Where should TreeView's Invoke method between two WinForms be handled?

Question

I have two WinForms (Setting and frmMain). I have a TreeView in Setting's form and I want to call its FillTree method in the second form frmMain.

I'm using the TreeView.Invoke for the threading purposes.

Here is my code for TreeView filling data in Setting's form :

TreeNode parentNode;
public void FillTree(DataTable dtGroups, DataTable dtGroupsChilds)
{
    treeViewGroups.Nodes.Clear();
    if (dtGroups == null) return;
    foreach (DataRow rowGroup in dtGroups.Rows)
    {
        parentNode = new TreeNode
        {
            Text = rowGroup["Groupname"].ToString(),
            Tag = rowGroup["Groupid"]
        };
        treeViewGroups.Invoke(new Add(AddParent), new object[] { parentNode });

        if (dtGroupsChilds == null) continue;
        foreach (DataRow rowUser in dtGroupsChilds.Rows)
        {
            if (rowGroup["Groupid"] == rowUser["Groupid"])
            {
                TreeNode childNode = new TreeNode
                {
                    Text = rowUser["Username"].ToString(),
                    Tag = rowUser["Phone"]
                };
                treeViewGroups.Invoke(new Add(AddParent), new object[] { childNode });
                System.Threading.Thread.Sleep(1000);
            }
        }
    }
    treeViewGroups.Update();
}

public delegate void Add(TreeNode tn);

public void AddParent(TreeNode tn)
{
    treeViewGroups.Nodes.Add(tn);
}

public void AddChild(TreeNode tn)
{
    parentNode.Nodes.Add(tn);
}

FillTree method from above code, I wants call it in my second form frmMain which I tried like so:

Settings settingsWindow;
public frmMain()
{
    InitializeComponent();

    settingsWindow = new Settings(this);
}

private void SomeMethod()
{
    //Two DataTables (dt1 and dt2) are passed from frmMain form

    settingWindow.FillTree(dt1, dt2);
}

When I call FillTree method it show me error like this:

Invoke or BeginInvoke cannot be called on a control until the window handle has been created.

I real wants to know, where should be handle TreeView's Invoke method in my second winform frmMain?

---

I'm following these links (but no avail):

1) Populating TreeView on a Background Thread

2) How to add object in treeview from another thread

3) How can i invoke a method called from backgroundworker dowork event?

---

Edited for Visualizing Problem

I have tried it with TreeView its not working then I Tried it with ListBox but the problem is still same.

Problem: I've a method (which populate my ListBox) in WinForm settingsWindow and I want to call that method in my second WinForm frmMain.

Settings form screenshot:

frmMain form screenshot:

Problem GIF :

Settings form Code for ListBox Populating :

public void PopulateGroupListData(DataTable dt)
{
    listGroups.DataSource = null;

    listGroups.DisplayMember = "GroupName";
    listGroups.ValueMember = "Groupid";
    listGroups.DataSource = dt;

    if (listGroups.Items.Count > 0)
        listGroups.SelectedItem = listGroups.Items[0];
}

Calling PopulateGroupListData in second form frmMain 's method:

void onCompleteReadFromServerStream(IAsyncResult iar)
{
    /... some code

    String[] arr1 = ServerMessage[1].Split('&');
    string Groupid1 = arr1[0];
    string GroupName1 = arr1[1];
    GroupsList.Rows.Add(Groupid1, GroupName1);
    settingsWindow.PopulateGroupListData(GroupsList);

    /... some code
}

Answer 1

Subscribe to HandleCreated event and fill the tree in the event handler.

public partial class Form1 : Form
{
    Form2 settingsWindow;

    public Form1()
    {
        InitializeComponent();
    }

    private void SettingsWindow_HandleCreated(object sender, EventArgs e)
    {
        var dt1 = new SampleTable1();
        var dt2 = new SampleTable2();

        settingsWindow.FillTree(dt1, dt2);
    }

    private void button1_Click(object sender, EventArgs e)
    {
        settingsWindow = new Form2();
        settingsWindow.HandleCreated += SettingsWindow_HandleCreated;
        settingsWindow.ShowDialog();
    }
}

---

As a bonus, we also fixed a couple problems with FillTree method, so the tree can be built correctly.

public void FillTree(DataTable dtGroups, DataTable dtGroupsChilds)
{
    treeViewGroups.Nodes.Clear();
    if (dtGroups == null) return;
    foreach (DataRow rowGroup in dtGroups.Rows)
    {
        parentNode = new TreeNode
        {
            Text = rowGroup["Groupname"].ToString(),
            Tag = rowGroup["Groupid"]
        };
        treeViewGroups.Invoke(new Add(AddParent), new object[] { parentNode });

        if (dtGroupsChilds == null) continue;
        foreach (DataRow rowUser in dtGroupsChilds.Rows)
        {
            if ((int)rowGroup["Groupid"] == (int)rowUser["Groupid"])
            {
                TreeNode childNode = new TreeNode
                {
                    Text = rowUser["Username"].ToString(),
                    Tag = rowUser["Phone"]
                };
                treeViewGroups.Invoke(new Add(AddChild), new object[] { childNode });
                //System.Threading.Thread.Sleep(1000);
            }
        }
    }
    treeViewGroups.Update();
}

