即使IList接口不是协变的，一个只读列表接口也理应如此。

And while IList isn't covariant, a read-only list interface would be.

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该服务提供一个有用的到流行邮件列表归档的查询接口。

This service provides a useful query interface to an archive of popular email lists.

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在清单6中查看图书列表服务的接口声明。

Check out the book list service's interface declaration in Listing 6.

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