Reflection pass by ref abnormal behavior

[Guest]

hi,
I am trying to call a method from a .net class dynamically and I am passing
a string reference to the method. Below is a sample of what I am doing.
Somehow the value of the parameter passed is not being updated. The wierd
thing is that if I change the variable type to Integer, it works.
====================================================
using System;
using System.Reflection;
using System.Collections;

namespace ReflectionTest
{
public class M
{
public void Method(ref string x)
{
x ="Changed value";
}
[STAThread]
public static void Main()
{
Hashtable h = new Hashtable();
h.Add("ABC", "Initial value"); //Set initial value
Type t = typeof(M);
M m = (M)Activator.CreateInstance(t);//Create instance of M
Type [] p = new Type[1];
p[0] = Type.GetType("System.String&");//typeof(ref string)
MethodInfo mi = t.GetMethod("Method", p);//Get the method
Object [] ps = {h["ABC"]}; //Create param array
mi.Invoke(m,ps);
Console.WriteLine(h["ABC"]); //**output is "Initial value"
//**But I expect it to be "Changed value"
}

}
}
====================================================

But if I change the variable type to Int, the value in the hashtable gets
updated. See sample below.
====================================================
using System;
using System.Reflection;
using System.Collections;

namespace ReflectionTest
{
public class M
{
public void Method(ref int x)
{
x =500;
}
[STAThread]
public static void Main()
{
Hashtable h = new Hashtable();
h.Add("ABC", 5); //Set initial value as 5
Type t = typeof(M);
M m = (M)Activator.CreateInstance(t);// Create instance of M
Type [] p = new Type[1];
p[0] = Type.GetType("System.Int32&");// typeof(ref int)
MethodInfo mi = t.GetMethod("Method", p);// Get the method
Object [] ps = {h["ABC"]}; // Create param array
mi.Invoke(m,ps);
Console.WriteLine(h["ABC"]); //*****output is 500
}

}
}
====================================================

Is this a kind of bug?

Thank you for your time.

Regards,
Sri Vobilisetti

 

[Mattias Sjögren]

Sri,

mi.Invoke(m,ps);

Insert

h["ABC"] = ps[0];

here

Console.WriteLine(h["ABC"]); //**output is "Initial value"
//**But I expect it to be "Changed value"

Mattias

 

[Jon Skeet [C# MVP]]

Thank you very much Mattias. This is not exactly I am thinking about, but
will use as a work around. By the way is there any way to get the value to
the hashtable directly. If you look at the other example in my question which
involves passing a int type. It gets the value into the hashtable without
having to retrieve it from the object[]. Do you know what the difference is
between the string and int implementation?

In the int implementation, I believe you're actually getting the boxed
value from the Hashtable, and changing the value within the box. It's
very interesting though - I hadn't seen that behaviour before. If you
change the code to use:

Object [] ps = {(int)h["ABC"]}; // Create param array

then there's no change in the hashtable because the value is unboxed
then reboxed.

I'm not even sure whether the behaviour you're showing is correct or
not...