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Can't binding in ToolTip

I'm working with ToolTip in WPF C#. I want to bind an element property to ToolTip content, but the output is empty. Initially, I used the binding below, and it did not work.
Content="{Binding ElementName=txtf1, Path=Text}"
So I found related solutions here, but the result did not change. Can anyone point me where I'm going wrong?
<StackPanel Height="Auto" Margin="15,10,15,0" Width="74">
<Image Source="/Image/AutoCAD.png" Width="50"/>
<TextBlock x:Name="txtf1" Text="Border19072021 Border19072021"/>
<StackPanel.ToolTip>
<ToolTip DataContext="{Binding Path=PlacementTarget.DataContext ,
RelativeSource={RelativeSource Self}}">
<Label Content="{Binding ElementName=txtf1, Path=Text}"/>
</ToolTip>
</StackPanel.ToolTip>
</StackPanel>

[...] initially, I use Content="{Binding ElementName=txtf1, Path=Text}" and it not work.
Yes, that is because each window in WPF has its own visual tree with elements and a ToolTip and other popups are actually displayed in a separate window. ElementName bindings do not work here (RelativeSource too), since these are different XAML namescopes.
The question you cited is the right approach, but you applied it the wrong way. In theory you can use the DataContext property as binding indirection for the ToolTip, but you should not.
<StackPanel DataContext="{Binding Text, ElementName=txtf1}" Height="Auto" Margin="15,10,15,0" Width="74">
<Image Source="/Image/AutoCAD.png" Width="50"/>
<TextBlock x:Name="txtf1" Text="Border19072021 Border19072021"/>
<StackPanel.ToolTip>
<ToolTip DataContext="{Binding Path=PlacementTarget.DataContext, RelativeSource={RelativeSource Self}}">
<Label Content="{Binding}"/>
</ToolTip>
</StackPanel.ToolTip>
</StackPanel>
The above code works, but the DataContext property serves a core purpose in WPF for data binding. Do not abuse this property for this purpose or you break data-binding down the visual tree. Apart from that, the binding on Label and ToolTip is redundant, as you can see below. There is another property called Tag that you can assign any value to.
Gets or sets an arbitrary object value that can be used to store custom information about this element.
<StackPanel Tag="{Binding Text, ElementName=txtf1}" Height="Auto" Margin="15,10,15,0" Width="74">
<Image Source="/Image/AutoCAD.png" Width="50"/>
<TextBlock x:Name="txtf1" Text="Border19072021 Border19072021"/>
<StackPanel.ToolTip>
<ToolTip>
<Label Content="{Binding Path=PlacementTarget.Tag, RelativeSource={RelativeSource AncestorType={x:Type ToolTip}}}"/>
</ToolTip>
</StackPanel.ToolTip>
</StackPanel>
You can use a RelativeSource binding to refer to the ToolTip.Tag directly from the label. If there are multiple bindings like this, you could consider creating custom attached properties for each of them and bind them the same way.

I reference here, How to binding other element in ToolTip
You have not carefully read the topic you are referring to.
ToolTip, ContextMenu, Popup (and some other elements) pop up over the contents of the Window without changing it.
Think for yourself: How can you introduce new elements into a Window without changing the visual tree of this Window?
Answer: this cannot be done in any way.
Therefore, these pop-up elements are implemented as the contents of a new small Window shown above the main Window (the one in which they were called).
And since these are DIFFERENT Windows, they have different visual trees.
Therefore, it is impossible to find elements from the main Window in the tree of the Pop-up Window (Bindings of the ElementName and FindAncestor types).
You can use an element reference (Binding Sourse = {x: Reference ...}), since references are resolved at compile time and without regard to the visual tree.
Example:
<StackPanel Height="Auto" Margin="15,10,15,0" Width="74">
<Image Source="/Image/AutoCAD.png" Width="50"/>
<TextBlock x:Name="txtf1" Text="Border19072021 Border19072021"/>
<StackPanel.ToolTip>
<ToolTip>
<Label Content="{Binding Source={x:Reference txtf1}, Path=Text}"/>
</ToolTip>
</StackPanel.ToolTip>
</StackPanel>
In addition, the Popup's DataContext inherits from the element in which It is created.
In your case, this is from StackPanel.
Therefore, you can simply set the default Bindings to receive data.
Example:
<StackPanel Height="Auto" Margin="15,10,15,0" Width="74">
<Image Source="/Image/AutoCAD.png" Width="50"/>
<TextBlock x:Name="txtf1" Text="{Binding SomeViewModelProperty}"/>
<StackPanel.ToolTip>
<ToolTip>
<Label Content="{Binding SomeViewModelProperty}"/>
</ToolTip>
</StackPanel.ToolTip>
</StackPanel>
The PlacementTarget property is used to change the target element for the popup.
DataContext inherits from this element.
If it is not specified, the DataContext is inherited from the element in which the flyout is specified.
You don't specify it, so it is null, which is what your binding returns.
Example:
<StackPanel DataContext="{Binding Text, ElementName=txtf1}" Height="Auto" Margin="15,10,15,0" Width="74">
<Image Source="/Image/AutoCAD.png" Width="50"/>
<TextBlock x:Name="txtf1" Text="{Binding SomeViewModelProperty}"/>
<StackPanel.ToolTip>
<ToolTip PlacementTarget="{Binding ElementName=txtf1}"
DataContext="{Binding Path=PlacementTarget.DataContext,
RelativeSource={RelativeSource Self}}">
<Label Content="{Binding SomeViewModelProperty}"/>
</ToolTip>
</StackPanel.ToolTip>
</StackPanel>

