[ConditionalAttribute("DEBUG")]定义Check()方法为conditional method，如果预处理符号（preprocessor symbol）没有定义，compiler不仅忽略该方法，而且忽略对该方法的调用，和#if DEBUG / #else / #endif有些类似。
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Its only purpose is to check at runtime whether or not the supplied preprocessor symbol is defined.
它唯一的目的是在运行时检查给定的预定义符号是否被定义。
A symbol that was not defined was used with a preprocessor directive.
未定义的符号与预处理器指令一起使用。
That is, a variable name should not be passed to a preprocessor directive and a symbol can only be evaluated by a preprocessor directive.
即，不应将变量名传递到预处理器指令，并且只能用预处理器指令计算符号。
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