e.KeyCode == Keys.Enter

if (e.KeyCode == Keys.Enter)
  {
 MessageBox.Show("Enter Key Pressed ");
  }

 
using System;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Enter)
            {
                MessageBox.Show("Enter key pressed");
            }
        }
    }
}