private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
{
if(e.KeyCode == Keys.Enter)
{
if(String.IsNullOrEmpty(textBox1.Text))
{
MessageBox("Textbox cannot be empty","Title", MessageBoxButtons.OK, MessageBoxIcon.Error);
textBox1.Focus();
}
else
{
}
}
}
}