Combobox in TreeViewItem

[gstercken]

It is possible to show combobox side by side with selected TreeViewItem?

Definitely! But in order to achieve this, you need make the combobox part of the TreeViewItem's header, instead of adding it to the child items.

In order to display the label string and the combobox side-by-side, you need to wrap both in an appropriate panel (for example, a stack panel).

Here's your updated XAML:

   <TreeView Name="treeView1">
        <TreeViewItem>
            <TreeViewItem.Header>
                <StackPanel Orientation="Horizontal">
                    <Label>aaa</Label>
                    <ComboBox Height="19">
                        <ComboBoxItem IsSelected="True">111</ComboBoxItem>
                        <ComboBoxItem>222</ComboBoxItem>
                        <ComboBoxItem>333</ComboBoxItem>
                    </ComboBox>
                </StackPanel>
            </TreeViewItem.Header>                
            <TreeViewItem Header="aaa1">
            </TreeViewItem>                
            <TreeViewItem Header="aaa2">
            </TreeViewItem>                
        </TreeViewItem>            
        <TreeViewItem Header="bbb">
            <TreeViewItem Header="bbb1" />
            <TreeViewItem Header="bbb2" />
        </TreeViewItem>
        <TreeViewItem Header="ccc" />
    </TreeView>

A real-world solution would rather use a ControlTemplate for applying this to all items at once - this is just to show how you can generally replace the item's header by any UIElement.