BeginInvoke copy parameter

Question

Just try to learn Invoke/BeginInvoke, I encountered in that problem.

       // Update UI
       public void UpdateForm(string value) {
        txtLog.AppendText(value + "\r\n");
       }


       // Thread function
       private void readSocket() {
        string row = "";

        while (socket.Connected) {                
            row = socket.readLine();            

            if (IsControlValid(this))
                BeginInvoke((MethodInvoker)delegate { UpdateForm(String.Copy(row)); });                    

        }
    }

Using Invoke method my UI update with the correct text, instead if I use BegineInvoke I see wrong text, ie some text repeatedly a lot of time. I know that call

BeginInvoke((MethodInvoker)delegate { UpdateForm(row); });

maybe "row" can be behavior like a shared variable rather than

BeginInvoke((MethodInvoker)delegate { UpdateForm(String.Copy(row)); });

I think that each BeginInvoke call create a "new" delegate, so using String.Copy must create another instance of string, but I see always wrong value (duplicates, ecc).

Where I'm wrong?

Why you need `string.Copy`? and what is the code of `UpdateForm`? — Alessandro D'Andria, Sep 26 '13 at 18:19
I just added string.Copy to prevent that variable "row" was changed during the UpdateForm(). UpdateForm() is a method that run need to run in UI to update some stuff (in this case only a TextBox) — Kaiser69, Sep 26 '13 at 19:57

score 6 · Accepted Answer · answered Sep 26 '13 at 18:19

6

maybe "row" can be behavior like a shared variable rather than

Yes, the row variable is captured - so whenever it's read, it's the latest value that's read. You should put the variable declaration inside the loop:

while (socket.Connected) {                
    string row = socket.readLine();

    if (IsControlValid(this)) {
        // Removed String.Copy call, as it was pointless
        BeginInvoke((MethodInvoker)delegate { UpdateForm(row); });
    }
}

Now you'll have a different variable for each iteration of the loop, so you won't be overwriting the previous value in one thread while you're reading it in the delegate.

answered Sep 26 '13 at 18:19

Jon Skeet

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It works! But I need to know two other things: 1. Isn't the same thing, create a new variable each loop, or pass as argument a COPY of the variable? 2. How much is bad in performance/waste memory, create a new variable for each string that I've receive, when I only need a reference to it. With Invoke I resolve that problem, but I want to know what's the best deal with BeginInvoke – Kaiser69 Sep 26 '13 at 19:56
@Kaiser69: No, it's not the same thing - because you're imagining that the variable's *value* is captured by the anonymous method. It isn't. The *variable itself* is captured. The problem isn't with the way you're calling `UpdateForm` - it's that `row` is evaluated at the point when it's called. As for the memory: almost certainly insignificant compared with anything else you'll be doing. – Jon Skeet Sep 26 '13 at 19:58
Yep, I'm thinking a loop that reads a socket. Read about 10000 rows with a verage 200chars. If I multiply that value for each useless "new string variable", I guess that was a big waste of memory. Well, we have PC with tons of GB, so use them :) – Kaiser69 Sep 26 '13 at 20:21
Another question. Just for curiosity, there is a way to do the same thing, using string.Copy()? – Kaiser69 Sep 26 '13 at 20:23
@Kaiser69: No - because you'd still be calling `String.Copy` from within the anonymous method, so it would still pick up the new value of `row`. And if you call it *outside* the anonymous method, it's pointless because strings are immutable. – Jon Skeet Sep 26 '13 at 20:42
Just perfect! Thanks. Now I'm know where I'm wrong. Before that disclosure, I thought that anonymous method was evaluated instantly when I call BeginInvoke, instead it's called "when he wants" :) – Kaiser69 Sep 26 '13 at 20:53
@Kaiser69: Exactly - because it's executed on the UI thread asynchronously. That's the difference between `BeginInvoke` and `Invoke`. – Jon Skeet Sep 26 '13 at 21:02

BeginInvoke copy parameter

1 Answers1