Wpf drop event not firing

Question

I have a user control simulating water mark text box. I need to allow drop text files on the user control. The PreviewDragEnter event works fine. This is the event where i want to verify that the file that been drag is actually a text file. The problem is that the drop event never fires. I want to catch the event when the user release the mouse button and decide to drop the file into the user control. I think it got something to do with the multiple controls inside the user control and the hierarchy. I really don't care which control will fire the drop event as long as it actually fire. This is the xaml (the control i want to target the drop event is name 'txt')

<UserControl x:Class="CrumbSearch.CustomControls.package_watermark"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             Height="45" Width="Auto" 
             FontFamily="B Yekan"             
             AllowDrop="true"
             Grid.IsSharedSizeScope="True"              
             xmlns:wpfwatermarktextbox="clr-namespace:WPFWaterMarkTextBox;assembly=WPFWaterMarkTextBox">
    <FrameworkElement.Resources>
        <ResourceDictionary>
            <FontFamily x:Key="simple_line_icon">/fonts/Simple-Line-Icons.ttf#simple-line-icons</FontFamily>
            <Style x:Key="SimpleTextBox" TargetType="{x:Type TextBox}">
                <Setter Property="AllowDrop" Value="True"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type TextBox}">
                            <Border Name="border" BorderBrush="{TemplateBinding BorderBrush}" 
                                    BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
                                <ScrollViewer Name="PART_ContentHost" Focusable="False" Margin="2,10,0,0"
                                              AllowDrop="True"
                                              HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden" />
                            </Border>
                            <ControlTemplate.Triggers>
                                <Trigger Property="UIElement.IsFocused" Value="True">
                                    <Setter TargetName="border" Property="Border.BorderBrush" Value="Transparent" />
                                </Trigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
            <Style x:Key="SimplePasswordBox" TargetType="{x:Type PasswordBox}">
                <Setter Property="AllowDrop" Value="True"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type PasswordBox}">
                            <Border Name="border" BorderBrush="{TemplateBinding BorderBrush}" 
                                    BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" 
                                    SnapsToDevicePixels="True">
                                <ScrollViewer Name="PART_ContentHost" 
                                              Focusable="False" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden" />
                            </Border>
                            <ControlTemplate.Triggers>
                                <Trigger Property="UIElement.IsFocused" Value="True">
                                    <Setter TargetName="border" Property="Border.BorderBrush" Value="#00000000" />
                                </Trigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </ResourceDictionary>
    </FrameworkElement.Resources>
    <Border Name="border" BorderThickness="0.5" CornerRadius="4" MouseEnter="border_MouseEnter"             
            MouseLeave="border_MouseLeave">
        <FrameworkElement.Style>
            <Style TargetType="{x:Type Border}">
                <Setter Property="BorderBrush" Value="White" />
                <Setter Property="AllowDrop" Value="true" />
            </Style>
        </FrameworkElement.Style>
        <Grid Name="MainGrid" PreviewMouseLeftButtonDown="MainGrid_PreviewMouseLeftButtonDown"               
              LostFocus="MainGrid_LostFocus">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="auto" />
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <Label Name="lbl2" Padding="5,0" FontSize="10" Content="Name" 
                   Margin="0,5,0,0"                   
                   HorizontalAlignment="Left" Foreground="White" FlowDirection="LeftToRight"
                   VerticalAlignment="Top" Height="13" Grid.IsSharedSizeScope="True" 
                   Opacity="0" Grid.Column="1" Background="{x:Null}" />
            <TextBlock Name="lbl" Text="WaterMark" HorizontalAlignment="Left" 
                       Foreground="#0EE7EC" FontSize="12"                        
                       Padding="5,5,5,0" FlowDirection="LeftToRight" FontFamily="Andalus"
                       TextWrapping="WrapWithOverflow" FontStyle="Italic"
                       Opacity="0.7" Grid.Column="1" 
                       VerticalAlignment="Center" Background="{x:Null}" />
            <TextBox Name="txt" Style="{DynamicResource SimpleTextBox}" FlowDirection="LeftToRight"                                                                        
                     AllowDrop="true" Background="{x:Null}" 
                     PreviewDragEnter="txt_PreviewDragEnter"
                     PreviewDrop="txt_PreviewDrop" Drop="txt_Drop"
                     TextWrapping="NoWrap" VerticalContentAlignment="Center" FontSize="14" 
                     VerticalScrollBarVisibility="Disabled" MaxLines="1" FontWeight="Medium" Grid.Column="1" 
                     BorderBrush="{x:Null}" TextChanged="TextBox_TextChanged"  Foreground="White"
                     PreviewGotKeyboardFocus="txt_PreviewGotKeyboardFocus" 
                     PreviewLostKeyboardFocus="txt_PreviewLostKeyboardFocus" />
            <PasswordBox Name="pass" Style="{DynamicResource SimplePasswordBox}" FontSize="14"                          
                         VerticalContentAlignment="Center" 
                         ScrollViewer.VerticalScrollBarVisibility="Disabled" Grid.Column="1" 
                         Visibility="Collapsed" BorderBrush="{x:Null}" 
                         Background="{x:Null}" PasswordChanged="PasswordBox_PasswordChanged" />
            <Label Name="LblIcon" Padding="0" FontFamily="{DynamicResource simple_line_icon}"                     
                   VerticalContentAlignment="Center" HorizontalContentAlignment="Center" 
                   Foreground="#FF6A6A6A" FontSize="14" Background="{x:Null}" />
        </Grid>
    </Border>
</UserControl>

Answer 1

Found the answer here: https://peteohanlon.wordpress.com/2009/09/28/textbox-dragdrop-in-wpf/ Apparently, Drag Drop into a TextBox in WPF doesn’t actually work. When you attempt to drag and drop an item into a TextBox, it refuses to cooperate and leaves the mouse cursor as the Drop denied cursor and you can’t drop into the field. (Incidentally, this behaviour also applies to RichTextBox and FlowDocument controls). The reason that you can’t drop into these fields, even if you set AllowDrop to true, is that these particular controls mark drag and drop events as handled, preventing you from handling them yourself.