如何在不知道类型的情况下转换泛型类型 [英] How to cast a generic type without knowing the type

问题描述

我试图在运行时创建一个通用对象.到目前为止，我已经能够创建它，但是我不知道如何投射它.我拥有的是枚举对象，我想生成一个EnumMapper，它将枚举值转换为自定义字符串以映射到旧数据库.

I am trying to create a generic object in runtime. So far I have been able to create it but I can't figure out how to cast it. What I have is the enum object and I want to generate the EnumMapper that converts the enum value to custom string to map to a legacy database.

Type enumType = myEnum.GetType();
Type enumMapperType = typeof(EnumMapper<>)
                      .GetGenericTypeDefinition().MakeGenericType(enumType);
var mapper = Activator.CreateInstance(enumMapperType); // OK
EnumMapper<> mapper = (EnumMapper<>) Activator.CreateInstance(enumMapperType); // Error

当我在调试器中检查对象时，它是按预期的方式创建的，但是如何转换它以便可以使用呢?

When I inspect the object in the debugger it is create as I expect, but how can I cast it so I can use it?

课程:

public class EnumMapper<T> : IEnumMapper<T>

界面:

public interface IEnumMapper<T>
{
    T EnumValue(string value);

    bool HasEnumValue(string stringValue);

    bool HasStringValue(T enumValue);

    string StringValue(T enumValue);
}

---

Error   2   ; expected  \EnumMapperTest.cs  36
Error   4   ; expected  \EnumMapperTest.cs  36
Error   1   Invalid expression term '>' \EnumMapperTest.cs  36
Error   3   Invalid expression term '>' \EnumMapperTest.cs  36
Error   34  Only assignment, call, increment, decrement, and new object expressions can be used as a statement  \EnumMapperTest.cs  36
Error   36  The name 'mapper' does not exist in the current context \EnumMapperTest.cs  36
Error   35  Using the generic type 'EnumMapper<T>' requires 1 type arguments    \EnumMapperTest.cs  36
Error   37  Using the generic type 'EnumMapper<T>' requires 1 type arguments    \EnumMapperTest.cs  36

推荐答案

据我所知，您所需的确切内容在C#中是不可能的.基本上，您需要根据常规非泛型参数的值(即，如果参数为 typeof(Enum1))与结果相比，方法具有不同的类型或返回的变量变量是 EnumMapper< Enum1> ，如果参数是 typeof(Enum2)，则结果变量是 EnumMapper< Enum2> .

As far as I know, the exact thing you need is not possible in C#. Basically, you need for a method to have different types or the returned variable, based on the value of a regular, non-generic, parameter, i.e. if the parameter is typeof(Enum1), than the result variable is EnumMapper<Enum1> and if the parameter is typeof(Enum2), than the result variable is EnumMapper<Enum2>.

您可以使用泛型参数来执行此操作，但是，由于泛型全部与编译时信息有关，并且仅在运行时具有该值，因此在这种情况下不能使用它们.

You can do this with generic parameters, however, since generics is all about compile-time information, and you only have the value at runtime, they cannot be used in this case.

您可以做的(以及我已经做的)是使用动态代码来解决此问题，请注意尽快进入静态类型的土地(动态确实具有传染性，有人说像微笑，有人说像病毒)

What you can do (and what I have done) is to use dynamic code to work around this problem, being careful to go into statically-typed land as soon as possible (dynamic is really contagious, some say like a smile, some say like a virus):

public dynamic GetMapperObject(Type enumType)
{
  Type enumMapperType = typeof(EnumMapper<>)
                           .GetGenericTypeDefinition()
                           .MakeGenericType(enumType);
  var mapper = Activator.CreateInstance(enumMapperType);
  return mapper;
}

调用代码如下:

var mapper = GetMapperObject(enumType);
//dynamic call, bind the resut to a statically typed variable
bool result = mapper.HasEnumValue("SomeStringValue") 

---

(旧答案，只是向问题添加了另一种间接访问级别:))

---

(old answer, just adds another level of indirection to the problem :) )

您可以将所有内容包装在通用方法中，如下所示:

You could wrap all that in a generic method, something like this:

public EnumMapper<T> GetMapperObject<T>()
{
  Type enumType = typeof(T);
  Type enumMapperType = typeof(EnumMapper<>)
                           .GetGenericTypeDefinition()
                           .MakeGenericType(enumType);
  var mapper = Activator.CreateInstance(enumMapperType);
  return mapper as EnumMapper<T>;
}

并使用

var mapper = GetMapperObject<EnumMapperTestEnum>();

但是，如果枚举只有一个值，则可以使用类型推断，例如:

However, if you only have a single value for the enum, you can use type inference, something like:

//everything is the same, just different signature
public EnumMapper<T> GetMapperByExample<T>(T item)
{
  Type enumType = typeof(T);
  Type enumMapperType = typeof(EnumMapper<>)
                           .GetGenericTypeDefinition()
                           .MakeGenericType(enumType);
  var mapper = Activator.CreateInstance(enumMapperType); // OK
  return mapper as EnumMapper<T>;
}

并使用类型推断对其进行调用

and call it with type inference

var mapper = GetMapperByExample(EnumMapperTestEnum.SomeValue); 

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