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c# - 具有匹配泛型参数的泛型类不是有效的基类

问题描述

为什么在下面的示例中Base<BaseResult<bool, object>, bool, object>不是有效的基类Concrete<object>？Base<BaseResult<bool, object>, bool, object>是 的基础SpecializedBase<bool, object>，并且SpecializedBase<bool, object>似乎是 的适当基础Concrete<object>？

public abstract class Base<TResult, TOutcome, TOutput>
        where TResult : BaseResult<TOutcome, TOutput>
        where TOutput : class
    {
    }

    public class BaseResult<TOutcome, TOutput>
    {
    }

    public abstract class SpecializedBase<TResult, TOutput> :
        Base<SpecializedResult<TOutput>, bool, TOutput>
        where TOutput : class
    {
    }

    public class SpecializedResult<TOutput> : BaseResult<bool, TOutput>
    {
    }

    public class Concrete<TOutput> : 
        SpecializedBase<SpecializedResult<TOutput>, TOutput>
        where TOutput : class
    {
    }

    static class Program {
        static void main()
        {
            // #1: works
            SpecializedBase<SpecializedResult<object>, object> obj;

            obj = new Concrete<object>();

            // #2: works
            Base<SpecializedResult<object>, bool, object> obj2;

            obj2 = new Concrete<object>();

            // #3: doesn't work:
            // Cannot implicitly convert type 'TypeTest.Concrete<object>' to 'TypeTest.Base<TypeTest.BaseResult<bool, object>, bool, object>'
            Base<BaseResult<bool, object>, bool, object> obj3;

            obj3 = new Concrete<object>();
        }
    }
}

标签： c#genericsinheritancepolymorphism

解决方案

让我们尝试逐步解析类型。

Concrete<object> 是 SpecializedBase<SpecializedResult<object>, object>。 SpecializedBase<SpecializedResult<object>, object> 是 Base<SpecializedResult<object>, bool, object>。

就是这样。Base直接继承自Object，因此最终派生最少的类型是Base<TResult, TOutcome, TOutput>set TResultto SpecializedResult<object>、TOutcometobool和TOutputto object。这种类型是不变的，因此Base<SpecializedResult<object>, bool, object> 不是 Base<BaseResult<bool, object>, bool, object>。

您可以在 MSDN docs、this SO question和this SO question中找到更多关于通用差异的信息（可能还有其他一千个地方，这是一个众所周知的复杂概念）。

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