SizeOf() method fails with ICustomMarshale r

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Hello,

When I try to implement a custom marshaler I get an exception when the
Marshal.SizeOf( ) method is invoked.
The message is:
"Type MyNS.PT_DATAFRA ME can not be marshaled as an unmanaged structure;
no meaningful size or offset can be computed."

Is custom marshaling possible with a struct? Perhaps a better question
is help, what am I doing wrong here?

I attached a code snippet to show the problem.

Thanks!

*************** ******

using System;
using System.Runtime. InteropServices ;
using System.Runtime. Serialization.F ormatters.Binar y;
using System.IO;
using System.Windows. Forms;

namespace MyNS
{
[Serializable()]
[StructLayout( LayoutKind.Sequ ential)]
struct PT_DATAFRAME
{
[MarshalAs(Unman agedType.Custom Marshaler, MarshalTypeRef =
typeof(MyNS.Dum myMarshaler))]
public string myValue; //Just a value that can be marshaled
};

public class AppMain
{
[STAThread]
static void Main()
{
PT_DATAFRAME Frame = new PT_DATAFRAME();
Frame.myValue = "Joehoo";
Int32 size=Marshal.Si zeOf( Frame );
/* Exception at this point:
Message:
Type MyNS.PT_DATAFRA ME can not be marshaled as an unmanaged
structure;
no meaningful size or offset can be computed.
??????????????? ???
*/
}
}

// This custom marshaler does not implement anything meaningful.
// Not any of the methods will be invoked.
public class DummyMarshaler : ICustomMarshale r
{
DummyMarshaler( ) { MessageBox.Show ("Never Shown"); }

public static ICustomMarshale r GetInstance( string cookie )
{
MessageBox.Show ("Never Shown");
return new DummyMarshaler( );
}

public object MarshalNativeTo Managed( System.IntPtr pNativeData )
{
MessageBox.Show ("Never Shown");
return pNativeData;
}

public IntPtr MarshalManagedT oNative( object ManagedObj )
{
MessageBox.Show ("Never Shown");
return (IntPtr) new System.Int32();
}

public void CleanUpNativeDa ta( IntPtr pNativeData )
{
MessageBox.Show ("Never Shown");
return;
}

public int GetNativeDataSi ze()
{
MessageBox.Show ("Never Shown");
return ( 10 ); // Just a number
}

public void CleanUpManagedD ata( object ManagedObj )
{
MessageBox.Show ("Never Shown");
return;
}
}

}