Compare the differences between value types and reference types in C#

CLR supports two types: value types and reference types. It seems that most types of FCL are reference types, but the most commonly used types are value types. Reference types are always allocated from the managed heap. When an object is instantiated using the new operator, the returned object memory address is stored in a variable. There are four psychological factors to understand when using reference types: 1. Memory must be allocated from the managed heap

2. Each object allocated on the heap has some additional members, these Members must be initialized before use.

　　　 3. Other bytes in the object are always set to zero.

　　　 4. When allocating objects from the managed heap, a garbage collection may be forced.

It can be seen that if the reference type is abused, it will inevitably cause overload. Looking at the value type again, it is a lightweight type. It is generally allocated on the thread stack and can also be embedded in a reference type object. The variable carrying it does not contain a pointer to the instance, and of course it is not controlled by the garbage collector, so the value The use of types relieves pressure on the managed heap and reduces the number of garbage collections during the application lifetime.

What are called ‘classes’ in C# are reference types, while value types are structures or enumerations. All structures are directly derived from the abstract type System.ValueType. And it itself is directly derived from Object, all enumerations are derived from the System.Enum abstract type, and Enum is derived from ValueType, and they are all linked together.

When designing your own type, when will you define it as a value type?

1. The type has the behavior of a primitive type (a data type directly supported by the compiler (like Int32, Int64, etc.))

2. The type does not need to inherit from any other type.

　　　3. The type does not derive any other types.

4. Since the value type needs to copy the corresponding fields when used as an actual parameter or return value, if the instance is too large, it will cause a certain performance loss.

Therefore, the type instance is required to be small (within 16 bytes) and the instance is larger, but it is not necessary to pass it as an actual parameter of the method and return it from the method.

Regarding the difference between value types and reference types, the following points are roughly listed:

1. Reference types are in boxed form, and value types are in unboxed form. Both can also be used. Convert to each other in a certain way (specific instructions will be discussed in detail in the next blog)

2. Value types should not introduce any new virtual methods. All methods cannot be abstract and are implicitly sealed. Prevent rewriting.

　 3. Assigning a value type variable to another value type variable will perform field-by-field copying. Assigning a variable of reference type to another reference type copies only the memory address. Two or more reference type variables can refer to the same object on the heap, and operations performed on one variable may affect the object referenced by another variable. In contrast, value type variables are independent and do not affect each other.

4. Unboxed value types are not allocated on the heap, so once an instance of this type is no longer active, the storage allocated for them will be released and will not wait for garbage collection

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