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[Communication] .NET/C# generic T covariant out and inverter in interface

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Posted on 8/3/2023 8:35:23 PM | | | |
Requirements: Recently, I was looking at the .NET source code and found that there is an out keyword in the interface generic, such as:public interface IStringLocalizer<out T>, and then studied it.

Generic type parameters in an interface can be declared as covariants or inverters. Covariance allows interface methods to have a return type that is more derived than the return type defined by the generic type parameter. The inverter allows the interface method to have a smaller degree of derivation from the argument type specified by the generic parameter. Generic interfaces with covariant or inverter generic type parameters are called "variants".

Covariance: Use the out keyword to declare the generic type parameter as a covariant.
Inverse: Use the in keyword to declare the generic type parameter as an inverter.

This screenshot from VS2022 is very descriptive - it illustrates what kind of limitations this brings to generics, as shown below:



On commissionFunc<in T, out TResult>This means that the parameter type is T and the return type is TResult.

Reference:
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