---

Answering your follow-up question: if Form2 is already visible, the exception you reported wouldn't happen because at this point the window's handle has been created, as we show below.

public partial class Form1 : Form
{
    Form2 settingsWindow;
    SampleTable1 dt1;
    SampleTable2 dt2;
    int groupid = 1;
    int userid = 101;

    public Form1()
    {
        InitializeComponent();

        dt1 = new SampleTable1();
        dt2 = new SampleTable2();

        dt1.AddGroup(groupid);
        dt2.AddUser(groupid, userid++);
        dt2.AddUser(groupid, userid++);

        dt1.AddGroup(++groupid);
        dt2.AddUser(groupid, userid++);
        dt2.AddUser(groupid, userid++);
        dt2.AddUser(groupid, userid++);
    }

    private void SettingsWindow_HandleCreated(object sender, EventArgs e)
    {
        settingsWindow.FillTree(dt1, dt2);
    }

    private void button1_Click(object sender, EventArgs e)
    {
        settingsWindow = new Form2(this);
        settingsWindow.HandleCreated += SettingsWindow_HandleCreated;
        settingsWindow.ShowDialog();
    }

    public void UpdateData(string groupname)
    {
        dt1.AddGroup(++groupid, groupname);
        dt2.AddUser(groupid, userid++);
        dt2.AddUser(groupid, userid++);

        settingsWindow.FillTree(dt1, dt2);
    }
}

Form2 stays the same you already have, just adding an event handler for the new button:

    private void button1_Click(object sender, EventArgs e)
    {
        form1.UpdateData(textBox1.Text);
    }

---

Answering your 2nd follow-up question: UpdateData was created for the specific use case of the user submitting a new group through Form2. I would rather have specific code for a different use case. Note that it would be fairly elementary to have UpdateData to consult with a server, or even better, with some abstract interface, that could be in a remote server or not (good design dictates it should be irrelevant/transparent...) and then return a string message to be presented in Form2. However, doing so wouldn't add any insight into the purposes of the limited scope of this sample. It would actually mud the waters about the root problem you're reporting and its corresponding solution. Let's not forget that without it, the reported exception in your original question comes right back at you...:O)

Answer 2

The code which you have shared doesn't help us to reproduce the problem. The only thing that someone can do is creating an example, showing you how to load a TreeView in another form in another thread (if thread is really necessary).

There are some points that you should consider:

• You should first show the form, then call Invoke method of the form or one of its controls. Otherwise you will receive 'Invoke or BeginInvoke cannot be called on a control until the window handle has been created.'

• When adding a lot of nodes to a TreeView, first call BeginUpdate then add all the nodes then call EndUpdate. This way it will be quite faster with less UI rendering.

• Condiser using async/await pattern to have a non-blocking UI.

• When you want to update UI thread from another thread, use Invoke.

• Don't do blocking time-consuming non-UI tasks in the UI thread.

• When using Invoke, keep in mind that the code is running in the UI thread. So don't call blocking time-consuming non-UI tasks in the Invoke. Just call the UI code in Invoke.

In the following example, I've created a form having a button. On click of button, I load data and then open another window which has a TreeView inside. Then loading tree with 10000 nodes:

private async void button1_Click(object sender, EventArgs e) 
{
    DataTable table = null;
    //Load data
    this.Text = "Loading ...";
    await Task.Run(async () => {
        await Task.Delay(2000); //Simulating a delay for loading data
        table = new DataTable();
        table.Columns.Add("C1");
        for (int i = 0; i < 10000; i++)
            table.Rows.Add(i.ToString());
    });
    this.Text = "Load data successfully.";

    //Show the other form
    var f = new Form();
    var tree = new TreeView();
    tree.Dock = DockStyle.Fill;
    f.Controls.Add(tree);
    f.Show();

    //Load Tree
    f.Text = "Loading tree...";
    await Task.Run(async () => {           
        await Task.Delay(2000); //Simulating a delay for processing
        Invoke(new Action(() => { tree.BeginUpdate(); }));
        foreach (DataRow row in table.Rows) {
            //DO NOT processing using invoke, just call UI code in INvoke.
            Invoke(new Action(() => { tree.Nodes.Add(row[0].ToString()); }));
        }
        Invoke(new Action(() => { tree.EndUpdate(); }));
    });
    f.Text = "Load tree successfully.";
}