c# - Logic to split message on different receive operations -

hello have asked 2 questions before , have not got answer unfortunately ,but fortunately got tumble weed badge stack on flow :d here question have 2 apps 1 client 1 server simple chat messages works stuck @ position have make both applications scalable un-limited number of users ,but when 2 or 3 users sends message client simultaneously got error in second block of code here code appreciated error comes @ else block in buffer.blockcopy,

public bool handlemessage(socketasynceventargs receivesendeventargs, dataholdingusertoken receivesendtoken, int32 remainingbytestoprocess)     {         bool incomingtcpmessageisready = false;          //create array we'll store complete message,          //if has not been created on previous receive op.         if (receivesendtoken.receivedmessagebytesdonecount == 0)         {                             receivesendtoken.thedataholder.datamessagereceived = new byte[receivesendtoken.lengthofcurrentincomingmessage];         }          // remember there receivesendtoken.receivedprefixbytesdonecount         // variable, allowed handle prefix when         // requires multiple receive ops. in same way, have          // receivesendtoken.receivedmessagebytesdonecount variable,         // helps handle message data, whether requires 1 receive         // operation or many.         if (remainingbytestoprocess + receivesendtoken.receivedmessagebytesdonecount == receivesendtoken.lengthofcurrentincomingmessage)         {             // if inside if-statement, got              // end of message. in other words,             // total number of bytes received message matched              // message length value got prefix             // write/append bytes received byte array in              // dataholder object using store our data.             buffer.blockcopy(receivesendeventargs.buffer, receivesendtoken.receivemessageoffset, receivesendtoken.thedataholder.datamessagereceived, receivesendtoken.receivedmessagebytesdonecount, remainingbytestoprocess);              incomingtcpmessageisready = true;         }          else         {             // if inside else-statement, means             // need receive op. still haven't got whole message,             // though have examined data received.             // not problem. in socketlistener.processreceive call             // startreceive receive op receive more data.              buffer.blockcopy(receivesendeventargs.buffer, receivesendtoken.receivemessageoffset, receivesendtoken.thedataholder.datamessagereceived, receivesendtoken.receivedmessagebytesdonecount, remainingbytestoprocess);              receivesendtoken.receivemessageoffset = receivesendtoken.receivemessageoffset - receivesendtoken.recprefixbytesdonethisop;              receivesendtoken.receivedmessagebytesdonecount += remainingbytestoprocess;         }          return incomingtcpmessageisready;     }