of type parameter constraints needs its own introductory where. Thethird valid example is interesting—if U is a value type, how can it derive from T, whichis a reference type? The answer is that T could be object or an interface that U implements.It’s a pretty nasty constraint, though.Now that you’ve got all the knowledge you need to read generic type declarations,let’s look at the type argument inference that I mentioned earlier. In listing 3.2 weexplicitly stated the type arguments to List.ConvertAll—but let’s now ask the compilerto work them out when it can, making it simpler to call generic methods.3.3.2 Type inference for type arguments of generic methodsSpecifying type arguments when you’re calling a generic method can often seempretty redundant. Usually it’s obvious what the type arguments should be, based onthe method arguments themselves. To make life easier, the C# 2 compiler is allowed tobe smart in tightly defined ways, so you can call the method without explicitly statingthe type arguments.Before we go any further, I should stress that this is only true for generic methods. Itdoesn’t apply to generic types. Now that we’ve got that cleared up, let’s look at the relevantlines from listing 3.3, and see how things can be simplified. Here are the linesdeclaring and invoking the method:static List MakeList (T first, T second)...List list = MakeList ("Line 1", "Line 2");Now look at the arguments we’ve specified—they’re both strings. Each of the parametersin the method is declared to be of type T. Even if we hadn’t got the partLicensed to Rhona Hadida