你可以使用“window_loaded”事件来聚焦你的文本框，这里有一个例子：

private void window_Loaded(object sender, RoutedEventArgs e)
    {
        textBox.Focus();
        textBox.SelectAll();
    }

我用一个变通方法解决了这个问题。在窗口启动期间设置TextBox的初始文本时，将激发OnTextBoxTextChanged事件。我只是抓住它，选择文本，然后删除事件。
与你的答案相比，黑暗圣堂武士的好处是，当我再次聚焦文本框时，例如。使用tab键，再次选择整个文本。

protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.GotFocus += OnTextBoxGotFocus;
        AssociatedObject.TextChanged += OnTextBoxTextChanged;
    }

    protected override void OnDetaching()
    {
        AssociatedObject.GotFocus -= OnTextBoxGotFocus;
        AssociatedObject.TextChanged -= OnTextBoxTextChanged;
        base.OnDetaching();
    }

    private void OnTextBoxGotFocus(object sender, RoutedEventArgs e)
    {
        AssociatedObject.SelectAll();
    }

    private void OnTextBoxTextChanged(object sender, RoutedEventArgs e)
    {
        AssociatedObject.SelectAll();
        AssociatedObject.TextChanged -= OnTextBoxTextChanged;
    }

为此，我创建了一个附加属性。

public static class TextBoxExtensions
{
    /// <summary>
    /// An attached property that, when set to true, causes a TextBox to select
    /// all its text when it receives focus. Assumes that the TextBox's lifecycle
    /// matches its parent user control's lifecycle.
    /// </summary>
    public static readonly DependencyProperty SelectTextOnFocusProperty = DependencyProperty.RegisterAttached(
        "SelectTextOnFocus",
        typeof(bool),
        typeof(TextBoxExtensions),
        new PropertyMetadata(false, OnSelectTextOnFocusChanged));

    public static bool GetSelectTextOnFocus(TextBox textBox) => (bool)textBox.GetValue(SelectTextOnFocusProperty);

    public static void SetSelectTextOnFocus(TextBox textBox, bool value) => textBox.SetValue(SelectTextOnFocusProperty, value);

    private static void OnSelectTextOnFocusChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is TextBox textBox && e.NewValue is bool b)
        {
            if (b) textBox.GotFocus += TextBox_GotFocus;
            else textBox.GotFocus -= TextBox_GotFocus;
        }

        void TextBox_GotFocus(object sender, RoutedEventArgs e)
        {
            textBox.SelectAll();
        }
    }
}

您在XAML中声明扩展的名称空间，我使用“ext”作为扩展。
然后，要应用它，只需设置属性。

<TextBox x:Name="_quantity" ext:TextBoxExtensions.SelectTextOnFocus="True" Grid.Column="1" Text="{Binding Quantity, UpdateSourceTrigger=PropertyChanged}" />

如果你只是在一个地方这样做，那就太过分了，但是如果你有一个表单，你想在很多文本框上都有这样的行为，那么这个带有样式的表单就很容易了。