C# in Depth

86 CHAPTER 3 Parameterized typing with genericsunder the covers—although we won’t be going so deep that you need a flashlight.We’ll have a look at what happens when you enumerate a generic collection usingforeach in C# 2, and round off the section by seeing how reflection in the .NETFramework is affected by generics.3.4.1 Static fields and static constructorsJust as instance fields belong to an instance, static fields belong to the type they’redeclared in. That is, if you declare a static field x in class SomeClass, there’s exactlyone SomeClass.x field, no matter how many instances of SomeClass you create, andno matter how many types derive from SomeClass. 7 That’s the familiar scenario fromC# 1—so how does it map across to generics?The answer is that each closed type has its own set of static fields. This is easiest tosee with an example. Listing 3.7 creates a generic type including a static field. We setthe field’s value for different closed types, and then print out the values to show thatthey are separate.Listing 3.7Proof that different closed types have different static fieldsclass TypeWithField{public static string field;public static void PrintField(){Console.WriteLine(field+": "+typeof(T).Name);}}...TypeWithField.field = "First";TypeWithField.field = "Second";TypeWithField.field = "Third";TypeWithField.PrintField();TypeWithField.PrintField();TypeWithField.PrintField();We set the value of each field to a different value, and print out each field along withthe name of the type argument used for that closed type. Here’s the output from listing3.7:First: Int32Second: StringThird: DateTimeSo the basic rule is “one static field per closed type.” The same applies for static initializersand static constructors. However, it’s possible to have one generic type nested7Well, one per application domain. For the purposes of this section, we’ll assume we’re only dealing with oneapplication domain. The concepts for different application domains work the same with generics as with nongenerictypes.Licensed to Rhona Hadida