Problem getting callback events from C code when using P/Invoke

[microcool1024]

I am having a problem getting a C callback to correctly activate in a C# program after using a dllimport.

Here is the declaration of the C code in the C header:

typedef BOOL (CALLBACK *HII_PaymentAcceptedProc)(BYTE bSrcNode, BYTE bNumber, BYTE bType, WORD wValue);
 
HIIFAPI UINT WINAPI HII_PaymentAccepted(BYTE bDestNode, BYTE bNumber, BYTE bType, WORD wValue);
 

Then in the demo C program I have it is called like this:

HII_InstallPaymentAcceptedProc(PaymentAcceptedProc);
 
 
static BOOL CALLBACK PaymentAcceptedProc(BYTE bSrcNode, BYTE bNumber, BYTE bType, WORD wValue)
{
    LPTSTR lpszType;
    UNREFERENCED_PARAMETER(bSrcNode);
 
    // Payment accepted event received
    switch(bType)
    {
        case 0:        lpszType = TEXT("Coin");        break;
        case 1:        lpszType = TEXT("Value Token"); break;
        case 2:        lpszType = TEXT("Vend Token");    break;
        case 3:        lpszType = TEXT("Known Slug");    break;
        case 4:        lpszType = TEXT("Note");        break;
        default:    lpszType = TEXT("Unknown");        break;
    }
 
    FormatEvent(hwndMain, TEXT("Payment Accepted, number %u, type %s, value %u\r\n"),
        bNumber, lpszType, wValue);
 
    return TRUE;
}
 

I then tried to translate this into C# like this:

public delegate bool HII_PaymentAcceptedProc(Byte bSrcNode, Byte bNumber, Byte bType, UInt16 wValue);
[DllImport("HiiF.dll")]
public static extern UInt32 HII_InstallPaymentAccepted(HII_PaymentAcceptedProc pfnProc);
 

Then call it like this:

public bool PaymentAcceptedProc(byte bSrcNode, byte bNumber, byte bType, UInt16 wValue)
{
            switch(bType)
    {
        case 0:        break;
        case 1:        break;
        case 2:        break;
        case 3:        break;
        case 4:        break;
        default:    break;
    }
return true;
        }
 
HII_Wrapper.HII_PaymentAcceptedProc PaymentAcceptedProcDelegate = new HII_Wrapper.HII_PaymentAcceptedProc(this.PaymentAcceptedProc);
 
HII_Wrapper.HII_InstallPaymentAcceptedProc(PaymentAcceptedProcDelegate);
 

However it never his the PaymentAcceptedProc function and I don't understand why. Are there any thoughts? Am I handling the delegates incorrectly?