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In the preceding code, you saw this line:
SomeVal v1 = new SomeVal(); // Allocated on stack
The way this line is written makes it look as if a SomeVal instance will be allocated on the managed heap . However, the C# compiler knows that SomeVal is a value type and produces code that allocates the SomeVal instance on the thread s stack . C# also ensures that all of the fields in the value type instance are zeroed .
Part II Designing Types
The preceding line could have been written like this instead:
SomeVal v1; // Allocated on stack
This line also produces IL that allocates the instance on the thread s stack and zeroes the fields . The only difference is that C# thinks that the instance is initialized if you use the new operator . The following code will make this point clear:
// These two lines compile because C# thinks that // v1's fields have been initialized to 0. SomeVal v1 = new SomeVal(); Int32 a = v1.x; // These two lines don't compile because C# doesn't think that // v1's fields have been initialized to 0. SomeVal v1; Int32 a = v1.x; // error CS0170: Use of possibly unassigned field 'x'
When designing your own types, consider carefully whether to define your types as value types instead of reference types . In some situations, value types can give better performance . In particular, you should declare a type as a value type if all the following statements are true:
The type acts as a primitive type . Specifically, this means that it is a fairly simple type that has no members that modify any of its instance fields . When a type offers no members that alter its fields, we say that the type is immutable . In fact, it is recommended that many value types mark all their fields as readonly (discussed in 7, "Constants and Fields") . The type doesn t need to inherit from any other type . The type won t have any other types derived from it .
The size of instances of your type is also a condition to take into account because by default, arguments are passed by value, which causes the fields in value type instances to be copied, hurting performance . Again, a method that returns a value type causes the fields in the instance to be copied into the memory allocated by the caller when the method returns, hurting performance . So, in addition to the previous conditions, you should declare a type as a value type if one of the following statements is true:
Instances of the type are small (approximately 16 bytes or less) . Instances of the type are large (greater than 16 bytes) and are not passed as method parameters or returned from methods .
The main advantage of value types is that they re not allocated as objects in the managed heap . Of course, value types have several limitations of their own when compared to reference types . Here are some of the ways in which value types and reference types differ:
5 Primitive, Reference, and Value Types
Value type objects have two representations: an unboxed form and a boxed form (discussed in the next section) . Reference types are always in a boxed form . Value types are derived from System.ValueType . This type offers the same methods as defined by System.Object . However, System.ValueType overrides the Equals method so that it returns true if the values of the two objects fields match . In addition, System.ValueType overrides the GetHashCode method to produce a hash code value by using an algorithm that takes into account the values in the object s instance fields . Due to performance issues with this default implementation, when defining your own value types, you should override and provide explicit implementations for the Equals and GetHashCode methods . I ll cover the Equals and GetHashCode methods at the end of this chapter . Because you can t define a new value type or a new reference type by using a value type as a base class, you shouldn t introduce any new virtual methods into a value type . No methods can be abstract, and all methods are implicitly sealed (can t be overridden) . Reference type variables contain the memory address of objects in the heap . By default, when a reference type variable is created, it is initialized to null, indicating that the reference type variable doesn t currently point to a valid object . Attempting to use a null reference type variable causes a NullReferenceException to be thrown . By contrast, value type variables always contain a value of the underlying type, and all members of the value type are initialized to 0 . Since a value type variable isn t a pointer, it s not possible to generate a NullReferenceException when accessing a value type . The CLR does offer a special feature that adds the notion of nullability to a value type . This feature, called nullable types, is discussed in 19, Nullable Value Types . When you assign a value type variable to another value type variable, a field-by-field copy is made . When you assign a reference type variable to another reference type variable, only the memory address is copied . Because of the previous point, two or more reference type variables can refer to a single object in the heap, allowing operations on one variable to affect the object referenced by the other variable . On the other hand, value type variables are distinct objects, and it s not possible for operations on one value type variable to affect another . Because unboxed value types aren t allocated on the heap, the storage allocated for them is freed as soon as the method that defines an instance of the type is no longer active . This means that a value type instance doesn t receive a notification (via a Finalize method) when its memory is reclaimed .
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