marshal-by-reference class

[tks423]

Hi,

 

I want to pass a parameter from one windows form to another.

 

//this function is in Form2, where c is a global public variable
private void button1_Click(object sender, System.EventArgs e)
{
 c = (int) numericUpDown1.Value + 
            (int) numericUpDown2.Value;

  Form1.PrintToTextBox(this);

}

//this is the function in Form1
public static void PrintToTextBox(Form2 obj)
{
 string b;
 b = obj.c.ToString();		
}

 

This is the error I get: Cannot pass 'WindowsApplication1.Form2.c' as ref or out, because 'WindowsApplication1.Form2.c' is a marshal-by-reference class

 

Any help would be much appreciated,

-Tim

Edited July 6, 2003 by divil

[_SBradley_]

Ah, yes. This is a good one. A more compact example of what you're experiencing is shown below:

 

[CS]

class Test : System.MarshalByRefObject

{

int c = 5;

 

static void Main()

{

Test t = new Test();

t.c.ToString();

t.f(ref t.c);

}

 

void f(ref int i)

{

}

}

[/CS]

 

The problem is that, when you call Int32.ToString(), the first parameter (the hidden 'this' parameter) is passed in the same way as a "ref int" parameter would be passed. (Compare this to the call to f(ref int).) The generated MSIL is the same for both Int32.ToString() parameter 0 and Test.f() parameter 1: they use 'ldflda'.

 

What I'm saying is that, so far as the compiler's concerned, when you do the "t.c.ToString()", the first parameter is essentially a "ref int" parameter. Since your class is derived from MarshalByRefObject (Form is ultimately derived from this class), passing this int member by-ref is illegal. If you were to comment out the derivation from MarshalByRefObject in the example above, the code would compile just fine.