C# in Depth

Type system characteristics45for a different type. You can try by adding a cast to give the compiler this extra (andincorrect) information, but if the compiler spots that it’s actually impossible for thatcast to work, it will trigger a compilation error—and if it’s theoretically allowed butactually incorrect at execution time, the CLR will throw an exception.Now that we know a little about how C# 1 fits into the bigger picture of type systems,I’d like to mention a few downsides of its choices. That’s not to say the choicesare wrong—just limiting in some ways. Often language designers have to choosebetween different paths that add different limitations or have other undesirable consequences.I’ll start with the case where you want to tell the compiler more information,but there’s no way of doing so.2.2.2 When is C# 1’s type system not rich enough?There are two common situations where you might want to expose more informationto the caller of a method, or perhaps force the caller to limit what they provide intheir arguments. The first involves collections, and the second involves inheritanceand overriding methods or implementing interfaces. We’ll examine each in turn.COLLECTIONS, STRONG AND WEAKHaving avoided the terms strong and weak for the C# type system in general, I’ll usethem when talking about collections. They’re used almost everywhere in this context,with little room for ambiguity. Broadly speaking, three kinds of collection types arebuilt into .NET 1.1:■■■Arrays—strongly typed—which are built into both the language and the runtimeThe weakly typed collections in the System.Collections namespaceThe strongly typed collections in the System.Collections.SpecializednamespaceArrays are strongly typed, 8 so at compile time you can’t set an element of a string[] tobe a FileStream, for instance. However, reference type arrays also support covariance,which provides an implicit conversion from one type of array to another, so long asthere’s a conversion between the element types. Checks occur at execution time tomake sure that the wrong type of reference isn’t actually stored, as shown in listing 2.3.Listing 2.3Demonstration of the covariance of arrays, and execution time type checkingstring[] strings = new string[5];object[] objects = strings;objects[0] = new object();BCApplies covariantconversionAttempts to storea plain “object”If you run listing 2.3, you will see an ArrayTypeMismatchException is thrown C. Thisis because the conversion from string[] to object[] B returns the original reference—bothstrings and objects refer to the same array. The array itself “knows” it is8At least, the language allows them to be. You can use the Array type for weakly typed access to arrays, though.Licensed to Rhona Hadida