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();
}

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();
    }
}

我们有它，所以第一次点击选择所有，另一次点击光标(我们的应用程序是设计用于平板电脑与笔)。

你可能会发现它很有用。

public class ClickSelectTextBox : TextBox
{
    public ClickSelectTextBox()
    {
        AddHandler(PreviewMouseLeftButtonDownEvent, 
          new MouseButtonEventHandler(SelectivelyIgnoreMouseButton), true);
        AddHandler(GotKeyboardFocusEvent, 
          new RoutedEventHandler(SelectAllText), true);
        AddHandler(MouseDoubleClickEvent, 
          new RoutedEventHandler(SelectAllText), true);
    }

    private static void SelectivelyIgnoreMouseButton(object sender, 
                                                     MouseButtonEventArgs e)
    {
        // Find the TextBox
        DependencyObject parent = e.OriginalSource as UIElement;
        while (parent != null && !(parent is TextBox))
            parent = VisualTreeHelper.GetParent(parent);

        if (parent != null)
        {
            var textBox = (TextBox)parent;
            if (!textBox.IsKeyboardFocusWithin)
            {
                // If the text box is not yet focussed, give it the focus and
                // stop further processing of this click event.
                textBox.Focus();
                e.Handled = true;
            }
        }
    }

    private static void SelectAllText(object sender, RoutedEventArgs e)
    {
        var textBox = e.OriginalSource as TextBox;
        if (textBox != null)
            textBox.SelectAll();
    }
}

哇!读完以上所有内容后，我发现自己不知所措。我从这篇文章中学到了一些东西，并尝试了一些完全不同的东西。当文本框获得焦点时，我使用这个来选择文本:

private void TextField_GotFocus(object sender, RoutedEventArgs e)
{
    TextBox tb = (sender as Textbox);
    if(tb != null)
    {
        e.Handled = true;
        tb.Focus();
        tb.SelectAll();
    }
}

将文本框的GotFocus属性设置为该方法。

运行应用程序并在文本框中单击一次，将突出显示文本框中已经存在的所有内容。

如果目标确实是在用户单击文本框时选择文本，这看起来很简单，涉及的代码也少了很多。只是说……

在MSDN上有一个非常好的非常简单的解决方案:

<TextBox
    MouseDoubleClick="SelectAddress"
    GotKeyboardFocus="SelectAddress"
    PreviewMouseLeftButtonDown="SelectivelyIgnoreMouseButton" />

下面是背后的代码:

private void SelectAddress(object sender, RoutedEventArgs e)
{
    TextBox tb = (sender as TextBox);
    if (tb != null)
    {
        tb.SelectAll();
    }
}

private void SelectivelyIgnoreMouseButton(object sender,
    MouseButtonEventArgs e)
{
    TextBox tb = (sender as TextBox);
    if (tb != null)
    {
        if (!tb.IsKeyboardFocusWithin)
        {
            e.Handled = true;
            tb.Focus();
        }
    }
}

尝试这个扩展方法，添加所需的行为到任何文本框控件。我还没有对它进行广泛测试，但它似乎满足了我的需求。

public static class TextBoxExtensions
{
    public static void SetupSelectAllOnGotFocus(this TextBox source)
    {
        source.GotFocus += SelectAll;
        source.PreviewMouseLeftButtonDown += SelectivelyIgnoreMouseButton;
    }

    private static void SelectAll(object sender, RoutedEventArgs e)
    {
        var textBox = e.OriginalSource as TextBox;
        if (textBox != null)
            textBox.SelectAll();
    }

    private static void SelectivelyIgnoreMouseButton(object sender, MouseButtonEventArgs e)
    {
        var textBox = (sender as TextBox);
        if (textBox != null)
        {
            if (!textBox.IsKeyboardFocusWithin)
            {
                e.Handled = true;
                textBox.Focus();
            }
        }
    }
}