How to create only binding not null controls in xaml?

I`d like select between combobox and textbox depending on ViewModel. In the view model i have two properties one of them should be null and the other not null.
enter image description here

I´m not sure if that´s exactly what you want to do. As CarbineCoder already said, some more code would be helpful. If you want to show only one of both controls you could create two properties called for example tbVisibility and cbVisibility. If you want to show the ComboBox, then cbVisibility has to be "visible" and tbVisibility has to be "hidden".
XAML would like that:
<StackPanel Orientation="Horizontal">
<!--This is visible if tbVisibility is "visible" and cbVisibility is "hidden"-->
<Label Width="100" Content="label" Visibility="{Binding tbVisibility}"></Label>
<TextBox Width="184" Text="Textbox" Visibility="{Binding tbVisibility}"></TextBox>
<!--This part is visible if cbVisibility is "visible" and tbVisibility is "hidden"-->
<Label Width="100" Content="label" Visibility="{Binding cbVisibility}"></Label>
<TextBox Width="184" Visibility="{Binding cbVisibility}"></TextBox>
</StackPanel>
You only have to define in you code which property is visibile and which is not.

Telerik RadComboBox Apply SelectionBoxTemplate OnLoad?

Basically, we have created our own custom MultiSelect-ComboBox using Telerik's RadComboBox and the SelectionBoxTemplate to make the custom text appear when multiple items are selected.
<ucControls:MultiSelectComboBoxBase
... >
<ucControls:RadComboBox
x:Name="RadCombo"
Text=""
Height="22"
HorizontalContentAlignment="Left"
HorizontalAlignment="Stretch"
DropDownClosed="RadCombo_DropDownClosed"
IsTextSearchEnabled="False"
IsEnabled="{Binding IsEnabled}"
CanAutocompleteSelectItems="False"
CanKeyboardNavigationSelectItems="False"
LostFocus="RadCombo_LostFocus">
<ucControls:RadComboBox.SelectionBoxTemplate>
<DataTemplate>
<Grid Background="Aqua">
<TextBlock x:Name="ComboBoxDisplay" Text="{Binding Text, ElementName=RadCombo}" />
</Grid>
</DataTemplate>
</ucControls:RadComboBox.SelectionBoxTemplate>
<ucControls:RadComboBox.ItemTemplate>
<DataTemplate>
<StackPanel>
<CheckBox x:Name="chkBox"
Content="{Binding ItemValue}"
Tag="{Binding ItemID}"
Height="16" Margin="2"
IsChecked="{Binding IsChecked, Mode=TwoWay}"
Visibility="{Binding IsSelectAllItem, Converter={StaticResource booleanToVisibilityConverter}, ConverterParameter=NOT}"
HorizontalAlignment="Stretch" VerticalAlignment="Top"
Checked="ChkBox_Checked" Unchecked="ChkBox_Unchecked"
IsEnabled="{Binding IsEnabled, Mode=TwoWay}"/>
<HyperlinkButton
VerticalAlignment="Center" Content="{Binding ItemValue}"
Visibility="{Binding IsSelectAllItem, Converter={StaticResource booleanToVisibilityConverter}}"
Style="{StaticResource HyperlinkButtonNoBorderStyle}" Margin="3,2,0,0"
Click="HyperlinkButton_Click"
IsEnabled="{Binding IsEnabled, Mode=TwoWay}" />
</StackPanel>
</DataTemplate>
</ucControls:RadComboBox.ItemTemplate>
</ucControls:RadComboBox>
It works great, but the only problem is that it appears that the SelectionBoxTemplate isn't actually applied when the radcombobox is loaded. It only applies when the box is clicked.When it loads it displays:
Then you click and it displays the dropdown:
Then you click away and it displays the correct text from the SelectionBoxTemplate:
Note I made the background color of the Textblock blue, just to show that it does not apply on the initial load.Ideally, when it loads it should automatically apply the SelectionBoxTemplate and display the correct information rather than having the click and then click away from the multiselectcombobox. Also, the TextBlock text is bound to the RadComboBox's text because in the code behind we set the RadComboBox's text. There is no issue with null values or the Text not being set before because I've looked in debugging mode and all the information is there, the selection box template is just not being applied until the box receives focus. Is there some sort of event that has to fire off before the template is applied, or is there someway I can force the template to be applied onload?

