具有动态字段数的C＃值类型

Question

The best way to explain my problem is with a code snippet:

enum Resource { ..., Size }
class ResourceVector 
{
    int[] values = new int[(int)Resource.Size];

    public static ResourceVector operator + (ResourceVector a, ResourceVector b)
    {...}

    ...
}

We are using this type everywhere as though it were a value type. Ie we are making the assumption that:

ResourceVector b = a;
b += c;

will not affect a , because that is how we are used to talking about vectors (and that is how a vector with a fixed number of fields does behave, if implemented as a struct).

However since that assumption is wrong, it has led to some extremely subtle bugs.

I am wondering if there is a way to make this behave as a value type, other than just expanding the members of Resource each into their own members in a struct ResourceVector (which requires touching every member of ResourceVector if we wish to add another Resource ).

Oh, just in case, we are working with C# 2.0. So no fancy-pants features :-)

Thanks.

Answer 1

I think you are confusing “value type” with “immutable type”. You actually want your ResourceVector to be an immutable type .

An immutable type is one that has no way to change its contents. Once constructed, an instance retains its value until it is garbage-collected. All operations such as addition return a new instance containing the result instead of modifying an existing instance.

string is the most well-known immutable class. All operations like Substring and Replace return a new string and don't modify the existing strings.

Once your type is properly immutable, semantically it doesn't matter so much anymore whether it is a value type or a reference type — but it matters to the performance. If you pass values around a lot, you should probably make it a reference type in order to avoid a lot of unnecessary copying.