WPF C# - User Control that contains a TabControl as a child within a TabItem

Question

I have a mainform which has a tabControl. I am attempting to create a userControl that can be added to a tabItem. I aim to add a tabControl to the userControl however I get this error:

'[Unknown]' property does not point to a DependencyObject in path '(0).(1)[1].(2)'.

The code is very simple within the userControl

    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <TabControl>
            <TabItem>
                
            </TabItem>
        </TabControl>
    </Grid>

How do I create the userControl so that it can contain a tabControl and then be added to the mainForm within a tabItem?

Answer 1

UserControl1.xmal

<UserControl x:Class="WpfApp1.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <TabControl x:Name="tabControl">

        </TabControl>
    </Grid>
</UserControl>

UserControl.cs

public partial class UserControl1 : UserControl
{
    public List<TabItem> TabItem { get; set; } = new List<TabItem>();
    public UserControl1()
    {
        InitializeComponent();
    }
    public override void OnApplyTemplate()
    {
        foreach (TabItem tabItems in TabItem)
            tabControl.Items.Add(tabItems);
        base.OnApplyTemplate();
    }
}

MainWindow.xaml

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApp1"
        Height="450" Width="800">
    <Grid>
        <local:UserControl1 Height="200" Width="200">
            <local:UserControl1.TabItem>
                <TabItem Header="Tab1">
                    <TextBox/>
                </TabItem>
                <TabItem Header="Tab2">
                    <TextBox/>
                </TabItem>
            </local:UserControl1.TabItem>
        </local:UserControl1>
    </Grid>
</Window>

In this case, the TextBox cannot be accessed through the x:Name="",

Because TextBox is a child of UserControl, not a child of MainWindow.