I had the same issue as you. I've managed to solve it by setting the SelectedIndex="0"
I've also read that the SelectionBoxTemplate only works when the RadComboBox is not editable: IsEditable="False"
<ucControls:RadComboBox
x:Name="RadCombo"
SelectedIndex="0"
IsEditable="False"
Height="22"
HorizontalContentAlignment="Left"
HorizontalAlignment="Stretch"
DropDownClosed="RadCombo_DropDownClosed"
IsEnabled="{Binding IsEnabled}"
LostFocus="RadCombo_LostFocus">
...
</ucControls:RadComboBox>

Is it possible to have same name on two components?

I am doing a project where I need a timer with drop shadow. I managed to figure out a way to do the drop shadow with 2 labels, but my problem now is that I need to give both the same name to act in the exact same way in all situation WITHOUT having duplicated code. Is this possible?
<Grid>
<Label x:Name="time" MouseDoubleClick="time_MouseDoubleClick" HorizontalAlignment="Center" FontFamily="Old English Text MT" Margin="0,5,0,0" FontSize="22" Content="00:00:00" FontWeight="Bold" Foreground="Black">
<Label.RenderTransform>
<TranslateTransform X="1.5" Y="1"/>
</Label.RenderTransform>
</Label>
<Label x:Name="time2" MouseDoubleClick="time_MouseDoubleClick" HorizontalAlignment="Center" FontFamily="Old English Text MT" Margin="0,5,0,0" FontSize="22" Content="00:00:00" FontWeight="Bold" Foreground="White"/>
</Grid>
This is how the code looks to make the drop shadow. Is there a way around this? Or is there a way like in HTML where I can define a "class" name?

You don't need two labels. You need DropShadowEffect:
<Label Content="00:00:00">
<Label.Effect>
<DropShadowEffect/>
</Label.Effect>
</Label>

It seems that you in actual fact need a drop-shaddow effect, another implementation to have two controls update based on the same property might lie in binding.
In the initialize method of the codebehind file set the DataContext.
DataContext = this;
Change your xaml to bind to a common string property.
<Grid>
<Label x:Name="time" MouseDoubleClick="time_MouseDoubleClick" HorizontalAlignment="Center" FontFamily="Old English Text MT" Margin="0,5,0,0" FontSize="22" Content="{Binding Time}" FontWeight="Bold" Foreground="Black">
<Label.RenderTransform>
<TranslateTransform X="1.5" Y="1"/>
</Label.RenderTransform>
</Label>
<Label x:Name="time2" MouseDoubleClick="time_MouseDoubleClick" HorizontalAlignment="Center" FontFamily="Old English Text MT" Margin="0,5,0,0" FontSize="22" Content="{Binding Time}" FontWeight="Bold" Foreground="White"/>
</Grid>
(The content property now includes binding)
Ensure that you have a property in the codebehind to support the binding
public string Time {get;set;}
Your code-behind (.cs file) should implement the INotifyPropertyChanged interface, and your property should Raise the Event.
Then by changing the Time property, the XAML will automatically update.

Handling Content property in WPF

I'm currently facing huge problem i.e I'm showing Image and some text in a image the problem is when I change content property of button from code, my image disappears and only assigned text is shown, I wan to retain image and just wann change the text, Any suggetions how to handle it
<Button x:Name="btnPrescan" Margin="8" Grid.Column="2" Click="btnPrescan_Click">
<StackPanel Orientation="Horizontal">
<Image Source="Icons\Scan_Start_Icon.png" Height="14" Width="23"/>
<TextBlock x:Name="tbButtonText" Text=" Prescan"/>
</StackPanel>
</Button>
and button looks something like this
Thanks

Bind the text to a backing property on your UserControl:
<Button x:Name="btnPrescan" Margin="8" Grid.Column="2" Click="btnPrescan_Click">
<StackPanel Orientation="Horizontal">
<Image Source="Icons\Scan_Start_Icon.png" Height="14" Width="23"/>
<!-- assumes DataContext is set appropriately -->
<TextBlock Text="{Binding ButtonText}"/>
</StackPanel>
</Button>
Then just change the backing property:
this.ButtonText = "New button text";

You should do this way,
((TextBlock)btnPrescan.GetTemplatedChild("tbButtonText")).Text = "Your Text"