C＃拳击枚举错误与泛型 [英] C# boxing enum error with generics

问题描述

我不明白这里发生了什么...

我有以下错误：
类型'TestApp.TestVal'不能用于通用类型或方法中的类型参数'T' 'TestApp.SomeClass< T>'。没有从'TestApp.TestVal'到'System.IComparable< TestApp.TestVal>'的拳击转换。 / em>

以下代码发生此错误：

  public枚举TestVal 
 {
首先，
第二，
第三个
} 
 
 public class SomeClass< T> 
其中T：IComparable< T> 
 {
 public T存储
 {
 get 
 {
 return storedval; 
} 
 set 
 {
 storedval = value; 
} 
} 
 private T storedval; 
} 
 
类程序
 {
 static void Main（string [] args）
 {
 //错误在下一行
 SomeClass< TestVal> t = new SomeClass< TestVal>（）; 
} 
} 
  

由于枚举是一个 int 默认情况下，int实现 IComparable< int> 接口似乎不应该有错误....

解决方案

首先，我不知道使用 IComparable< T> 有一个枚举... IEquatable< T> ，但是比较？

作为一个更安全的选择;而不是使用通用约束来命名 IComparable< T> ，可以在类中使用 Comparer&T; .Default 。这有利于支持 IComparable< T> 和 IComparable - 这意味着您传播的限制较少。 / p>

例如：

  public class SomeClass< T> {// note no constraint 
 public int ExampleCompareTo（T other）{
 return Comparer< T> .Default.Compare（Stored，other）; 
} 
 ... [snip] 
} 
  

可以使用枚举：

  SomeClass< TestVal> t = new SomeClass< TestVal>（）; 
 t.Stored = TestVal.First; 
 int i = t.ExampleCompareTo（TestVal.Second）; // -1 
  

I don't understand what is going on here...

I've got the following error: The type 'TestApp.TestVal' cannot be used as type parameter 'T' in the generic type or method 'TestApp.SomeClass<T>'. There is no boxing conversion from 'TestApp.TestVal' to 'System.IComparable<TestApp.TestVal>'.

This error happens for the following code:

public enum TestVal
{
    First,
    Second,
    Third
}

public class SomeClass<T>
    where T : IComparable<T>
{
    public T Stored
    {
        get
        {
            return storedval;
        }
        set
        {
            storedval = value;
        }
    }
    private T storedval;
}

class Program
{
    static void Main(string[] args)
    {
        //Error is on the next line
        SomeClass<TestVal> t = new SomeClass<TestVal>(); 
    }
}

Since the enum is an int by default and int's implement the IComparable<int> interface it seems like there shouldn't be an error....

解决方案

Firstly, I'm not sure whether it is sensible to use IComparable<T> with an enum... IEquatable<T>, sure - but comparison?

As a safer alternative; rather than mandate the IComparable<T> with the generic constraint, perhaps use Comparer<T>.Default inside the class. This has the advantage of supporting IComparable<T> and IComparable - and it means you have less constraints to propagate.

For example:

public class SomeClass<T> { // note no constraint
    public int ExampleCompareTo(T other) {
        return Comparer<T>.Default.Compare(Stored, other);
    }
    ... [snip]
}

This works fine with the enum:

SomeClass<TestVal> t = new SomeClass<TestVal>();
t.Stored = TestVal.First;
int i = t.ExampleCompareTo(TestVal.Second); // -1

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