That means a reference type can have a null pointer, but a value type cannot have a null value.
这意味着引用类型可能有空指针，但是数值类型不能有空值。
Reference types - same as pointer types, except that a reference type cannot be NULL.
引用类型—与指针类型相同，只是引用类型不能为NULL。
A null reference pointer was passed to the stub.
向占位程序传送了空的索引指针。
If a pointer pointing to an object not in the graph, then the object is not a reachable object, and then garbage collector sets the slot to null in the short weak reference table.
如果一个指针指向一个不在图中的对象，那么此对象就是一个不可到达的对象，垃圾收集器就将短弱引用表相应的槽置空。
If a pointer pointing to an object not in the graph, then the object is not a reachable object, and then garbage collector sets the slot to null in the long weak reference table.
如果一个指针指向一个不在图中的对象(注意此时图中已包含Finalize可到达的对象)，那么此对象就是一个不可到达的对象，垃圾收集器就将长弱引用表相应的槽置空。
If a pointer pointing to an object not in the graph, then the object is not a reachable object, and then garbage collector sets the slot to null in the long weak reference table.
如果一个指针指向一个不在图中的对象(注意此时图中已包含Finalize可到达的对象)，那么此对象就是一个不可到达的对象，垃圾收集器就将长弱引用表相应的槽置空。
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