C# 通用静态构造函数

Question

是否会为传递给泛型参数的每种类型运行泛型类上的静态构造函数，例如:

 class SomeGenericClass<T>
 {
      static List<T> _someList;
      static SomeGenericClass()
      {
          _someList = new List<T>();
      }
 }

使用这种方法有什么缺点吗？

Answer 1

是的，静态构造函数将为每个封闭类类型(指定类型参数时创建的类型)调用一次。查看C# 3 specification , §10.12 静态构造函数。

The static constructor for a closed class type executes at most once in a given application domain.

还有:

Because the static constructor is executed exactly once for each closed constructed class type, it is a convenient place to enforce run-time checks on the type parameter that cannot be checked at compile-time via constraints (§10.1.5). For example, the following type uses a static constructor to enforce that the type argument is an enum:

class Gen<T> where T: struct
{
    static Gen() {
        if (!typeof(T).IsEnum) {
            throw new ArgumentException("T must be an enum");
        }
    }
}

阅读§4.4.2 开放和封闭类型也很重要:

At run-time, all of the code within a generic type declaration is executed in the context of a closed constructed type that was created by applying type arguments to the generic declaration. Each type parameter within the generic type is bound to a particular run-time type. The run-time processing of all statements and expressions always occurs with closed types, and open types occur only during compile-time processing.

Each closed constructed type has its own set of static variables, which are not shared with any other closed constructed types.

这个您可以自己运行的程序演示了静态构造函数被调用了三次，而不仅仅是一次:

public class Program
{
    class SomeGenericClass<T>
    {
        static SomeGenericClass()
        {
            Console.WriteLine(typeof(T));
        }
    }

    class Baz { }

    static void Main(string[] args)
    {
        SomeGenericClass<int> foo = new SomeGenericClass<int>();
        SomeGenericClass<string> bar = new SomeGenericClass<string>();
        SomeGenericClass<Baz> baz = new SomeGenericClass<Baz>();
    }
}

输出:

System.Int32System.StringProgram+Baz