Question

For some reason i have a hard time calling a delegate method from within a thread in Windows service. Don’t think environment makes that much of a difference, but want you to understand why i can’t just step through this code in debugger (trace-logging is quite possible though). Below is a simplified outline; all i want is to execute a callback from the main class to deal with received data (my desire is to separate data-processing implementation [involving too many other dependencies] from BaseSvc‘s library code):

public class    BaseSvc         // library class (separate DLL)
{
    public delegate     bool    dlProcCmd( byte[] bData );
    public static   dlProcCmd   fnProcCmd;

    public static void      Start( )                                // 1
//  public static void      Start( BaseSvc.dlProcCmd ProcCmd )      // 2
    {
//      BaseSvc.fnProcCmd=  ProcCmd;                                // 2
        ..
        bListen=    true;
        thTcpComm=  new Thread( new ThreadStart( TcpComm ) );
        thTcpComm.Start( );
    }
    ..
    public static void      TcpComm( )
    {
        ..
        while(  bListen  )
        {
            ..
            if(  fnProcCmd != null  )
            {
                Utils.Log( fnProcCmd.Method.ToString( ) );
                    // prints:Boolean ProcCmd(Byte[])
                fnProcCmd( bData );     // throws:
                    // Void TcpComm():  System.NullReferenceException
                    // Object reference not set to an instance of an object.
            }
        }
    }
    ..
}

..

public partial class    WinSvc : ServiceBase        // SCM-compatible .exe
{
    /// <summary>Handles Start command</summary>
    protected override void     OnStart( string[] args )
    {
        BaseSvc.fnProcCmd=  ProcCmd;                                // 1a
//      BaseSvc.fnProcCmd=  new BaseSvc.dlProcCmd( ProcCmd );       // 1b
//      BaseSvc.fnProcCmd+= new BaseSvc.dlProcCmd( ProcCmd );       // 1c
        BaseSvc.Start( );                                           // 1
//      BaseSvc.Start( new BaseSvc.dlProcCmd( ProcCmd ) );          // 2
        ..
    }

    public bool     ProcCmd( byte[] bData )
    {
        ..
        return  true;
    }
}

At first i thought smth is wrong with declaration or initialization of the delegate method, so various tries are marked with 1a, 1b, 1c and 2. However, in either scenario i end up with a trace printout confirming that if( fnProcCmd != null ) check passed, followed by a NullRefException. bData argument is initialized at that moment, 100% guaranteed and confirmed by tracing, so that leaves only fnProcCmd as the culprit. But it is initialized, is it not!? What am i missing here??

Could it be that the execution context for WinSvc class is different from that of BaseSvc, and/or that TcpComm( ) executes in its own thread? I’d expect a more intuitive error in that case..

I must be doing smth wrong. What should i do to enable such a callback?

UPDATE: After Henk’s suggestion i commented out ProcCmd‘s body and the problem disappeared, => issue is within that code and has nothing to do with the delegate mechanism. Sorry for confusion.
(BTW, using delegate via scenario 1a works fine.)

Answer 1

Looking at this part:

        if(  fnProcCmd != null  )
        {
            Utils.Log( fnProcCmd.Method.ToString( ) );
                // prints:Boolean ProcCmd(Byte[])
            fnProcCmd( bData );     // throws:
                // Void TcpComm():  System.NullReferenceException
                // Object reference not set to an instance of an object.
        }

The null-checking on fnProcCmd is extensive and convincing so it must be something happening inside ProcCmd() .

To home in on the error, try it with an empty ProcCmd() :

 public bool     ProcCmd( byte[] bData )
 {
    //    ...
    //
    //    ...
    return  true;
 }