![]() It's important to note that local variables don't exist until a value is assigned to them in newer versions of C#. Changing one instance won't change the other.Īll variables (including fields and properties) always have space allocated for them as long as they exist. To the programmer, it appears as though a shallow copy of the object is made. When an object is passed by value, its contents, rather than a reference to the object, are passed along. ![]() You probably have a good understanding of what happens when an object is passed by reference. The price you pay for that performance gain is that user code can see a partially initialized structure.Ī struct is usually passed by value, while a class is always passed by reference. Summing up: calling a ctor is optional for value types because no new memory needs to be allocated when initializing an instance of a value type and because skipping the constructor call means that you get to skip a short-term allocation and a copy. Doing so of course means that you run the risk of having a variable of value type that can be observed to be in a partially initialized state by user code. All you need to do is ensure that the storage is properly initialized, and you can often do that without calling a constructor. Value types do not need a new allocation all they need is initialization. The new value is going to go somewhere, and you already have obtained that storage by some other means. You don't have to call "new" on a struct because there is no need to allocate the "final" storage the final storage already exists. "new" is the operator that knows how to do that. You need new memory allocated from the long-term storage and you need to pass a reference to that storage to the constructor. You have to allocate a class with "new" because of those three things on the list. Why are we forced to allocate a class with "new", instead of simply being able to initialize the fields as with a struct? So now we can address your question, which you actually have asked backwards. That is, the compiler can generate code that simply passes a reference to the final storage location to the constructor, thereby saving one allocation and one copy.) (Note that the compiler is allowed to optimize these three steps into one step if the compiler can determine that doing so never exposes a partially-constructed struct to user code. Remember, variables of value type store the actual value. After the constructor runs, the value that was in the short-term storage location is copied to the storage location for the value, wherever that happens to be. Second, the constructor is passed a reference to the short term storage location. First, the memory manager allocates space from short term storage. When you "new" a value type, three things happen. Third, that reference is passed back to the caller. Second, a reference to that space is passed to the constructor, which initializes the instance. First, the memory manager allocates space from long term storage. When you "new" a reference type, three things happen. Why are we not forced to instantiate a struct with "new", like when using a class?
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