how to create group items in ComboBox in C#?

If you are willing to do the work to create a UserControl, you can combine a ListView, which supports Groups, with a TextBox, for displaying the selected List Item. It's actually pretty easy to do, but, if you are relatively new to C# and Windows Forms, and have no experience with UserControls, I would suggest you carefully analyze your available time, and motivation, before taking this on.

The strategy would be like this:

1. create a new UserControl

2. put a ListView in it docked to the Bottom

3. put a TextBox in it docked to the Top.

4. choose a Font for the TextBox so it covers up the necessary column you must have to display items.

5. get the design looking right.

6. create the necessary logic to:

a. present the UserControl, initially, so its Height is equal to the height of the TextBox.

b. when the TextBox gets a Click, resize the UserControl to the full height it was in design-mode. That will display the ListView and its Groups, and Items.

c. when an Item in the ListView is selected, then put the Text of that Item in the TextBox, and set the UserControl height back to the height of the TextBox.

Of course, you have to expose, for use by consumers of the UserControl, any data from the UserControl necessary.

If you are really interested in this, and have time to work hard, let me know, and I'll post a rough code sketch.

... edit #1 ...

7. raising a custom event from the UserControl when a a ListViewItem is selected: assume the UserControl with ListView and TextBox is named 'LVCombo.

a. define a custom EventArgs class to hold data generated when the Event is raised:

public class LVEventArgs : EventArgs
{
    public int CurrentIndex;
    public string CurrentText;

    public LVEventArgs(int i, string s)
    {
        CurrentIndex = i;
        CurrentText = s;
    }
}

b. define the custom Event:

// the Event declaration
public event EventHandler<LVEventArgs> LVSelectedIndexChanged;

// the Event code
protected virtual void OnLVSelectedIndexChanged(LVEventArgs e)
{
    EventHandler<LVEventArgs> handler = LVSelectedIndexChanged;
    if (handler != null) handler(this, e);
}

c. in the ListView SelectedIndexChanged EventHandler create the custom data to be transmitted, and invoke the Event:

private void listView1_SelectedIndexChanged(object sender, EventArgs e)
{
    // handle potential no selection error
    if (listView1.SelectedItems.Count == 0) return;

    int ndx = listView1.SelectedIndices[0];
    string txt = listView1.Items[ndx].Text;

    // write the selected value to the TextBox
    textBox1.Text = txt; ;

    // collapse the UserControl
    this.Height = textBox1.Height;

    // invoke the Event
    OnLVSelectedIndexChanged(new LVEventArgs(ndx, txt));
}

8. Example of subscribing to the custom Event from the Form that hosts the UserControl:

... assume the UserControl on the Form is named 'lvCombo1 ...

private void Form1_Load(object sender, EventArgs e)
{
    lvCombo1.LVSelectedIndexChanged += lvCombo1_LVSelectedIndexChanged;
}

// note that we use the custom EventArgs here
private void lvCombo1_LVSelectedIndexChanged(object sender, LVEventArgs e)
{
    // write the information out to a TextBox on the Form, 'textBox1
    textBox1.Text = string.Format("index: {0} value: {1}", e.CurrentIndex, e.CurrentText);
}

Note that in step #7b. we used the generic Event syntax available from .NET 2.0; this syntax lets us define the necessary Event "plumbing" without defining a Delegate.

Good resources for understanding creating, raising, and consuming, Events in C#: on CodeProject: [^], on MSDN: [^], StackOverFlow: [^]

... end edit #1 ...