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how to add a customtext to datepicker text box

i designed a textbox foe date picker but i cant add a custom text inside the textbox before choosing date.i want to add "select a date"
<Grid>
<DatePicker x:Name="DateGviya" SelectedDate="{Binding CurrentDate}" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="97,77,0,0">
<DatePicker.Resources>
<Style TargetType="{x:Type DatePickerTextBox}">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<TextBox x:Name="PART_TextBox" Text="{Binding Path=SelectedDate, StringFormat='dd/MM/yyyy hh:mm:ss tt', RelativeSource={RelativeSource AncestorType={x:Type DatePicker}}}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</DatePicker.Resources>
</DatePicker>
</Grid>
result is like this:

You could use the TargetNullValue property of the binding:
<DatePicker x:Name="DateGviya" SelectedDate="{Binding CurrentDate}" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="97,77,0,0">
<DatePicker.Resources>
<Style TargetType="{x:Type DatePickerTextBox}">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<TextBox x:Name="PART_TextBox"
Text="{Binding Path=SelectedDate,
StringFormat='dd/MM/yyyy hh:mm:ss tt',
RelativeSource={RelativeSource AncestorType={x:Type DatePicker}},
TargetNullValue='Select a Date'}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</DatePicker.Resources>
</DatePicker>

How to change the display data format of the WPF DatePicker

I got this sample code from this forum.
<DatePicker SelectedDate="{Binding MainReportEndDate, Mode=TwoWay}"
DataContext="{Binding DataContext, RelativeSource= {RelativeSource FindAncestor, AncestorType={x:Type GroupBox}}}"
DisplayDateStart="1/01/20" DisplayDateEnd="12/31/22"
FirstDayOfWeek="Monday"">
<DatePicker.Resources>
<Style TargetType="{x:Type DatePickerTextBox}">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<TextBox x:Name="PART_TextBox"
Text="{Binding Path=SelectedDate,
RelativeSource={RelativeSource AncestorType={x:Type DatePicker}},
StringFormat={}{0:MMM yy}}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</DatePicker.Resources>
</DatePicker>
But it doesn't work. I want the same result as the one in the picture "Jul 20"

Try this
<TextBox x:Name="PART_TextBox"
Text="{Binding Path=SelectedDate, StringFormat='dd MMM yyyy',
RelativeSource={RelativeSource AncestorType={x:Type DatePicker}}}" />

WPF How to bind a control which is defined in a style from the element which uses the style?

I'm new to wpf and xaml,
trying to understand the basic concepts by writing a simple app using MVVM.
One thing I can't wrap my head around is
How to bind a control eg. textBox which is defined in a style to a viewModel from the control which uses this style?
Style Example (I like to bind the textBox named "SearchBox"):
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Style TargetType="{x:Type TextBox}"
x:Key="FlatSearchBox">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<Border CornerRadius="10"
Background="#353340"
Width="200" Height="40">
<Grid>
<Rectangle StrokeThickness="1"/>
<TextBox Margin="1"
Text="{TemplateBinding Text}"
BorderThickness="0"
Background="Transparent"
VerticalContentAlignment="Center"
Padding="5"
Foreground="#CFCFCF"
x:Name="SearchBox"/>
<TextBlock Grid.Column="1"
IsHitTestVisible="False"
Text="Search"
VerticalAlignment="Center"
HorizontalAlignment="Left"
Margin="10,0,0,0"
FontSize="11"
Foreground="DarkGray">
<TextBlock.Style>
<Style TargetType="{x:Type TextBlock}">
<Style.Triggers>
<DataTrigger Binding="{Binding Text, ElementName=SearchBox}" Value="">
<Setter Property="Visibility" Value="Visible"/>
</DataTrigger>
</Style.Triggers>
<Setter Property="Visibility" Value="Hidden"/>
</Style>
</TextBlock.Style>
</TextBlock>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Part of XAML in which the style is applied to a textBox:
<TextBox Grid.Row="2" Grid.Column="2"
VerticalAlignment="Center"
FontSize="20"
Style="{StaticResource FlatSearchBox}"
Text="{Binding CurrentFinanceModel.Value, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource currencyConverter}}"/>
The text binding should apply to the textBox named "SearchBox" in the style.
Hope the question is clear and I did not make a mistake, because it's my first question here :)
Thanks!

