如何指定 .NET 泛型约束中不允许的类型？

发布于 2024-11-29 20:11:48 字数 157 浏览 3 评论 0原文

是否可以在泛型类上指定不允许某些类型的约束？我不知道这是否可能，如果可能，我不确定语法是什么。就像：

public class Blah<T> where : !string {
}

我似乎找不到任何允许这种约束的符号。

原文

Is it possible to specify a constraint on a generic class that disallows certain types? I don't know if it is possible and if it is, I am not sure what the syntax would be. Something like:

public class Blah<T> where : !string {
}

I can't seem to find any notation that would allow such a constraint.

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故人的歌 2024-12-06 20:11:48

您可以获得的最接近的是运行时约束。

编辑：最初我将运行时检查放在构造函数调用中。这实际上不是最佳的，因为它会在每次实例化时产生开销；我相信将检查放在 static 构造函数中会更明智，该构造函数将针对用作 Blah< 的 T 参数的每个类型调用一次。 T> 类型：

public class Blah<T> {
    static Blah() {
        // This code will only run ONCE per T, rather than every time
        // you call new Blah<T>() even for valid non-string type Ts
        if (typeof(T) == typeof(string)) {
            throw new NotSupportedException("The 'string' type argument is not supported.");
        }
    }
}

显然不理想，但是如果您将此约束放在适当的位置并记录string不是受支持的类型参数的事实（例如，通过XML 注释），你应该有所收获接近编译时约束的有效性。

The closest you can get is a run-time constraint.

Edit: Originally I put the run-time check in the constructor call. That's actually not optimal, as it incurs overhead on every instantiation; I believe it would be much more sensible to put the check in the static constructor, which will be invoked once per type used as the T parameter for your Blah<T> type:

public class Blah<T> {
    static Blah() {
        // This code will only run ONCE per T, rather than every time
        // you call new Blah<T>() even for valid non-string type Ts
        if (typeof(T) == typeof(string)) {
            throw new NotSupportedException("The 'string' type argument is not supported.");
        }
    }
}

Obviously not ideal, but if you put this constraint in place and document the fact that string is not a supported type argument (e.g., via XML comments), you should get somewhere near the effectiveness of a compile-time constraint.

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