Mangler en event der trigger på "Enter" i en tekstboks.

brug keydown og kontroller om det er enter der er trykket

Simpelt eksempel:
using System;
using System.Windows.Forms;

namespace WindowsApplication1
{
    /// <summary>
    /// Summary description for Form1.
    /// </summary>
    public class Form1 : System.Windows.Forms.Form
    {       
        private System.Windows.Forms.TextBox textBox;
       
        public Form1()
        {
            InitializeComponent();
        }
       
        private void InitializeComponent()
        {
            Controls.Add(textBox = new TextBox());
            textBox.KeyPress += new KeyPressEventHandler(textBox_keyPress);
        }
       
        protected void textBox_keyPress(object sender,KeyPressEventArgs e)
        {
            if ( e.KeyChar == 13 )
                MessageBox.Show(this,"You pressed Enter !");
        }

        [STAThread]
        static void Main()
        {
            Application.Run(new Form1());
        }
       
    }
}