BeginInvoke copy parameter [duplicate]

Question

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Captured variable in a loop in C# (10 answers)

Closed 1 year ago.

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?

Answer 1

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.