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ebook Addison -Essential CSharp 4.0_4

Mặc dù một số ngôn ngữ hỗ trợ khả năng truy cập các loại giá trị trên đống trực tiếp, điều này có thể trong C # chỉ khi các loại giá trị được truy cập như một lĩnh vực trên một loại tài liệu tham khảo. Kể từ khi giao diện được các loại tài liệu tham khảo, unboxing và sao chép có thể tránh được, khi truy cập vào giá trị đóng hộp thông qua giao diện của nó. | Boxing 345 The result is that no copy back from the heap to the stack occurs. Instead the modified heap data is ready for garbage collection while the data in angle remains unmodified. In the last case the cast to lAngle occurs with the data on the heap already so no copy occurs. MoveTo updates the -Hours value and the code behaves as desired. ADVANCED TOPIC Unboxing Avoided As discussed earlier the unboxing instruction does not include the copy back to the stack. Although some languages support the ability to access value types on the heap directly this is possible in C only when the value type is accessed as a field on a reference type. Since interfaces are reference types unboxing and copying can be avoided when accessing the boxed value via its interface. When you call an interface method on a value type the instance must be a variable because the method might mutate the value. Since unboxing produces a managed address the runtime has a storage location and hence a variable. As a result the runtime simply passes that managed address on an interface and no unboxing operation is necessary. Listing 8.7 added an interface implementation to the Angle struct. Listing 8.8 uses the interface to avoid unboxing. Listing 8.8 Avoiding Unboxing and Copying int number object thing number 42 Boxing thing number No unbox instruction. string text IFormattable thing .ToString X null Console.WriteLine text Interfaces are reference types anyway so calling an interface member does not even require unboxing. Furthermore calling a struct s ToString method that overrides object s ToString method does not Chapter 8 Value Types require an unbox. When compiling it is clear that a struct s overriding ToString method will always be called because all value types are sealed. The result is that the C compiler can instruct a direct call to the method without unboxing. Enums Compare the two code snippets shown in Listing 8.9. Listing 8.9 Comparing an Integer Switch to an Enum Switch int .