CSharp-变体

Show you the code.^[1][2][3][4]

// Variant type parameters could be declared in interfaces or delegates only!

interface ICovariant<out T>
{
}

class Covariant<T> : ICovariant<T>
{
}

interface IContravariant<in T>
{
}

class Contravariant<T> : IContravariant<T>
{
}

interface IInvariant<T>
{
}

class Invariant<T> : IInvariant<T>
{
}

class Program
{
    private static void Covariant( /* out */)
    {
        ICovariant<object> obj = new Covariant<object>();
        ICovariant<string> str = new Covariant<string>();

        // You can assign "Derived" to "Base"
        obj = str;
        str = obj; // error
    }

    private static void Contravariant( /* in */)
    {
        IContravariant<object> obj = new Contravariant<object>();
        IContravariant<string> str = new Contravariant<string>();

        // You can assign "Base" to "Derived"
        str = obj;
        obj = str; // error
    }

    private static void Invariant( /* none */)
    {
        IInvariant<object> obj = new Invariant<object>();
        IInvariant<string> str = new Invariant<string>();

        // You can't do any assign 
        obj = str; // error
        str = obj; // error
    }
}

---

1. Microsoft Learn, Covariance and Contravariance in Generics 与 C# language specification §19.2.3 Variant type parameter lists 支撑：C# 的变体类型参数只出现在接口和委托类型上，并通过 in / out 标注协变、逆变或不变。 ↩︎

2. Microsoft Learn out generic modifier 说明 out 表示协变类型参数，允许把更派生的类型实参作为更宽泛的泛型接口或委托使用；这对应示例里 ICovariant<string> 可赋值给 ICovariant<object>。 ↩︎

3. Microsoft Learn in generic modifier 说明 in 表示逆变类型参数，允许把更宽泛的类型实参作为更具体的泛型接口或委托使用；这对应示例里 IContravariant<object> 可赋值给 IContravariant<string>。 ↩︎

4. Microsoft Learn Creating Variant Generic Interfaces 与 C# language specification 的 variance conversion 规则共同支撑：未标注 in / out 的类型参数是不变的，变体转换需要满足对应的协变、逆变或恒等转换条件。 ↩︎