Try:
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlCommand cmd = new SqlCommand("SELECT COUNT(iD) FROM myTable WHERE id=@ID", con))
{
cmd.Parameters.AddWithValue("@ID", textbox1.Text);
int count - cmd.ExecuteScalar();
if (count != 0)
{
}
else
{
}
}
}