C#

using System;

namespace WindowsApplication1
{
	public class ValidatingTextBox : System.Windows.Forms.TextBox
	{
		private bool			BlockTextChanged = false;
		private const string	EnterString = @"\r\n";

		public ValidatingTextBox()
		{
		}

		protected override void OnTextChanged(EventArgs e)
		{
			if(!BlockTextChanged)
			{
				BlockTextChanged = true;
				if(this.Text != string.Empty)
				{
					for(int x = Text.IndexOf(EnterString,0); x<this.Text.Length && x>-1; x = Text.IndexOf(EnterString,x))
					{
						SendEnterAtIndex(x);
					}
				}
				BlockTextChanged = false;
			}
			base.OnTextChanged(e);
		}

		private void SendEnterAtIndex(int index)
		{
			this.Text = this.Text.Remove(index, EnterString.Length);
			this.SelectionStart = index;
			this.Text = this.Text.Insert(index, System.Environment.NewLine);	
		}
	}
}