C# in Depth

Beyond the basics77NOTEC# vs. CLI standards—There is a discrepancy between the C# and CLIstandards when it comes to value types and constructors. The CLI specificationstates that value types can’t have parameterless constructors, butthere’s a special instruction to create a value without specifying anyparameters. The C# specification states that all value types have a defaultparameterless constructor, and it uses the same syntax to call both explicitlydeclared constructors and the parameterless one, relying on the compilerto do the right thing underneath. You can see this discrepancy atwork when you use reflection to find the constructors of a value type—you won’t see a parameterless one.Again, let’s look at a quick example, this time for a method. Just to show how it’s useful,I’ll give the implementation of the method too.public T CreateInstance() where T : new(){return new T();}This method just returns a new instance of whatever type you specify, providingthat it has a parameterless constructor. So CreateInstance(); and Create-Instance(); are OK, but CreateInstance(); isn’t, because stringdoesn’t have a parameterless constructor.There is no way of constraining type parameters to force other constructor signatures—forinstance, you can’t specify that there has to be a constructor taking a singlestring parameter. It can be frustrating, but that’s unfortunately just the way it is.Constructor type constraints can be useful when you need to use factory-like patterns,where one object will create another one as and when it needs to. Factories oftenneed to produce objects that are compatible with a certain interface, of course—andthat’s where our last type of constraint comes in.DERIVATION TYPE CONSTRAINTSThe final (and most complicated) kind of constraint lets you specify another type thatthe type argument must derive from (in the case of a class) or implement (in the caseof an interface). 4 For the purposes of constraints, types are deemed to derive fromthemselves. You can specify that one type argument must derive from another, too—this is called a type parameter constraint and makes it harder to understand the declaration,but can be handy every so often. Table 3.2 shows some examples of generic typedeclarations with derivation type constraints, along with valid and invalid examples ofcorresponding constructed types.The third constraint of T : IComparable is just one example of using a generictype as the constraint. Other variations such as T : List (where U is another type4Strictly speaking, an implicit reference conversion is OK too. This allows for a constraint such as where T :IList to be satisfied by Circle[]. Even though Circle[] doesn’t actually implement IList, there is an implicit reference conversion available.Licensed to Rhona Hadida