Covariance vs. Contravariance: How Do They Differ in Programming?
Jan 25, 2025 am 02:56 AM
Coordination and inverter are two vital concepts in programming. They define how the functions are applied to different types of objects to ensure type security and flexibility.
Cooperation: Keep assignment compatibility
Coordination is suitable for mapping functions that maintain the compatibility of objects. This means that if the object of the type X can be assigned to the object of the type Y, the result of applying the mapping function to X can always be assigned to the result of applying the function to Y.
For example, consider a set of animals, including Animal, Tiger, FRUIT and Banana. By defining a mapping function from Animal to IenuMerable, we get the following types:
<code>{ Animal, Tiger, Fruit, Banana } → { IEnumerable<Animal>, IEnumerable<Tiger>, IEnumerable<Fruit>, IEnumerable<Banana> }</code>and INUMERABLE
? INUMERABLE
Different from the coordination, the inverter reverses the assignment compatibility relationship. This means that if the object of the type X can be assigned to the object of the type Y, the result of applying the mapping function to Y can be assigned to the result of the application of the function to X. Consider the collection of IComparable type:
In C#, we observe the following compatible relationship:
<code>{ IComparable<Tiger>, IComparable<Animal>, IComparable<Fruit>, IComparable<Banana> } → { IC<Tiger>, IC<Animal>, IC<Fruit>, IC<Banana> }</code>The application mapping function reverses the direction of compatibility. As a result, IC
? ic
, IC<code>Animal ? Tiger Tiger ? Tiger Banana ? Banana Fruit ? Banana Fruit ? Fruit</code>? ic
, and so on. This behavior ensures that the device that can compare Animal can compare the Tiger, otherwise it is not the other.
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