Why I can set same Tabindex for many control in C# WPF

Tab index controls the order in which controls are focused when tabbing. If 2 controls have the same index then which would Windows choose? You can have duplicate indice BTW but you shouldn't have dups in the same parent control. For example in a tab control each tab page will have its own tab index ordering and they can overlap but all the controls on a single page should have separate indice.

The Tab Layout feature you are talking about for Winforms has actually been broken for well over a year. When they started the migration to the new designer it was disabled so we had to do things the hard way. It was recently fixed in VS 2022 back to what it was before. AFAIK WPF has never had this feature nor does it exist now. In general WPF will figure out the tab order. If you want a specific tab order then you'll have to set it yourself manually.

If you want to visually set the TabIndex order of a control, you can right-click the control and choose Properties.

Set the TabIndex for the control on the Properties window：

Update:

Reference documentation: TabIndex

A tab index can consist of any valid integer greater than or equal to zero, lower numbers being earlier in the tab order. If more than one control on the same parent control has the same tab index, the z-order of the controls determines the order to cycle through the controls.

Xaml:

<StackPanel>  
     <TextBox x:Name="Textbox0" TabIndex="0"  Height="23" Margin="5,5,0,0" TextWrapping="Wrap" Text="TextBox" Width="505"/>  
            <TextBox x:Name="Textbox1" TabIndex="4"  Height="23" Margin="5,5,0,0" TextWrapping="Wrap" Text="TextBox"  Width="505"/>  
            <TextBox x:Name="Textbox2" TabIndex="1"  Height="23" Margin="5,5,0,0" TextWrapping="Wrap" Text="TextBox"  Width="505"/>  
            <Button x:Name="Textbox3" TabIndex="3" Content="Button" Height="40" Margin="5" Width="505"/>  
            <Button x:Name="Textbox5" TabIndex="2" Content="Button" Height="40" Margin="5"  Width="505"/>  
            <Button x:Name="Textbox6" TabIndex="2" Content="Button"  Height="40" Margin="5" Width="505"/>  
</StackPanel>  

The result:
Focus on the 2 controls with a tabindex value of 2, first focus on Textbox 5, and then focus on Textbox 6.

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thanks all ,

is this helpful ? :

 public static void SettingTabindexha(params FrameworkElement[] FRELM)  
        {  
            bool onTimedone = false;  
            for (int i = -2147483647; i <= 2147483647; i++)  
            {  
                if (!onTimedone)  
                {  
                    foreach (var ElementITM in FRELM)  
                    {  
                        if (ElementITM is TextBox) ((TextBox)ElementITM).TabIndex = i;  
                        if (ElementITM is DataGrid) ((DataGrid)ElementITM).TabIndex = i;  
                        if (ElementITM is ComboBox) ((ComboBox)ElementITM).TabIndex = i;  
                        if (ElementITM is CheckBox) ((CheckBox)ElementITM).TabIndex = i;  
                        if (ElementITM is Button) ((Button)ElementITM).TabIndex = i;  
                        if (ElementITM is RadioButton) ((RadioButton)ElementITM).TabIndex = i;  
                        i++;  
                    }  
                    onTimedone = true;  
                }  
                else { break; }  
            }  
        }  
  
//Usage :  
SettingTabindexha(  
TextBox1,  
TextBox2,  
TextBox3  
       );