C# delegate method with return value

Question

I am making a C# program which is runnning 2 threads. The main UI thread, and a seperate network thread i make myself.

I have figured out, that if i need to change something in the UI from the network thread, i will need to call a deligate method, which works just fine:

// Declared as a global variable
public delegate void ListBoxFirst();

//Then call this from the network thread
listBox1.BeginInvoke(new ListBoxFirst(InitListBox));

//Which points to this
public void InitListBox() {
     listBox1.SelectedIndex = 0;
}

Now i need to be able to read a UI value (a listbox.selectedindex) from the network thread, but it shows the error "cannot implicitly convert type 'system.iasyncresult' to 'int'" if i try it the same way (of course with "int" instead of "void", and a "int a = " before the listbox1.begininvoke). I have googled a lot but im pretty new to C# so i get really lost.

Any help would be much appriciated

That's running asynchronously. You can't return read any return value, because it's not done yet. — SLaks
– SLaks, Commented Jul 25, 2012 at 22:07

user1285334 · Accepted Answer · 2012-07-26 08:38:15Z

2

I figured it out:

public int ReadListBoxIndex()
    {
        int count = 0;
        listBox1.Invoke(new MethodInvoker(delegate
        {
            count = listBox1.SelectedIndex;
        }));
        return count;
    }

Called with a regular

int count = ReadListBoxIndex();

answered Jul 26, 2012 at 8:38

user1285334

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Embedd_0913 · Accepted Answer · 2012-07-25 22:25:51Z

0

You need to call EndInvoke() to get the result.

Some code:

 public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            listBox1.Items.AddRange(new[] { "AS","Ram"});           
        }

        protected override void OnLoad(EventArgs e)
        {
            listBox1.BeginInvoke(new Action(GetResult));
        }

        private void GetResult()
        {
            if (InvokeRequired)
            {
                Invoke(new Action(GetResult));
            }
            listBox1.SelectedIndex = 0;
        }
    }

edited Jul 25, 2012 at 22:25

answered Jul 25, 2012 at 22:09

Embedd_0913

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1 Comment

user1285334 Over a year ago

Well, i think you misunderstood my question. I did figure out how to set the index to 0. What i need, is to READ the value, not set it.

DeepSpace · Accepted Answer · 2015-05-29 16:07:30Z

0

This works also, where tn is the treenode you want added to cross thread and tnret is the treenode that is returned.

_treeView.Invoke(new Action(() => tnret = tn.Nodes.Add( name, name )));

edited May 29, 2015 at 16:07

DeepSpace

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answered May 29, 2015 at 15:28

Ogmios Ogmios

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