更多琐事而不是真正重要：为什么 Activator.CreateInstance() 没有 new() 约束？

发布于 2024-10-20 05:34:01 字数 645 浏览 4 评论 0原文

我认为有人可能能够回答这个问题，这是出于好奇而提出的问题：

来自 System.Activator 的通用 CreateInstance 方法，在 .NET v2 中引入泛型参数没有类型限制，但需要激活类型的默认构造函数，否则会引发 MissingMethodException 。对我来说，很明显这个方法应该有一个类型约束，比如

Activator.CreateInstance<T>() where T : new() {
   ...
}

只是一个遗漏或潜伏在这里的一些轶事？

更新

正如所指出的，编译器不允许您编写

private T Create<T>() where T : struct, new()
error CS0451: The 'new()' constraint cannot be used with the 'struct' constraint

但是，请参阅注释结构可以用作指定 new() 约束的泛型方法的类型参数。在这种情况下，给定的答案似乎是不限制该方法的唯一有效理由......

感谢您查看此内容！

原文

I think there are people who may be able to answer this, this is a question out of curiosity:

The generic CreateInstance method from System.Activator, introduced in .NET v2 has no type constraints on the generic argument but does require a default constructor on the activated type, otherwise a MissingMethodException is thrown. To me it seems obvious that this method should have a type constraint like

Activator.CreateInstance<T>() where T : new() {
   ...
}

Just an omission or some anecdote lurking here?

Update

As pointed out, the compiler does not allow you to write

private T Create<T>() where T : struct, new()
error CS0451: The 'new()' constraint cannot be used with the 'struct' constraint

However, see comments a struct can be used as type argument to a generic method specifying a new() constraint. Under this circumstance the given answer seems the only valid reason to not constrain the method...

Thanks for looking over this!

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℡寂寞咖啡 2024-10-27 05:34:01

我可能是错的，但我认为它的主要好处是它允许您执行以下操作：

// Simple illustration only, not claiming this is awesome code!
class Cache<T>
{
    private T _instance;

    public T Get()
    {
        if (_instance == null)
        {
            _instance = Create();
        }

        return _instance;
    }

    protected virtual T Create()
    {
        return Activator.CreateInstance<T>();
    }
}

请注意，如果 Activator.CreateInstance 有一个 where T : new( ) 约束，那么上面的 Cache 类也需要该约束，这会过于严格，因为 Create 是虚拟方法和某些派生类可能希望使用不同的实例化方式，例如调用类型的内部构造函数或使用静态生成器方法。

I could be wrong, but the main benefit as I see it is that it allows you to do something like this:

// Simple illustration only, not claiming this is awesome code!
class Cache<T>
{
    private T _instance;

    public T Get()
    {
        if (_instance == null)
        {
            _instance = Create();
        }

        return _instance;
    }

    protected virtual T Create()
    {
        return Activator.CreateInstance<T>();
    }
}

Note that if Activator.CreateInstance<T> had a where T : new() constraint, then the Cache<T> class above would also need that constraint, which would be overly restrictive since Create is a virtual method and some derived class might want to use a different means of instantiation, such as calling a type's internal constructor or using a static builder method.

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