Agregar un marcador de posición a un TextBox

En el ejemplo siguiente se muestra cómo mostrar el texto del marcador de posición en un TextBox cuando TextBox está vacío. TextBox Cuando tiene texto, el texto del marcador de posición está oculto. El texto del marcador de posición ayuda a los usuarios a comprender qué tipo de entrada TextBox espera.

En este artículo, aprenderá a:

• Cree una propiedad adjunta para proporcionar un texto de muestra.
• Cree un adorno para mostrar el texto del marcador de posición.
• Agregue la propiedad adjunta a un TextBox control .

Creación de una propiedad adjunta

Con las propiedades adjuntas, puede anexar valores a un control . Esta característica se usa mucho en WPF, como cuando se establecen Grid.Row propiedades o Panel.ZIndex en un control. Para obtener más información, vea Información general sobre las propiedades adjuntas. En este ejemplo se utilizan propiedades adjuntas para agregar texto de marcador de posición al TextBox.

1. Agregue una nueva clase a su proyecto denominado TextBoxHelper y ábralo.

2. Agregue los siguientes espacios de nombres:

   using System.Linq;
   using System.Windows;
   using System.Windows.Controls;
   using System.Windows.Documents;
   using System.Windows.Media;
   

   Imports System.Linq
   Imports System.Security.Cryptography
   Imports System.Windows
   Imports System.Windows.Controls
   Imports System.Windows.Documents
   Imports System.Windows.Media
   

3. Cree una nueva propiedad de dependencia denominada Placeholder.

   Esta propiedad de dependencia usa el delegado de devolución de llamada cambiado por la propiedad.

   public static string GetPlaceholder(DependencyObject obj) =>
       (string)obj.GetValue(PlaceholderProperty);
   
   public static void SetPlaceholder(DependencyObject obj, string value) =>
       obj.SetValue(PlaceholderProperty, value);
   
   public static readonly DependencyProperty PlaceholderProperty =
       DependencyProperty.RegisterAttached(
           "Placeholder",
           typeof(string),
           typeof(TextBoxHelper),
           new FrameworkPropertyMetadata(
               defaultValue: null,
               propertyChangedCallback: OnPlaceholderChanged)
           );
   

   Public Shared Function GetPlaceholder(obj As DependencyObject) As String
       Return obj.GetValue(PlaceholderProperty)
   End Function
   
   Public Shared Sub SetPlaceholder(obj As DependencyObject, value As String)
       obj.SetValue(PlaceholderProperty, value)
   End Sub
   
   Public Shared ReadOnly PlaceholderProperty As DependencyProperty =
       DependencyProperty.RegisterAttached(
           "Placeholder",
           GetType(String),
           GetType(TextBoxHelper),
           New FrameworkPropertyMetadata(
               defaultValue:=Nothing,
               propertyChangedCallback:=AddressOf OnPlaceholderChanged)
           )
   

4. Cree el método OnPlaceholderChanged para integrar la propiedad vinculada con un TextBox.

   private static void OnPlaceholderChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
   {
       if (d is TextBox textBoxControl)
       {
           if (!textBoxControl.IsLoaded)
           {
               // Ensure that the events are not added multiple times
               textBoxControl.Loaded -= TextBoxControl_Loaded;
               textBoxControl.Loaded += TextBoxControl_Loaded;
           }
   
           textBoxControl.TextChanged -= TextBoxControl_TextChanged;
           textBoxControl.TextChanged += TextBoxControl_TextChanged;
   
           // If the adorner exists, invalidate it to draw the current text
           if (GetOrCreateAdorner(textBoxControl, out PlaceholderAdorner adorner))
               adorner.InvalidateVisual();
       }
   }
   

   Private Shared Sub OnPlaceholderChanged(d As DependencyObject, e As DependencyPropertyChangedEventArgs)
       Dim textBoxControl = TryCast(d, TextBox)
   
