C# in Depth

78 CHAPTER 3 Parameterized typing with genericsTable 3.2Examples of derivation type constraintsDeclarationclass Samplewhere T : Streamstruct Samplewhere T : IDisposableclass Samplewhere T : IComparableclass Samplewhere T : UConstructed type examplesValid: SampleSampleInvalid: SampleSampleValid: SampleSampleInvalid: SampleValid: SampleInvalid: SampleValid: SampleSampleInvalid: Sampleparameter) and T : IList are also fine. You can specify multiple interfaces,but only one class. For instance, this is fine (if hard to satisfy):class Sample where T : Stream,IEnumerable,IComparableBut this isn’t:class Sample where T : Stream,ArrayList,IComparableNo type can derive directly from more than one class anyway, so such a constraintwould usually either be impossible (like this one) or part of it would be redundant(specifying that the type had to derive from both Stream and MemoryStream, for example).One more set of restrictions: the class you specify can’t be a struct, a sealed class(such as string), or any of the following “special” types:■■■■System.ObjectSystem.EnumSystem.ValueTypeSystem.DelegateDerivation type constraints are probably the most useful kind, as they mean you canuse members of the specified type on instances of the type parameter. One particularlyhandy example of this is T : IComparable, so that you know you can comparetwo instances of T meaningfully and directly. We’ll see an example of this (as well asdiscuss other forms of comparison) in section 3.3.3.COMBINING CONSTRAINTSI’ve mentioned the possibility of having multiple constraints, and we’ve seen them inaction for derivation constraints, but we haven’t seen the different kinds beingLicensed to Rhona Hadida