The Datatype class must provide some object representation of the type that the validator can use for equality comparison and hash code calculation.

datatype类必须提供类型的某种对象表示，以便验证器可用来进行相等性比较和计算散列码。

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Only properties you designate as key properties participate in tests of equality between anonymous type instances, or calculation of hash code values.

如果两个执行个体只宣告非索引键属性，但名称、型别、顺序和值都相等，则这两个执行个体还是不相等。

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