       If textBoxControl IsNot Nothing Then
   
           If Not textBoxControl.IsLoaded Then
   
               'Ensure that the events are not added multiple times
               RemoveHandler textBoxControl.Loaded, AddressOf TextBoxControl_Loaded
               AddHandler textBoxControl.Loaded, AddressOf TextBoxControl_Loaded
   
           End If
   
           RemoveHandler textBoxControl.TextChanged, AddressOf TextBoxControl_TextChanged
           AddHandler textBoxControl.TextChanged, AddressOf TextBoxControl_TextChanged
   
           'If the adorner exists, invalidate it to draw the current text
           Dim adorner As PlaceholderAdorner = Nothing
           If GetOrCreateAdorner(textBoxControl, adorner) Then
               adorner.InvalidateVisual()
           End If
   
       End If
   
   End Sub
   

   Se llama a este método de dos maneras cuando cambia el valor de la propiedad adjunta:

   • Cuando la propiedad adjunta se agrega por primera vez a TextBox, se llama a este método. Esa acción proporciona una oportunidad para que la propiedad adjunta se integre con los eventos del control.
   • Cada vez que se cambia esta propiedad, el Adorner se puede invalidar para actualizar el texto del marcador de posición visual.

   El GetOrCreateAdorner método se crea en la sección siguiente.

5. Cree los controladores de eventos para el TextBox.

   private static void TextBoxControl_Loaded(object sender, RoutedEventArgs e)
   {
       if (sender is TextBox textBoxControl)
       {
           textBoxControl.Loaded -= TextBoxControl_Loaded;
           GetOrCreateAdorner(textBoxControl, out _);
       }
   }
   
   private static void TextBoxControl_TextChanged(object sender, TextChangedEventArgs e)
   {
       if (sender is TextBox textBoxControl
           && GetOrCreateAdorner(textBoxControl, out PlaceholderAdorner adorner))
       {
           // Control has text. Hide the adorner.
           if (textBoxControl.Text.Length > 0)
               adorner.Visibility = Visibility.Hidden;
   
           // Control has no text. Show the adorner.
           else
               adorner.Visibility = Visibility.Visible;
       }
   }
   

   Private Shared Sub TextBoxControl_Loaded(sender As Object, e As RoutedEventArgs)
       Dim textBoxControl As TextBox = TryCast(sender, TextBox)
   
       If textBoxControl IsNot Nothing Then
           RemoveHandler textBoxControl.Loaded, AddressOf TextBoxControl_Loaded
           GetOrCreateAdorner(textBoxControl, Nothing)
       End If
   End Sub
   
   Private Shared Sub TextBoxControl_TextChanged(sender As Object, e As TextChangedEventArgs)
       Dim textBoxControl As TextBox = TryCast(sender, TextBox)
       Dim adorner As PlaceholderAdorner = Nothing
   
       If textBoxControl IsNot Nothing AndAlso GetOrCreateAdorner(textBoxControl, adorner) Then
   
           If textBoxControl.Text.Length > 0 Then
               'Control has text. Hide the adorner.
               adorner.Visibility = Visibility.Hidden
           Else
               'Control has no text. Show the adorner.
               adorner.Visibility = Visibility.Visible
           End If
   
       End If
   End Sub
   

   Se controla el evento Loaded de manera que el adorno pueda crearse una vez que se haya aplicado la plantilla del control. El controlador se quita después de que se genere el evento y se cree el adorno.

   El evento TextChanged se gestiona para garantizar que el adorno se oculte o visualice dependiendo de si Text tiene un valor asignado.

Crear un adorno

Adorner es un objeto visual enlazado a un control y representado en un AdornerLayer. Para más información, consulte Información general sobre adornos.

