TextBlockAttach.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using HandyControl.Data;
using HandyControl.Tools.Helper;

namespace HandyControl.Controls
{
    public class TextBlockAttach
    {
        public static readonly DependencyProperty AutoTooltipProperty = DependencyProperty.RegisterAttached(
            "AutoTooltip", typeof(bool), typeof(TextBlockAttach), new PropertyMetadata(ValueBoxes.FalseBox, OnAutoTooltipChanged));

        private static void OnAutoTooltipChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is TextBlock ctl)
            {
                if ((bool) e.NewValue)
                {
                    UpdateTooltip(ctl);
                    ctl.SizeChanged += TextBlock_SizeChanged;
                }
                else
                {
                    ctl.SizeChanged -= TextBlock_SizeChanged;
                }
            }
        }

        public static void SetAutoTooltip(DependencyObject element, bool value)
            => element.SetValue(AutoTooltipProperty, ValueBoxes.BooleanBox(value));

        public static bool GetAutoTooltip(DependencyObject element)
            => (bool) element.GetValue(AutoTooltipProperty);

        private static void TextBlock_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            if (sender is TextBlock textBlock)
            {
                UpdateTooltip(textBlock);
            }
        }

        private static void UpdateTooltip(TextBlock textBlock)
        {
            textBlock.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
            var width = textBlock.DesiredSize.Width - textBlock.Margin.Left - textBlock.Margin.Right;

            if(textBlock.RenderSize.Width > width || textBlock.ActualWidth < width || CalcTextWidth(textBlock) > width)
            {
                ToolTipService.SetToolTip(textBlock, textBlock.Text);
            }
            else
            {
                ToolTipService.SetToolTip(textBlock, null);
            }
        }

        private static double CalcTextWidth(TextBlock textBlock)
        {
            var formattedText = TextHelper.CreateFormattedText(textBlock.Text, textBlock.FlowDirection,
                new Typeface(textBlock.FontFamily, textBlock.FontStyle, textBlock.FontWeight, textBlock.FontStretch),
                textBlock.FontSize);

            return formattedText.WidthIncludingTrailingWhitespace;
        }
    }
}