泛型构造要求类型“”IConditionalOperator“”具有公共默认构造函数

Question

我想创建一个类型层次结构，它应该实现泛型单元测试，使用抽象类型作为测试装置。同样的模式在C#中工作得很好，但是我遇到了一个编译器错误：“泛型构造要求类型'IConditionalOperator‘有一个公共默认构造函数。”：

   type Color =
      | White = 0xFFFFFF
      | Silver = 0xC0C0C0
      | Gray = 0x808080

   type IConditionalOperator =
      abstract member GetName : Color -> string

   type PatternMatching() =
      interface IConditionalOperator with
         member this.GetName color =
               match color with
               | Color.White -> "White"
               | Color.Silver -> "Silver"
               | Color.Gray -> "Gray"
               | _ -> "None"

   [<AbstractClass>]
   type ConditionalOperatorTest<'T when 'T :> IConditionalOperator and 'T : (new : unit -> 'T)>() =
      member this.ConditionalOperator_ReturnsName(name: string) = None

   type PatternMatchingTest() =
      inherit ConditionalOperatorTest<IConditionalOperator>()

相应的C#代码可以正常工作，如下所示：

   public enum Color
   {
      White = 0xFFFFFF,
      Silver = 0xC0C0C0,
      Gray = 0x808080,
   }

   public interface IConditionalOperator
   {
      string GetName(Color color);
   }

   public class IfStatement : IConditionalOperator
   {
      public string GetName(Color color)
      {
         if (color == Color.White)
         {
            return "White";
         }
         else if (color == Color.Silver)
         {
            return "Silver";
         }
         else if (color == Color.Gray)
         {
               return "Gray";
         }
         else
         {
            return "None";
         }
      }
   }

   public abstract class ConditionalOperatorTest<T> where T : IConditionalOperator, new()
   {
      public void ConditionalOperator_ReturnsName(Color color, string name) {}
   }

   public class IfStatementTest : ConditionalOperatorTest<IfStatement> {}

Answer 1

您已经定义了ConditionalOperatorTest<'T>，因此它要求'T具有默认构造函数，这意味着'T不能是一个接口。因此，不要将IConditionalOperator指定为'T，而是使用具体的PatternMatching类型：

type PatternMatchingTest() =
    inherit ConditionalOperatorTest<PatternMatching>()

进行此更改后，F#版本现在与原始C#代码相匹配：ConditionalOperatorTest<IfStatement>