泛型方法和類型轉換

Question

我讀了以下問題（我會解決這個問題的相同的方式給出答案）：Passing derived type as argument to abstract class

但爲什麼不能找到派生類中的value屬性？即使我添加一個類型轉換是不可能的：

public abstract class baseClass 
{ 
    public abstract float function<T>(T a, T b) where T:baseClass; 
} 

public class derived: baseClass 
{ 
    public override float function<derived>(derived a, derived b) 
    { 
     // Here value is not found 
     return a.value + b.value; 
    } 

    public float value; 
} 

實例與類型轉換也沒有工作（和建議冗餘類型轉換所示）：

public abstract class baseClass 
{ 
    public abstract float function<T>(T a, T b) where T:baseClass; 
} 

public class derived: baseClass 
{ 
    public override float function<derived>(derived a, derived b) 
    { 
     // Here value is not found even with type cast 
     return ((derived)a).value + ((derived)b).value; 
    } 

    public float value; 
} 

Answer 1

因爲你聲明該方法的泛型類型參數。編譯器不明白這應該是derived類型。它只知道你已經引入了一個新的泛型類型參數。

你想要什麼叫做F-bound polymorphism，其中類型參數是實現類，遞歸定義的：

public abstract class BaseClass<T> where T : BaseClass<T> 
{ 
    public abstract float Function(T a, T b); 
} 

public class Derived : BaseClass<Derived> 
{ 
    public override float Function(Derived a, Derived b) 
    { 
     return a.Value + b.Value; 
    } 

    public float Value { get; set; } 
}