Choosing Between Class and Struct

One of the basic design decisions every framework designer faces is whether to design a type as a class (a reference type) or as a struct (a value type). Good understanding of the differences in the behavior of reference types and value types is crucial in making this choice.

The first difference between reference types and value types we will consider is that reference types are allocated on the heap and garbage-collected, whereas value types are allocated either on the stack or inline in containing types and deallocated when the stack unwinds or when their containing type gets deallocated. Therefore, allocations and deallocations of value types are in general cheaper than allocations and deallocations of reference types.
簡單來說struct的allocate跟deallocate比Class來的便宜Cheaper. 指記憶體的消耗

Next, arrays of reference types are allocated out-of-line, meaning the array elements are just references to instances of the reference type residing on the heap. Value type arrays are allocated inline, meaning that the array elements are the actual instances of the value type. Therefore, allocations and deallocations of value type arrays are much cheaper than allocations and deallocations of reference type arrays. In addition, in a majority of cases value type arrays exhibit much better locality of reference.
The next difference is related to memory usage. Value types get boxed when cast to a reference type or one of the interfaces they implement. They get unboxed when cast back to the value type. Because boxes are objects that are allocated on the heap and are garbage-collected, too much boxing and unboxing can have a negative impact on the heap, the garbage collector, and ultimately the performance of the application. In contrast, no such boxing occurs as reference types are cast.
Next, reference type assignments copy the reference, whereas value type assignments copy the entire value. Therefore, assignments of large reference types are cheaper than assignments of large value types.
因為Struct有box unbox,所以如果是很龐大的Class會比較便宜.

Finally, reference types are passed by reference, whereas value types are passed by value. Changes to an instance of a reference type affect all references pointing to the instance. Value type instances are copied when they are passed by value. When an instance of a value type is changed, it of course does not affect any of its copies. Because the copies are not created explicitly by the user but are implicitly created when arguments are passed or return values are returned, value types that can be changed can be confusing to many users. Therefore, value types should be immutable.
最後, refernce type與value type的不同就是 一個pass by ref ,另一個pass by value.
ref type的實際值改變時所有的ref都跟著變.  value type都是獨立的instances,改變值時其他並不會變.


As a rule of thumb, the majority of types in a framework should be classes. There are, however, some situations in which the characteristics of a value type make it more appropriate to use structs.
所以選擇class or struct要視情況而定

✓ CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.
如果你要用的type很小而且短命,或是崁入別的物件內 就用struct.

X AVOID defining a struct unless the type has all of the following characteristics:
避免使用struct ,除非以下條件都達到
It logically represents a single value, similar to primitive types (int, double, etc.).
用來表示single value,就像是int double之類的.
It has an instance size under 16 bytes.
實作的size小於16 bytes
It is immutable.
固定的值,不變動的
It will not have to be boxed frequently.
不需要常常封裝
In all other cases, you should define your types as classes.
如在其他狀況下,你就應該用class而非struct

Portions © 2005, 2009 Microsoft Corporation. All rights reserved.

Reprinted by permission of Pearson Education, Inc. from Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries, 2nd Edition by Krzysztof Cwalina and Brad Abrams, published Oct 22, 2008 by Addison-Wesley Professional as part of the Microsoft Windows Development Series.