public class TextboxTrimSpaceing : TextBox
    {

        private string myVar;

        new public string Text
        {
            get { return myVar; }
            set { myVar = value.TrimEnd().TrimStart(); }//Control is not cmg here
        }

    }

<local:TextboxTrimSpaceing x:Name="TrimSpaceing" Text="   avi  aaa   "></local:TextboxTrimSpaceing>

  var i = TrimSpaceing.Text; //Getting Null

public class TextboxTrimSpacing : TextBox
{
    private bool _trim = true;
    protected override void OnTextChanged(TextChangedEventArgs e)
    {
        base.OnTextChanged(e);
        if(_trim)
        {
            _trim = false;
            Text = Text?.Trim();
            _trim = true;
        }
    }
}