1. Abra la TextBoxHelper clase .

2. Agregue el código siguiente para crear el GetOrCreateAdorner método .

   private static bool GetOrCreateAdorner(TextBox textBoxControl, out PlaceholderAdorner adorner)
   {
       // Get the adorner layer
       AdornerLayer layer = AdornerLayer.GetAdornerLayer(textBoxControl);
   
       // If null, it doesn't exist or the control's template isn't loaded
       if (layer == null)
       {
           adorner = null;
           return false;
       }
   
       // Layer exists, try to find the adorner
       adorner = layer.GetAdorners(textBoxControl)?.OfType<PlaceholderAdorner>().FirstOrDefault();
   
       // Adorner never added to control, so add it
       if (adorner == null)
       {
           adorner = new PlaceholderAdorner(textBoxControl);
           layer.Add(adorner);
       }
   
       return true;
   }
   

   Private Shared Function GetOrCreateAdorner(textBoxControl As TextBox, ByRef adorner As PlaceholderAdorner) As Boolean
   
       'Get the adorner layer
       Dim layer As AdornerLayer = AdornerLayer.GetAdornerLayer(textBoxControl)
   
       'If nothing, it doesn't exist or the control's template isn't loaded
       If layer Is Nothing Then
           adorner = Nothing
           Return False
       End If
   
       'Layer exists, try to find the adorner
       adorner = layer.GetAdorners(textBoxControl)?.OfType(Of PlaceholderAdorner)().FirstOrDefault()
   
       'Adorner never added to control, so add it
       If adorner Is Nothing Then
           adorner = New PlaceholderAdorner(textBoxControl)
           layer.Add(adorner)
       End If
   
       Return True
   
   End Function
   

   Este método proporciona una manera segura de agregar o recuperar el Adorner. Los adornos requieren seguridad extra porque se agregan al control AdornerLayer, el cual es posible que no exista. Cuando se aplica una propiedad adjunta XAML a un control, la plantilla del control aún no se ha aplicado para crear el árbol visual, por lo que la capa de adorno no existe. La capa del adorno debe recuperarse una vez que se haya cargado el control. Es posible que también falte la capa de adorno si se aplica una plantilla que omite la capa de adorno al control.

3. Agregue una clase secundaria denominada PlaceholderAdorner a la clase TextBoxHelper.

   public class PlaceholderAdorner : Adorner
   {
       public PlaceholderAdorner(TextBox textBox) : base(textBox) { }
   
       protected override void OnRender(DrawingContext drawingContext)
       {
           TextBox textBoxControl = (TextBox)AdornedElement;
   
           string placeholderValue = TextBoxHelper.GetPlaceholder(textBoxControl);
   
           if (string.IsNullOrEmpty(placeholderValue))
               return;
   
           // Create the formatted text object
           FormattedText text = new FormattedText(
                                       placeholderValue,
                                       System.Globalization.CultureInfo.CurrentCulture,
                                       textBoxControl.FlowDirection,
                                       new Typeface(textBoxControl.FontFamily,
                                                    textBoxControl.FontStyle,
                                                    textBoxControl.FontWeight,
                                                    textBoxControl.FontStretch),
                                       textBoxControl.FontSize,
                                       SystemColors.InactiveCaptionBrush,
                                       VisualTreeHelper.GetDpi(textBoxControl).PixelsPerDip);
   
           text.MaxTextWidth = System.Math.Max(textBoxControl.ActualWidth - textBoxControl.Padding.Left - textBoxControl.Padding.Right, 10);
           text.MaxTextHeight = System.Math.Max(textBoxControl.ActualHeight, 10);
   
           // Render based on padding of the control, to try and match where the textbox places text
           Point renderingOffset = new Point(textBoxControl.Padding.Left, textBoxControl.Padding.Top);
   
           // Template contains the content part; adjust sizes to try and align the text
           if (textBoxControl.Template.FindName("PART_ContentHost", textBoxControl) is FrameworkElement part)
           {
               Point partPosition = part.TransformToAncestor(textBoxControl).Transform(new Point(0, 0));
               renderingOffset.X += partPosition.X;
               renderingOffset.Y += partPosition.Y;
   
               text.MaxTextWidth = System.Math.Max(part.ActualWidth - renderingOffset.X, 10);
               text.MaxTextHeight = System.Math.Max(part.ActualHeight, 10);
           }
   
