可以向逆变委托分配同一类型的其他委托，但需使用派生程度较小的泛型类型参数。
A contravariant delegate can be assigned another delegate of the same type, but with a less derived generic type parameter.
可以向逆变委托分配同一类型的其他委托，但需使用派生程度较小的泛型类型参数。
A contravariant delegate can be assigned another delegate of the same type, but with a less derived generic type parameter.
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