# If statement frustration with UdpClient.Receive

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I have an if statement to catch empty messages that are sent from the server via UDP. Nothing is done with these messages. This part of my program is not complete but that's not what concerns me. What does concern my is the fact that some how, empty messages are being send to a function that will throw an error if an empty message is received. Please tell me what I am doing wrong. I've been baffled by this for hours.

Obviously, this code it listening for messages
 while (this.IsListening) { message = this.UDPClient.Receive(ref sourceIPE); if (sourceIPE.Address.Equals(this.IPE.Address) && message.Length > Message.HeaderSize) { ThreadStart Starter = delegate { this.HandleMessage(message); }; Thread handlerThread = new Thread(Starter); handlerThread.Start(); } } 

this code handles the received messages. "switch ((MessageType)message[0])" throws and error every now and then because message.Length is 0. I followed that stack back to ThreadStart Starter = delegate { this.HandleMessage(message); }; This is the only place in my code that calls OnMessageRecieved(byte[] message);
 public void OnMessageRecieved(byte[] message) { switch ((MessageType)message[0]) { case MessageType.Error: break; case MessageType.Ping: this.PingResponse(message); break; } } 

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Is HandleMessage() and OnMessageRecieved() the same? If not, what does HandleMessage() look like? And what's Message.HeaderSize?

Edit: Are zero-length arrays even legal?
Edit2: Apparently they are. I suppose it makes sense for some situations.

Edit3: In other comments, you should probably stop spawning new threads for each recieved message, that's going to be pretty inefficient and probably resource intensive. Can't you use ThreadPool or something instead?

Edit4: The thread might be the problem. I'm always confused about anonymous functions accessing variables from outer scope. Your problem is possibly that it takes a while to start the thread, so before the delegate is called, the value of the message variable in the outer scope has changed. Edited by DvDmanDT

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yes, I am going to use a ThreadPool. I was just doing a test run when I came across this error. I've been doing some research into passing arrays as parameters. Because they are reference type variables passed by value, any changes made to the original value or parameterized value will change the in memory value. My next thoughts were to send the array to another function and then start the thread there but because we're still dealing with a reference type, we will have the same issue. What options do I have at this point that might work better than message.CopyTo()? I feel like creating a new array every time a message is received is going to be pretty resource intensive as well. It would work fine for testing purposes but I would prefer a system that is optimized for speed.

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UDPClient.Recieve() probably already allocates a new array per se..
The problem is that your code will be translated to something like this (I've not verified that this is how it actually works, but I'm pretty sure it behaves as if it was working like this anyway!)
[source]
class UdpListener
{
class __ANON123
{
public byte[] message;
public UdpListener __owner;
public void __ANON_METHOD_1() // this is your anonymous delegate handler
{
__owner.HandleMessage(message);
}
}
void Listen()
{
__ANON123 __locals = new __ANON123();
__locals.__owner = this;
while (this.IsListening)
{
__locals.message = this.UDPClient.Receive(ref sourceIPE);
{
ThreadStart Starter = locals.__ANON_METHOD_1;
}
}
}
// body for HandleMessage() here..
}
[/source]
Now, as you can see, that __locals.message might be overwritten with a reference to a new array before your delegate is called. If I'm correct, simply wrapping the three lines that start the thread in another method will probably solve the issue as a new __ANON123 will be created each time that function is called, instead of the same one being used over and over again and therefore it's values overwritten. Edited by DvDmanDT

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The problem is likely what DvDmanDT is hinting at: Lexical scoping in closures for C#.
Specifically, the "message" in your lambda is bound *to the variable message*, not to the value that it happens to have when you instantiate the delegate/lambda function.
If you wrap the instantiation in a local copy that has shorter scope, it will likely work. This doesn't copy the data of the message, but it binds the delegate to the variable "copy" that goes out of scope, and thus can't be re-assigned later.

Perhaps something like this:
 while (whatever) { message = whatever(); if (message.Length > 0) { byte[] copy = message; (new Thread(() => this.HandleMessage(copy))).Start(); } }  Edited by hplus0603

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