           // Draw the text
           drawingContext.DrawText(text, renderingOffset);
       }
   }
   

   Public Class PlaceholderAdorner
       Inherits Adorner
   
       Public Sub New(adornedElement As UIElement)
           MyBase.New(adornedElement)
       End Sub
   
       Protected Overrides Sub OnRender(drawingContext As DrawingContext)
           Dim textBoxControl As TextBox = DirectCast(AdornedElement, TextBox)
   
           Dim placeholderValue As String = TextBoxHelper.GetPlaceholder(textBoxControl)
   
           If String.IsNullOrEmpty(placeholderValue) Then
               Return
           End If
   
           'Create the formatted text object
           Dim text As New FormattedText(
               placeholderValue,
               System.Globalization.CultureInfo.CurrentCulture,
               textBoxControl.FlowDirection,
               New Typeface(textBoxControl.FontFamily,
                            textBoxControl.FontStyle,
                            textBoxControl.FontWeight,
                            textBoxControl.FontStretch),
               textBoxControl.FontSize,
               SystemColors.InactiveCaptionBrush,
               VisualTreeHelper.GetDpi(textBoxControl).PixelsPerDip)
   
           text.MaxTextWidth = Math.Max(textBoxControl.ActualWidth - textBoxControl.Padding.Left - textBoxControl.Padding.Right, 10)
           text.MaxTextHeight = Math.Max(textBoxControl.ActualHeight, 10)
   
           'Render based on padding of the control, to try and match where the textbox places text
           Dim renderingOffset As New Point(textBoxControl.Padding.Left, textBoxControl.Padding.Top)
   
           'Template contains the content part; adjust sizes to try and align the text
           Dim part As FrameworkElement = TryCast(textBoxControl.Template.FindName("PART_ContentHost", textBoxControl), FrameworkElement)
   
           If part IsNot Nothing Then
               Dim partPosition As Point = part.TransformToAncestor(textBoxControl).Transform(New Point(0, 0))
               renderingOffset.X += partPosition.X
               renderingOffset.Y += partPosition.Y
   
               text.MaxTextWidth = Math.Max(part.ActualWidth - renderingOffset.X, 10)
               text.MaxTextHeight = Math.Max(part.ActualHeight, 10)
           End If
   
           ' Draw the text
           drawingContext.DrawText(text, renderingOffset)
       End Sub
   
   End Class
   

   Un adorno hereda de la clase Adorner. Este adorno concreto invalida el OnRender(DrawingContext) método para dibujar el texto del marcador de posición. Vamos a desglosar el código:

   • En primer lugar, compruebe que el texto del marcador de posición existe llamando a TextBoxHelper.GetPlaceholder(textBoxControl).
   • Cree un objeto FormattedText. Este objeto contiene toda la información sobre qué texto se dibuja en el objeto visual.
   • Las propiedades FormattedText.MaxTextWidth y FormattedText.MaxTextHeight se establecen a la región del control. También se establecen un valor mínimo de 10 para asegurarse de que el FormattedText objeto es válido.
   • renderingOffset almacena la posición del texto dibujado.
   • Usa la PART_ContentHost si la plantilla del control lo declara. Esta parte representa dónde se dibuja el texto en la plantilla del control. Si se encuentra esa parte, modifique el renderingOffset para reflejar su posición.
   • Dibuja el texto llamando a DrawText(FormattedText, Point) y pasando el objeto FormattedText y la posición del texto.

Aplicar la propiedad adjunta

Una vez definida la propiedad adjunta, su espacio de nombres debe importarse en el XAML y, a continuación, hacer referencia a él en un control TextBox. El código siguiente asigna el espacio de nombres DotnetDocsSample de .NET al espacio de nombres lXML .

