MyTextBox.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Controls;
using System.Windows.Media;

namespace Rendertest
{
    public class MyTextbox : TextBox
    {
        public MyTextbox()
        {
            Text = "text";
        }

        protected override void OnRender(System.Windows.Media.DrawingContext drawingContext)
        {
            //base.OnRender(drawingContext);
            this.BorderBrush = Brushes.Red;
        }
    }
}