If the evaluation of the test expression is true and does not result in a dynamic or type error, the element is considered locally valid.
如果测试表达式的计算结果为true并且没有导致动态的或类型错误，那么则认为元素在逻辑上是有效的。
If the expression can be parsed but not evaluated, an object is returned but does not contain a valid value.
如果表达式可以进行分析但无法计算，则将返回不包含有效值的对象。
The expression to the left of the equals sign is not a valid target for an assignment.
等号左边的表达式，对于赋值，并不是一个有效目标。
The embedded expression syntax allowed in document and element literals is not valid XML.
文件和项目常值中允许的内嵌运算式语法不是有效的XML。
If the expression can be parsed but not evaluated, an object is returned but will not contain a valid value.
如果表达式可以进行分析但无法计算，则将返回不包含有效值的对象。
The constraint will not become valid until an expression is specified for the constraint.
为该约束指定表达式后，该约束才生效。
Error: the expression to the left of equals sign is not a valid target for an assignment.
错误：表达式等号的左边不是有效的赋值目标。
Error: the expression to the left of equals sign is not a valid target for an assignment.
错误：表达式等号的左边不是有效的赋值目标。
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