Conversions from derived type to base type are allowed.

允许从派生类型型到基类类型的转换.

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Contravariance is the ability to use a less derived type.

“逆变”则是指能够使用派生程度更小的类型.

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The derived type and the base type must have the same content type.

派生类型和基类型必须具有相同的内容类型.

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