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等价Java泛型循环引用C#

generics c#

Stefano Balzarotti · 技术社区 · 7 年前

我正在用C#移植一个Java库,发现这个奇怪的类使用泛型循环引用:

public static class UserAgentAnalyzerDirectBuilder<UAA extends UserAgentAnalyzerDirect, B extends UserAgentAnalyzerDirectBuilder<UAA, B>> 
{
   // Implementation
}

这些代码可以很容易地翻译成C#:

public class UserAgentAnalyzerDirectBuilder<UAA, B> where UAA: UserAgentAnalyzerDirect where B: UserAgentAnalyzerDirectBuilder<UAA, B>
{
  // Implementation
}

假设我的等价类是正确的,我需要实现实例化那个奇怪类的方法。

在Java中,我有以下代码:

public static UserAgentAnalyzerDirectBuilder<? extends UserAgentAnalyzer, ? extends UserAgentAnalyzerDirectBuilder<?, ?>> newBuilder() {
    return new UserAgentAnalyzerDirectBuilder<>(new UserAgentAnalyzer());
}

在C#我试图用:

public static UserAgentAnalyzerDirectBuilder<UAA, UserAgentAnalyzerDirectBuilder<UAA, B>> newBuilder<UAA, B>() 
            where UAA: UserAgentAnalyzerDirect
            where B: UserAgentAnalyzerDirectBuilder<UAA, B>
        {
            return new UserAgentAnalyzerDirectBuilder<UAA, UserAgentAnalyzerDirectBuilder<UAA, B>> (new UserAgentAnalyzer());
        }

1 回复 | 直到 7 年前

Ondrej Tucny 7 年前

你看到的不是循环引用。类型参数约束只允许传入一个类型,该类型是该约束指定的泛型类型的后代。

public class UserAgentAnalyzerDirect { }

public class UserAgentAnalyzerDirectBuilder<UAA, B> 
    where UAA : UserAgentAnalyzerDirect 
    where B : UserAgentAnalyzerDirectBuilder<UAA, B>
{
    // this method is supposed to implement the effect of the 
    // constructor in the original Java code
    public void SetUAA(UAA a) { }

    // further implementation
}

public static UserAgentAnalyzerDirectBuilder<UAA, B> NewBuilder<UAA, B>()
    where UAA : UserAgentAnalyzerDirect, new()
    where B : UserAgentAnalyzerDirectBuilder<UAA, B>, new()
{
    // Unlike in Java, C# allows instantiating generic type parameters only using 
    // a parameter-less constructor. Hence we use the SetUAA method here instead.
    var a = new UAA();
    var b = new B();
    b.SetUAA(a);
    return b;
}

public class CustomUserAgentAnalyzerDirect : UserAgentAnalyzerDirect { }

public class CustomUserAgentAnalyzerDirectBuilder : UserAgentAnalyzerDirectBuilder<CustomUserAgentAnalyzerDirect, CustomUserAgentAnalyzerDirectBuilder> { }

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