<Window x:Class="DotnetDocsSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:l="clr-namespace:DotnetDocsSample"
        Title="Recipe Tracker" Width="400" SizeToContent="Height">
    <StackPanel Margin="10">
        <TextBlock FontSize="20" TextWrapping="Wrap">Welcome to Recipe Tracker! To get started, create a new account.</TextBlock>
        
        <Label Padding="0,5">Name</Label>
        <TextBox l:TextBoxHelper.Placeholder="Ex. Jeffry Goh" />
        
        <Label Padding="0,5">Email</Label>
        <TextBox l:TextBoxHelper.Placeholder="jeffry@contoso.com" />
        
        <Label Padding="0,5">Password</Label>
        <PasswordBox />
        
        <Button HorizontalAlignment="Right" Width="100" Margin="0,10,0,5">Submit</Button>
    </StackPanel>
</Window>

La propiedad adjunta se agrega a TextBox usando la sintaxis xmlNamespace:Class.Property.

Ejemplo completo

El código siguiente es la clase completa TextBoxHelper .

public static class TextBoxHelper
{
    public static string GetPlaceholder(DependencyObject obj) =>
        (string)obj.GetValue(PlaceholderProperty);

    public static void SetPlaceholder(DependencyObject obj, string value) =>
        obj.SetValue(PlaceholderProperty, value);

    public static readonly DependencyProperty PlaceholderProperty =
        DependencyProperty.RegisterAttached(
            "Placeholder",
            typeof(string),
            typeof(TextBoxHelper),
            new FrameworkPropertyMetadata(
                defaultValue: null,
                propertyChangedCallback: OnPlaceholderChanged)
            );

    private static void OnPlaceholderChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is TextBox textBoxControl)
        {
            if (!textBoxControl.IsLoaded)
            {
                // Ensure that the events are not added multiple times
                textBoxControl.Loaded -= TextBoxControl_Loaded;
                textBoxControl.Loaded += TextBoxControl_Loaded;
            }

            textBoxControl.TextChanged -= TextBoxControl_TextChanged;
            textBoxControl.TextChanged += TextBoxControl_TextChanged;

            // If the adorner exists, invalidate it to draw the current text
            if (GetOrCreateAdorner(textBoxControl, out PlaceholderAdorner adorner))
                adorner.InvalidateVisual();
        }
    }

    private static void TextBoxControl_Loaded(object sender, RoutedEventArgs e)
    {
        if (sender is TextBox textBoxControl)
        {
            textBoxControl.Loaded -= TextBoxControl_Loaded;
            GetOrCreateAdorner(textBoxControl, out _);
        }
    }

    private static void TextBoxControl_TextChanged(object sender, TextChangedEventArgs e)
    {
        if (sender is TextBox textBoxControl
            && GetOrCreateAdorner(textBoxControl, out PlaceholderAdorner adorner))
        {
            // Control has text. Hide the adorner.
            if (textBoxControl.Text.Length > 0)
                adorner.Visibility = Visibility.Hidden;

            // Control has no text. Show the adorner.
            else
                adorner.Visibility = Visibility.Visible;
        }
    }

    private static bool GetOrCreateAdorner(TextBox textBoxControl, out PlaceholderAdorner adorner)
    {
        // Get the adorner layer
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(textBoxControl);

        // If null, it doesn't exist or the control's template isn't loaded
        if (layer == null)
        {
            adorner = null;
            return false;
        }

        // Layer exists, try to find the adorner
        adorner = layer.GetAdorners(textBoxControl)?.OfType<PlaceholderAdorner>().FirstOrDefault();

        // Adorner never added to control, so add it
        if (adorner == null)
        {
            adorner = new PlaceholderAdorner(textBoxControl);
            layer.Add(adorner);
        }

        return true;
    }

    public class PlaceholderAdorner : Adorner
    {
        public PlaceholderAdorner(TextBox textBox) : base(textBox) { }

        protected override void OnRender(DrawingContext drawingContext)
        {
            TextBox textBoxControl = (TextBox)AdornedElement;

            string placeholderValue = TextBoxHelper.GetPlaceholder(textBoxControl);

            if (string.IsNullOrEmpty(placeholderValue))
                return;

