WPF TextBox实现按字节长度限制输入功能

public class MaxByteAttachedProperty : DependencyObject
 {
 public enum Encode
 {
  Default,ASCII,UTF8,UTF32,UTF7,BigEndianUnicode,Unicode
 }


 private static string GetPreText(DependencyObject obj)
 {
  return (string)obj.GetValue(PreTextProperty);
 }

 private static void SetPreText(DependencyObject obj,string value)
 {
  obj.SetValue(PreTextProperty,value);
 }

 // Using a DependencyProperty as the backing store for PreText. This enables animation,styling,binding,etc...
 private static readonly DependencyProperty PreTextProperty =
  DependencyProperty.RegisterAttached("PreText",typeof(string),typeof(MaxByteAttachedProperty),new PropertyMetadata(""));


 public static int GetMaxByteLength(DependencyObject obj)
 {
  return (int)obj.GetValue(MaxByteLengthProperty);
 }

 public static void SetMaxByteLength(DependencyObject obj,int value)
 {
  obj.SetValue(MaxByteLengthProperty,value);
 }

 // Using a DependencyProperty as the backing store for MaxByteLength. This enables animation,etc...
 public static readonly DependencyProperty MaxByteLengthProperty =
  DependencyProperty.RegisterAttached("MaxByteLength",typeof(int),new PropertyMetadata(OnTextBoxPropertyChanged));

 private static void OnTextBoxPropertyChanged(DependencyObject d,DependencyPropertyChangedEventArgs e)
 {
  TextBox tb = d as TextBox;
  if (tb == null)
  {
  return;
  }
  tb.TextChanged += Tb_TextChanged;
 }

 private static void Tb_TextChanged(object sender,TextChangedEventArgs e)
 {
  TextBox tb = sender as TextBox;
  if (IsOutMaxByteLength(tb.Text,tb))
  {
  tb.Text = GetPreText(tb);
  tb.Select(tb.Text.Length,0);
  return;
  }
 }

 public static Encode GetEncodeModel(DependencyObject obj)
 {
  return (Encode)obj.GetValue(EncodeModelProperty);
 }

 public static void SetEncodeModel(DependencyObject obj,Encode value)
 {
  obj.SetValue(EncodeModelProperty,value);
 }

 // Using a DependencyProperty as the backing store for EncodeM. This enables animation,etc...
 public static readonly DependencyProperty EncodeModelProperty =
  DependencyProperty.RegisterAttached("EncodeModel",typeof(Encode),new PropertyMetadata(Encode.UTF8,OnEncodeModelChanged));
 private static void OnEncodeModelChanged(DependencyObject d,DependencyPropertyChangedEventArgs e)
 {
  SetEM(d,GetEncodeModel(d));
 }

 private static Encoding GetEncodingModel(DependencyObject obj)
 {
  return (Encoding)obj.GetValue(EncodingModelProperty);
 }

 private static void SetEncodingModel(DependencyObject obj,Encoding value)
 {
  obj.SetValue(EncodingModelProperty,value);
 }

 // Using a DependencyProperty as the backing store for EncodingModel. This enables animation,etc...
 private static readonly DependencyProperty EncodingModelProperty =
  DependencyProperty.RegisterAttached("EncodingModel",typeof(Encoding),new PropertyMetadata(Encoding.UTF8));

 private static void SetEM(DependencyObject obj,Encode e)
 {
  switch (e)
  {
  case Encode.Default:
   SetEncodingModel(obj,Encoding.Default);
   break;
  case Encode.ASCII:
   SetEncodingModel(obj,Encoding.ASCII);
   break;
  case Encode.UTF8:
   SetEncodingModel(obj,Encoding.UTF8);
   break;
  case Encode.UTF32:
   SetEncodingModel(obj,Encoding.UTF32);
   break;
  case Encode.UTF7:
   SetEncodingModel(obj,Encoding.UTF7);
   break;
  case Encode.BigEndianUnicode:
   SetEncodingModel(obj,Encoding.BigEndianUnicode);
   break;
  case Encode.Unicode:
   SetEncodingModel(obj,Encoding.Unicode);
   break;
  default:
   break;
  }
 }

 private static bool IsOutMaxByteLength(string txt,DependencyObject obj)
 {
  int txtLength = GetEncodingModel(obj).GetBytes(txt).Length;//文本长度
  if (GetMaxByteLength(obj) >= txtLength)
  {
  SetPreText(obj,txt);
  return false;
  }
  return true;
 }
 }