我认为这很有效:

private void ValueText_GotFocus(object sender, RoutedEventArgs e)
{
    TextBox tb = (TextBox)e.OriginalSource;
    tb.Dispatcher.BeginInvoke(
        new Action(delegate
            {
                tb.SelectAll();
            }), System.Windows.Threading.DispatcherPriority.Input);
}

如果你想实现它作为一个扩展方法:

public static void SelectAllText(this System.Windows.Controls.TextBox tb)
{
    tb.Dispatcher.BeginInvoke(
        new Action(delegate
        {
            tb.SelectAll();
        }), System.Windows.Threading.DispatcherPriority.Input);
}

在GotFocus事件中:

private void ValueText_GotFocus(object sender, RoutedEventArgs e)
{
    TextBox tb = (TextBox)e.OriginalSource;
    tb.SelectAllText();
}

我发现上面的解决方案是因为几个月前我正在寻找一种方法来将焦点设置到给定的UIElement上。我在某处发现了下面的代码(特此给予)，它工作得很好。尽管它与OP的问题没有直接关系，但我还是发布了它，因为它演示了使用Dispatcher处理UIElement的相同模式。

// Sets focus to uiElement
public static void DelayedFocus(this UIElement uiElement)
{
    uiElement.Dispatcher.BeginInvoke(
    new Action(delegate
    {
        uiElement.Focusable = true;
        uiElement.Focus();
        Keyboard.Focus(uiElement);
    }),
    DispatcherPriority.Render);
}

#region TextBoxIDCard selection
private bool textBoxIDCardGotFocus = false;
private void TextBoxIDCard_GotFocus(object sender, RoutedEventArgs e)
{
    this.TextBoxIDCard.SelectAll();
}

private void TextBoxIDCard_LostFocus(object sender, RoutedEventArgs e)
{
    textBoxIDCardGotFocus = false;
}

private void TextBoxIDCard_PreviewMouseDown(object sender, MouseButtonEventArgs e)
{
    if (textBoxIDCardGotFocus == false)
    {
        e.Handled = true;
        this.TextBoxIDCard.Focus();
        textBoxIDCardGotFocus = true;
    }
} 
#endregion

下面是为方便您实现答案解决方案的Blend行为:

一个用于附加到单个文本框:

public class SelectAllTextOnFocusBehavior : Behavior<TextBox>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.GotKeyboardFocus += AssociatedObjectGotKeyboardFocus;
        AssociatedObject.GotMouseCapture += AssociatedObjectGotMouseCapture;
        AssociatedObject.PreviewMouseLeftButtonDown += AssociatedObjectPreviewMouseLeftButtonDown;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        AssociatedObject.GotKeyboardFocus -= AssociatedObjectGotKeyboardFocus;
        AssociatedObject.GotMouseCapture -= AssociatedObjectGotMouseCapture;
        AssociatedObject.PreviewMouseLeftButtonDown -= AssociatedObjectPreviewMouseLeftButtonDown;
    }

    private void AssociatedObjectGotKeyboardFocus(object sender,
        System.Windows.Input.KeyboardFocusChangedEventArgs e)
    {
        AssociatedObject.SelectAll();
    }

    private void AssociatedObjectGotMouseCapture(object sender,
        System.Windows.Input.MouseEventArgs e)
    {
        AssociatedObject.SelectAll();   
    }

    private void AssociatedObjectPreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        if(!AssociatedObject.IsKeyboardFocusWithin)
        {
            AssociatedObject.Focus();
            e.Handled = true;
        }
    }
}

一个用于附加到包含多个文本框的容器的根:

public class SelectAllTextOnFocusMultiBehavior : Behavior<UIElement>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.GotKeyboardFocus += HandleKeyboardFocus;
        AssociatedObject.GotMouseCapture += HandleMouseCapture;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        AssociatedObject.GotKeyboardFocus -= HandleKeyboardFocus;
        AssociatedObject.GotMouseCapture -= HandleMouseCapture;
    }

    private static void HandleKeyboardFocus(object sender,
        System.Windows.Input.KeyboardFocusChangedEventArgs e)
    {
        var txt = e.NewFocus as TextBox;
        if (txt != null)
            txt.SelectAll();
    }

    private static void HandleMouseCapture(object sender,
        System.Windows.Input.MouseEventArgs e)
    {
        var txt = e.OriginalSource as TextBox;
        if (txt != null)
            txt.SelectAll();
    }
}

我搜索了很多解决方案，我发现了两个解决方案选择但是，问题是，当我们做右键单击，并从文本框选择部分文本后进行剪切/复制，它选择了所有甚至我选择了部分文本。解决办法就在这里。只需在键盘选择事件中添加以下代码。这对我很管用。

private static void SelectContentsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (d is TextBox)
    {
        TextBox textBox = d as TextBox;
        if ((e.NewValue as bool?).GetValueOrDefault(false))
        {
            textBox.GotKeyboardFocus += OnKeyboardFocusSelectText;                 
        }
        else
        {
            textBox.GotKeyboardFocus -= OnKeyboardFocusSelectText;

        }
    }
}


private static void OnKeyboardFocusSelectText(object sender, KeyboardFocusChangedEventArgs e)
{
    if (e.KeyboardDevice.IsKeyDown(Key.Tab))
        ((TextBox)sender).SelectAll();
}

从这里开始:

在App.xaml.cs文件中注册全局事件处理程序:

protected override void OnStartup(StartupEventArgs e)
{
    EventManager.RegisterClassHandler(typeof(TextBox),TextBox.GotFocusEvent,
    new RoutedEventHandler(TextBox_GotFocus));

    base.OnStartup(e);
}

那么处理程序就像这样简单:

private void TextBox_GotFocus(object sender, RoutedEventArgs e)
{
    (sender as TextBox).SelectAll();
}

我知道这是非常古老的，但这是我的解决方案，它是基于表达式/微软交互性和交互名称空间。

首先，我按照此链接中的说明将交互性触发器放入样式中。

然后就归结为这个

<Style x:Key="baseTextBox" TargetType="TextBox">
  <Setter Property="gint:InteractivityItems.Template">
    <Setter.Value>
      <gint:InteractivityTemplate>
        <gint:InteractivityItems>
          <gint:InteractivityItems.Triggers>
            <i:EventTrigger EventName="GotKeyboardFocus">
              <ei:CallMethodAction MethodName="SelectAll"/>
            </i:EventTrigger>
            <i:EventTrigger EventName="PreviewMouseLeftButtonDown">
              <ei:CallMethodAction MethodName="TextBox_PreviewMouseLeftButtonDown"
                TargetObject="{Binding ElementName=HostElementName}"/>
            </i:EventTrigger>
          </gint:InteractivityItems.Triggers>
        </gint:InteractivityItems>
      </gint:InteractivityTemplate>
    </Setter.Value>
  </Setter>
</Style>

这

public void TextBox_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
  TextBox tb = e.Source as TextBox;
  if((tb != null) && (tb.IsKeyboardFocusWithin == false))
  {
    tb.Focus();
    e.Handled = true;
  }
}

在我的例子中，我有一个用户控件，其中的文本框有一个隐藏代码。隐藏代码具有处理程序函数。我在XAML中为用户控件设置了一个名称，并将该名称用于元素。这对我来说非常有效。简单地应用样式的任何文本框，你想有所有的文本选择时，点击在文本框。

当TextBox上的GotKeyboardFocus事件触发时，第一个CallMethodAction调用文本框的SelectAll方法。

我希望这能有所帮助。