            // Create the formatted text object
            FormattedText text = new FormattedText(
                                        placeholderValue,
                                        System.Globalization.CultureInfo.CurrentCulture,
                                        textBoxControl.FlowDirection,
                                        new Typeface(textBoxControl.FontFamily,
                                                     textBoxControl.FontStyle,
                                                     textBoxControl.FontWeight,
                                                     textBoxControl.FontStretch),
                                        textBoxControl.FontSize,
                                        SystemColors.InactiveCaptionBrush,
                                        VisualTreeHelper.GetDpi(textBoxControl).PixelsPerDip);

            text.MaxTextWidth = System.Math.Max(textBoxControl.ActualWidth - textBoxControl.Padding.Left - textBoxControl.Padding.Right, 10);
            text.MaxTextHeight = System.Math.Max(textBoxControl.ActualHeight, 10);

            // Render based on padding of the control, to try and match where the textbox places text
            Point renderingOffset = new Point(textBoxControl.Padding.Left, textBoxControl.Padding.Top);

            // Template contains the content part; adjust sizes to try and align the text
            if (textBoxControl.Template.FindName("PART_ContentHost", textBoxControl) is FrameworkElement part)
            {
                Point partPosition = part.TransformToAncestor(textBoxControl).Transform(new Point(0, 0));
                renderingOffset.X += partPosition.X;
                renderingOffset.Y += partPosition.Y;

                text.MaxTextWidth = System.Math.Max(part.ActualWidth - renderingOffset.X, 10);
                text.MaxTextHeight = System.Math.Max(part.ActualHeight, 10);
            }

            // Draw the text
            drawingContext.DrawText(text, renderingOffset);
        }
    }
}

Public Class TextBoxHelper

    Public Shared Function GetPlaceholder(obj As DependencyObject) As String
        Return obj.GetValue(PlaceholderProperty)
    End Function

    Public Shared Sub SetPlaceholder(obj As DependencyObject, value As String)
        obj.SetValue(PlaceholderProperty, value)
    End Sub

    Public Shared ReadOnly PlaceholderProperty As DependencyProperty =
        DependencyProperty.RegisterAttached(
            "Placeholder",
            GetType(String),
            GetType(TextBoxHelper),
            New FrameworkPropertyMetadata(
                defaultValue:=Nothing,
                propertyChangedCallback:=AddressOf OnPlaceholderChanged)
            )

    Private Shared Sub OnPlaceholderChanged(d As DependencyObject, e As DependencyPropertyChangedEventArgs)
        Dim textBoxControl = TryCast(d, TextBox)

        If textBoxControl IsNot Nothing Then

            If Not textBoxControl.IsLoaded Then

                'Ensure that the events are not added multiple times
                RemoveHandler textBoxControl.Loaded, AddressOf TextBoxControl_Loaded
                AddHandler textBoxControl.Loaded, AddressOf TextBoxControl_Loaded

            End If

            RemoveHandler textBoxControl.TextChanged, AddressOf TextBoxControl_TextChanged
            AddHandler textBoxControl.TextChanged, AddressOf TextBoxControl_TextChanged

            'If the adorner exists, invalidate it to draw the current text
            Dim adorner As PlaceholderAdorner = Nothing
            If GetOrCreateAdorner(textBoxControl, adorner) Then
                adorner.InvalidateVisual()
            End If

        End If

    End Sub

    Private Shared Sub TextBoxControl_Loaded(sender As Object, e As RoutedEventArgs)
        Dim textBoxControl As TextBox = TryCast(sender, TextBox)