Custom tooltip validation error WPF

I'm trying to create a custom tooltip. The problem is that I can not display the error text. This code works perfectly (a simple tooltip)
<Style TargetType="{x:Type TextBox}"
BasedOn="{StaticResource {x:Type TextBox}}">
<Setter Property="Validation.ErrorTemplate">
<Setter.Value>
<ControlTemplate>
<Grid>
<Polygon Fill="Red"
Margin="0,2,2,0"
Points="10,10 10,0 0,0"
VerticalAlignment="Top"
HorizontalAlignment="Right"
ToolTip="{Binding ElementName=adorner, Path=AdornedElement.(Validation.Errors).CurrentItem.ErrorContent}">
</Polygon>
<AdornedElementPlaceholder x:Name="adorner" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
But this code, not work, not show the error
<Style TargetType="{x:Type TextBox}"
BasedOn="{StaticResource {x:Type TextBox}}">
<Setter Property="Validation.ErrorTemplate">
<Setter.Value>
<ControlTemplate>
<Grid>
<Polygon Fill="Red"
Margin="0,2,2,0"
Points="10,10 10,0 0,0"
VerticalAlignment="Top"
HorizontalAlignment="Right">
<Polygon.ToolTip>
<ToolTip Content="{Binding ElementName=adorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}"
BorderThickness="1"
Foreground="White"
Background="Red" />
</Polygon.ToolTip>
</Polygon>
<AdornedElementPlaceholder x:Name="adorner" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>

The Tooltip itself and the AdornedElementPlaceholder reside in different namescopes so binding using an ElementName won't work.
But you can set the Tag property of the Polygon to the ErrorContent and bind the Content property of the Tooltip to the Tag property of its PlacementTarget (which is the Polygon). This works:
<Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource {x:Type TextBox}}">
<Setter Property="Validation.ErrorTemplate">
<Setter.Value>
<ControlTemplate>
<Grid>
<Polygon Fill="Red"
Margin="0,2,2,0"
Points="10,10 10,0 0,0"
VerticalAlignment="Top"
HorizontalAlignment="Right"
Tag="{Binding ElementName=adorner, Path=AdornedElement.(Validation.Errors).CurrentItem.ErrorContent}">
<Polygon.ToolTip>
<ToolTip Content="{Binding PlacementTarget.Tag, RelativeSource={RelativeSource Self}}"
BorderThickness="1"
Foreground="White"
Background="Red" />
</Polygon.ToolTip>
</Polygon>
<AdornedElementPlaceholder x:Name="adorner" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

The call to (Validation.Errors)[0] will be troublesome as you are referencing a particular object from a list that will update and change therefore breaking the binding.
The first example where you use (Validation.Errors).CurrentItem. is more suitable as it matches your first implementation which works.
I ran into issues with this a few years back, basically never use an index in a binding unless you are absolutely 100% sure it will not change.

How to select date and time in WPF

i'm looking for a control, that allow select both date and time. Default DatePicker doesn't allow it. But custom controls (like Extended WPFToolkit) are expensive, anothers are not free...

At the moment I posted it I came to a solution. So I leave it here if someone needs it:
<DatePicker Name="Picker">
<DatePicker.Resources>
<Style TargetType="DatePickerTextBox">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate>
<TextBox Text="{Binding Path=SelectedDate, RelativeSource={RelativeSource AncestorType=DatePicker}, StringFormat={}{0:dd/MM/yyyy hh:mm:ss}}"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</DatePicker.Resources>
</DatePicker>