public IFormatProvider ContentFormatProvider { get; set; }

Public Property ContentFormatProvider As IFormatProvider

<dx:SimpleButton
   HorizontalAlignment="Center"
   VerticalAlignment="Center"
   Glyph="{dx:SvgImageSource Uri=Glyph_Message.svg}">
   <dx:Badge.Badge>
       <dx:Badge Content="150" BadgeKind="Error"
                 ContentStringFormat="{}{0:length2}"
                 ContentFormatProvider="{local:BadgeFormatProvider}"/>
   </dx:Badge.Badge>
</dx:SimpleButton>

public class BadgeFormatProvider : MarkupExtension, IFormatProvider, ICustomFormatter {
   static readonly Regex formatExpression = new Regex(@"length(\d+)", RegexOptions.Compiled);

   public object GetFormat(Type formatType) { return this; }
   public override object ProvideValue(IServiceProvider serviceProvider) { return this; }
   public string Format(string format, object arg, IFormatProvider formatProvider) {
       if(arg==null)
           return String.Empty;

       var match = formatExpression.Match(format);
       if(!match.Success)
           return String.Format(CultureInfo.CurrentCulture, format ?? "{0}", arg);

       var length = int.Parse(match.Groups[1].Value);
       var inputString = Convert.ToString(arg);
       if (inputString.Length <= length)
           return inputString;

       if (decimal.TryParse(inputString, out _))
           return (Math.Pow(10, length) - 1) + "+";

       return inputString.Substring(0, length) + "...";
   }
}

Public Class BadgeFormatProvider
    Inherits MarkupExtension
    Implements IFormatProvider, ICustomFormatter

    Shared ReadOnly formatExpression As Regex = New Regex("length(\d+)", RegexOptions.Compiled)

    Public Function GetFormat(ByVal formatType As Type) As Object
        Return Me
    End Function

    Public Overrides Function ProvideValue(ByVal serviceProvider As IServiceProvider) As Object
        Return Me
    End Function

    Public Function Format(ByVal format As String, ByVal arg As Object, ByVal formatProvider As IFormatProvider) As String
        If arg Is Nothing Then Return String.Empty
        Dim match = formatExpression.Match(format)
        If Not match.Success Then Return String.Format(CultureInfo.CurrentCulture, If(format, "{0}"), arg)
        Dim length = Integer.Parse(match.Groups(1).Value)
        Dim inputString = Convert.ToString(arg)
        If inputString.Length <= length Then Return inputString
        If Decimal.TryParse(inputString, __) Then Return (Math.Pow(10, length) - 1) & "+"
        Return inputString.Substring(0, length) & "..."
    End Function
End Class