        If textBoxControl IsNot Nothing Then
            RemoveHandler textBoxControl.Loaded, AddressOf TextBoxControl_Loaded
            GetOrCreateAdorner(textBoxControl, Nothing)
        End If
    End Sub

    Private Shared Sub TextBoxControl_TextChanged(sender As Object, e As TextChangedEventArgs)
        Dim textBoxControl As TextBox = TryCast(sender, TextBox)
        Dim adorner As PlaceholderAdorner = Nothing

        If textBoxControl IsNot Nothing AndAlso GetOrCreateAdorner(textBoxControl, adorner) Then

            If textBoxControl.Text.Length > 0 Then
                'Control has text. Hide the adorner.
                adorner.Visibility = Visibility.Hidden
            Else
                'Control has no text. Show the adorner.
                adorner.Visibility = Visibility.Visible
            End If

        End If
    End Sub


    Private Shared Function GetOrCreateAdorner(textBoxControl As TextBox, ByRef adorner As PlaceholderAdorner) As Boolean

        'Get the adorner layer
        Dim layer As AdornerLayer = AdornerLayer.GetAdornerLayer(textBoxControl)

        'If nothing, it doesn't exist or the control's template isn't loaded
        If layer Is Nothing Then
            adorner = Nothing
            Return False
        End If

        'Layer exists, try to find the adorner
        adorner = layer.GetAdorners(textBoxControl)?.OfType(Of PlaceholderAdorner)().FirstOrDefault()

        'Adorner never added to control, so add it
        If adorner Is Nothing Then
            adorner = New PlaceholderAdorner(textBoxControl)
            layer.Add(adorner)
        End If

        Return True

    End Function

    Public Class PlaceholderAdorner
        Inherits Adorner

        Public Sub New(adornedElement As UIElement)
            MyBase.New(adornedElement)
        End Sub

        Protected Overrides Sub OnRender(drawingContext As DrawingContext)
            Dim textBoxControl As TextBox = DirectCast(AdornedElement, TextBox)

            Dim placeholderValue As String = TextBoxHelper.GetPlaceholder(textBoxControl)

            If String.IsNullOrEmpty(placeholderValue) Then
                Return
            End If

            'Create the formatted text object
            Dim text As New FormattedText(
                placeholderValue,
                System.Globalization.CultureInfo.CurrentCulture,
                textBoxControl.FlowDirection,
                New Typeface(textBoxControl.FontFamily,
                             textBoxControl.FontStyle,
                             textBoxControl.FontWeight,
                             textBoxControl.FontStretch),
                textBoxControl.FontSize,
                SystemColors.InactiveCaptionBrush,
                VisualTreeHelper.GetDpi(textBoxControl).PixelsPerDip)

            text.MaxTextWidth = Math.Max(textBoxControl.ActualWidth - textBoxControl.Padding.Left - textBoxControl.Padding.Right, 10)
            text.MaxTextHeight = Math.Max(textBoxControl.ActualHeight, 10)

            'Render based on padding of the control, to try and match where the textbox places text
            Dim renderingOffset As New Point(textBoxControl.Padding.Left, textBoxControl.Padding.Top)

            'Template contains the content part; adjust sizes to try and align the text
            Dim part As FrameworkElement = TryCast(textBoxControl.Template.FindName("PART_ContentHost", textBoxControl), FrameworkElement)

            If part IsNot Nothing Then
                Dim partPosition As Point = part.TransformToAncestor(textBoxControl).Transform(New Point(0, 0))
                renderingOffset.X += partPosition.X
                renderingOffset.Y += partPosition.Y

                text.MaxTextWidth = Math.Max(part.ActualWidth - renderingOffset.X, 10)
                text.MaxTextHeight = Math.Max(part.ActualHeight, 10)
            End If

            ' Draw the text
            drawingContext.DrawText(text, renderingOffset)
        End Sub

    End Class
End Class

Consulte también

• Información general sobre TextBox
• Información general sobre adornos
• Introducción a las propiedades adjuntas