不要使用DependencyPropertyDescriptor来注册一个属性改变的回调。如果你不小心（你的代码就是这种情况），它将创建一个（潜在的严重）每个示例内存泄漏。
最好在static构造函数中重写dependency属性的元数据，以注册新的回调（参见下面的示例）。
TextBlock不支持自定义字符间距。TextBlock完全依赖提供的字体（或字样）来计算实际文本宽度。由于MeasureOverride是密封的，因此在更改Text值或任何与文本相关的属性（例如FontSize等）后，通常必须强制使用新的Width。
这意味着，要正确实现大小调整行为，您必须重写所有相关属性的属性元数据，以触发实际NewTextBlock宽度的重新计算。您可以遵循以下示例中TextBlock.TextProperty属性的元数据重写。
要计算格式化字符串的宽度，通常使用FormattedText类（见下文），在这种情况下，必须将当前字符间距包括在计算中。

public class NewTextBlock : TextBlock
{
  public double LetterSpacing
  {
    get => (double)GetValue(LetterSpacingProperty);
    set => SetValue(LetterSpacingProperty, value);
  }

  public static readonly DependencyProperty LetterSpacingProperty = DependencyProperty.Register(
    "LetterSpacing", 
    typeof(double), 
    typeof(NewTextBlock), 
    new FrameworkPropertyMetadata(default(double), OnLetterSpacingChanged));

  static NewTextBlock() 
  {
    TextProperty.OverrideMetadata(
      typeof(NewTextBlock), 
      new FrameworkPropertyMetadata(default(string), OnTextChanged));
  }

  public NewTextBlock() 
  {
    this.TextEffects = new TextEffectCollection();
  }

  private static void OnTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    => (d as NewTextBlock).ApplyTextEffects();

  private static void OnLetterSpacingChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) 
    => (d as NewTextBlock).ApplyTextEffects();

  private void ApplyTextEffects()
  {
    if (string.IsNullOrWhiteSpace(this.Text))
    {
      return;
    }

    this.TextEffects.Clear();
    for (int characterIndex = 1; characterIndex <= this.Text.Length; characterIndex++)
    {
      var transform = new TranslateTransform(this.LetterSpacing, 0);
      var effect = new TextEffect
      {
        Transform = transform,
        PositionStart = characterIndex,
        PositionCount = this.Text.Length
      };

      this.TextEffects.Add(effect);
      if (effect.CanFreeze)
      {
        effect.Freeze();
      }
    }

    AdjustCurrentWidth();
  }

  private void AdjustCurrentWidth()
  {
    var typeface = new Typeface(this.FontFamily, this.FontStyle, this.FontWeight, this.FontStretch);
    double pixelsPerDip = VisualTreeHelper.GetDpi(this).PixelsPerDip;
    var formattedText = new FormattedText(this.Text, CultureInfo.CurrentCulture, FlowDirection.LeftToRight, typeface, this.FontSize, this.Foreground, pixelsPerDip);
    double totalLetterSpacingWidth = (this.Text.Length - 1) * this.LetterSpacing;
    double totalTextWidth = formattedText.WidthIncludingTrailingWhitespace + totalLetterSpacingWidth;

    this.Width = totalTextWidth;
  }
}

也许我错过了这个STextBlock的要点，但在我看来，您可以（也许应该）只使用Glyphs框架元素来代替。
https://learn.microsoft.com/en-us/dotnet/desktop/wpf/advanced/introduction-to-the-glyphrun-object-and-glyphs-element?view=netframeworkdesktop-4.8
https://learn.microsoft.com/en-us/dotnet/api/system.windows.documents.glyphs?view=windowsdesktop-7.0
乙二醇

<!-- "Hello World!" with explicit character widths for proportional font -->
<Glyphs 
   FontUri             = "C:\WINDOWS\Fonts\ARIAL.TTF"
   FontRenderingEmSize = "36"
   UnicodeString       = "Hello World!"
   Indices             = ",80;,80;,80;,80;,80;,80;,80;,80;,80;,80;,80"
   Fill                = "Maroon"
   OriginX             = "50"
   OriginY             = "225"
/>

<!-- "Hello World!" with fixed-width font -->
<Glyphs 
   FontUri             = "C:\WINDOWS\Fonts\COUR.TTF"
   FontRenderingEmSize = "36"
   StyleSimulations    = "BoldSimulation"
   UnicodeString       = "Hello World!"
   Fill                = "Maroon"
   OriginX             = "50"
   OriginY             = "300"
/>

我从@BionicCode的代码中找到了另一个解决方案

protected override void OnRender(DrawingContext drawingContext)
        {
            base.OnRender(drawingContext);

            if (!string.IsNullOrEmpty(Text))
            {                
                double PositionX = Padding.Left;
                double PositionY = Padding.Top;
                foreach (var i in Text)
                {
                    var FT = new FormattedText(i.ToString(), CultureInfo.CurrentUICulture, FlowDirection.LeftToRight, new Typeface(FontFamily,FontStyle,FontWeight,FontStretch), FontSize, Foreground);                    
                    drawingContext.DrawText(FT, new Point(PositionX, PositionY));
                    PositionX += FT.Width+LetterSpacing;
                    var t = Width;                    
                }
            }
        }