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问使用动态泛型类型参数创建泛型类实例
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Stack Overflow用户

提问于 2017-05-10 09:48:18

回答 2查看 9.6K关注 0票数 5

我需要创建这样一个泛型类的实例：

 Type T = Type.GetType(className).GetMethod(functionName).ReturnType;
 var comparer = new MyComparer<T>(); // ERROR: "The type or namespace name 'T' could not be found"

我找到了这个只有反射才有可能的answer。但是使用反射，我得到了对象，我需要将其转换为泛型类型。我试过像这样

 Type myGeneric = typeof(MyComparer<>);
 Type constructedClass = myGeneric.MakeGenericType();
 object created = Activator.CreateInstance(constructedClass);
 var comparer = (T)Convert.ChangeType(created, T);// ERROR: "The type or namespace name 'T' could not be found"

但也有同样的错误。如何解决这个问题？

下面是一个完整的例子：

    public static bool Test(string className, string functionName, object[] parameters, object correctResult)
    {
        var method = Type.GetType(className).GetMethod(functionName);
        Type T = method.ReturnType;
        var myResult = method.Invoke(null, parameters);
        dynamic myResultAsT = Convert.ChangeType(myResult, T);
        dynamic correctResultAsT = Convert.ChangeType(correctResult, T);
        var comparer = new MyComparer<T>();    // Problem is here!!!       
        return comparer.Equals(myResultAsT, correctResultAsT);            
    }

其思想是进行单元测试，它将调用带有参数的函数，并将其结果与正确的结果进行比较。但是我需要自定义比较器，所以我实现了MyComparer，因为编译器错误而不能使用它。

public class MyComparer<T> : IEqualityComparer<T>
{
    public bool Equals(T x, T y){/* some implementation*/}
}

generics

reflection

casting

回答 2

Stack Overflow用户

回答已采纳

发布于 2017-05-11 09:18:12

我找到了非常简单的解决问题的方法。不需要将object转换为特定类型的T，只需使用dynamic关键字而不是强制转换。

   Type myGeneric = typeof(MyComparer<>);
   Type constructedClass = myGeneric.MakeGenericType(T);
   object created = Activator.CreateInstance(constructedClass);
   dynamic comparer = created; // No need to cast created object to T

然后，我可以正常地使用比较器调用它的方法，例如：

   return comparer.Equals(myResultAsT, correctResultAsT);

根据LueTm注释，很可能再次使用反射并调用比较器方法，但是这个解决方案看起来要容易得多。

票数 6

Stack Overflow用户

发布于 2017-05-10 10:10:56

看起来你就快到了

// t is a variable, so make it lowercase. This is where some of the confusion comes from
Type t = Type.GetType(className).GetMethod(functionName).ReturnType;
Type myGeneric = typeof(IEqualityComparer<>);

// You need to provide the generic type to make it generic with
// You want IEqualityComparer<T>, so:
Type constructedClass = myGeneric.MakeGenericType(t); 

// Now create the object
object created = Activator.CreateInstance(constructedClass);

// This is tricky without more context...
// You could try this, but to tell you more I would need to know where you use
// the comparer instance. Are you using it in a LINQ query, or in a Sort()?
// If so just cast it to a IEqualityComparer<YourType>, and
// make YourType whaterver you need it to be in the list or the query...
var comparer = (IEqualityComparer<object>)created;