从 'System.Int32' 到 'System.Nullable`1[[System.Int32, mscorlib]] 的无效转换

Invalid cast from #39;System.Int32#39; to #39;System.Nullable`1[[System.Int32, mscorlib]](从 System.Int32 到 System.Nullable`1[[System.Int32, mscorlib]] 的无效转换)

问题描述

Type t = typeof(int?); //will get this dynamically
object val = 5; //will get this dynamically
object nVal = Convert.ChangeType(val, t);//getting exception here

我在上面的代码中得到了 InvalidCastException.对于上面我可以简单地写 int?nVal = val，但上面的代码是动态执行的.

I am getting InvalidCastException in above code. For above I could simply write int? nVal = val, but above code is executing dynamically.

我得到一个值(不可为空的类型，如 int、float 等)包裹在一个对象(此处为 val)中，我必须通过将其转换为另一种类型(可以或不能)将其保存到另一个对象是它的可空版本).当

I am getting a value(of non nullable type like int, float, etc) wrapped up in an object (here val), and I have to save it to another object by casting it to another type(which can or cannot be nullable version of it). When

从 'System.Int32' 到 'System.Nullable`1[[System.Int32,mscorlib，版本=4.0.0.0，文化=中性，PublicKeyToken=b77a5c561934e089]]'.

Invalid cast from 'System.Int32' to 'System.Nullable`1[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]'.

一个 int，应该可以转换/类型转换为 nullable int，这里有什么问题?

An int, should be convertible/type-castable to nullable int, what is the issue here ?

推荐答案

你必须使用 Nullable.GetUnderlyingType 来获取 Nullable 的底层类型.

You have to use Nullable.GetUnderlyingType to get underlying type of Nullable.

这是我用来克服 Nullable 的 ChangeType 限制的方法

This is the method I use to overcome limitation of ChangeType for Nullable

public static T ChangeType<T>(object value) 
{
   var t = typeof(T);

   if (t.IsGenericType && t.GetGenericTypeDefinition().Equals(typeof(Nullable<>))) 
   {
       if (value == null) 
       { 
           return default(T); 
       }

       t = Nullable.GetUnderlyingType(t);
   }

   return (T)Convert.ChangeType(value, t);
}

非泛型方法:

public static object ChangeType(object value, Type conversion) 
{
   var t = conversion;

   if (t.IsGenericType && t.GetGenericTypeDefinition().Equals(typeof(Nullable<>))) 
   {
       if (value == null) 
       { 
           return null; 
       }

       t = Nullable.GetUnderlyingType(t);
   }

   return Convert.ChangeType(value, t);
}

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