无法在当前状态下写入位置参数。编写器无法以特性的形式写入位置参数，因为编写器是为写入元素而启动的；编写器也无法展开位置参数，原因是包含位置参数的标记扩展中缺少默认构造函数。尝试在节点流中向前移动位置参数成员，将其放置在 XamlXmlWriter 仍可以写入特性的位置。

Cannot write positional parameters in the current state. The writer cannot write the positional parameters in attribute form because the writer has started to write elements, nor can the writer expand the positional parameters due to the lack of a default constructor on the markup extension that contains the positional parameters. Try moving the positional parameter member earlier in the node stream, to a place where XamlXmlWriter can still write attributes.

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