激活器的可空问题.CreateInstance和Convert.ChangeType

Question

我有一个第三方库，它使用反射设置给定的对象属性，如下所示。(这是简化版)

public void Set(object obj, string prop, object value) {
    var propInf = obj.GetType().GetProperty(prop);
    value = Convert.ChangeType(value, propInf.PropertyType);
    propInf.SetValue(obj, value, null);
}

我们有一个可空属性的类

class Test
{
    public int? X { get; set; }
}

当我写下面的代码时，它说它不能将int转换为int?

var t = new Test();
Set(t, "X", 1);

由于Nullable不实现IConvertible，所以它是有意义的。然后我决定编写一个方法，返回给定值类型对象的可空版本。

public object MakeNullable(object obj) {
    if(obj == null || !obj.GetType().IsValueType)
        throw new Exception("obj must be value type!");
    return Activator.CreateInstance(
        typeof(Nullable<>).MakeGenericType(obj.GetType()), 
        new[] { obj });
}

我希望这样使用这个方法。

var t = new Test();
Set(t, "X", MakeNullable(1));

但它仍然说不能将int转化为int?。当我调试typeof(Nullable<>).MakeGenericType(obj.GetType())等于int?，但Activator.CreateInstace返回int值不是int?

所以这是我的情况…任何帮助吗?

Answer 1

应该可以:

public static object ChangeType(object value, Type conversionType)
{
   if (conversionType.IsGenericType && conversionType.GetGenericTypeDefinition() == typeof(Nullable<>))
   {
       if (value == null)
           return null;
       var nullableConverter = new NullableConverter(conversionType);
       conversionType = nullableConverter.UnderlyingType;
    }
    return Convert.ChangeType(value, conversionType);
 }

Answer 2

试试下面的代码…

public void Set(object obj, string prop, object value)
      {
          //var propInf = obj.GetType().GetProperty(prop);
          //value = Convert.ChangeType(value, propInf.PropertyType);
          //propInf.SetValue(obj, value, null);
          var property = obj.GetType().GetProperty(prop);
          if (property != null)
          {
              Type t = Nullable.GetUnderlyingType(property.PropertyType)
                           ?? property.PropertyType;
              object safeValue = (value == null) ? null
                                                 : Convert.ChangeType(value, t);
              property.SetValue(obj, safeValue, null);
          }
      }

Convert.ChangeType()在可空类型上失败通过LukeH

更新:隐藏基方法。

namespace ConsoleApplication2
{
using System;
using System.Collections.Generic;
using System.Linq;
internal class Program
{
    private static void Main(string[] args)
    {
        EnterPoint t = new EnterPoint();
        t.BeginProcess();
    }
}
public class EnterPoint
{
    public void BeginProcess()
    {
        var t = new Test(); //Object
        try
        {
            baseDllMethod m = new baseDllMethod(); //Your DLL
            m.Set(t, "X", 1); //Problem Method, this give you error
        }
        catch (Exception ex)
        {
            Console.WriteLine("ERROR EN DLL METHOD");    
        }

        xyz_D der = new xyz_D(); //Derivated method
        der.Set(t, "X", 1) ; //HIDE BASE METHOD
    }
}
public class baseDllMethod //YOUR DLL HERE
{
    public void Set(object obj, string prop, object value)
    {
        var propInf = obj.GetType().GetProperty(prop);
        value = Convert.ChangeType(value, propInf.PropertyType);
        propInf.SetValue(obj, value, null);
    }
}
public class xyz_D : baseDllMethod //Inherits
{
    public new void Set(object obj, string prop, object value) //Hide base method
    {
        var property = obj.GetType().GetProperty(prop);
        if (property != null)
        {
            Type t = Nullable.GetUnderlyingType(property.PropertyType)
                         ?? property.PropertyType;
            object safeValue = (value == null) ? null
                                               : Convert.ChangeType(value, t);
            property.SetValue(obj, safeValue, null);
        }
    }
}
public class Test //Your Object
{
    public int? X { get; set; }
}

}