using (SqlConnection conn = new SqlConnection("server=.;database=Northwind;uid=sa;")) {
conn.Open();
SqlCommand cmd = conn.CreateCommand();
cmd.CommandText = "SELECT TOP 1 Region FROM Customers WHERE Region IS NULL";
result = cmd.ExecuteScalar(); // 返回 NULL
writer.WriteLine(DBNull.Value == result);
writer.WriteLine(DBNull.Value.Equals(result));
writer.WriteLine(Object.Equals(DBNull.Value, result));
writer.WriteLine(Object.ReferenceEquals(DBNull.Value, result));
writer.WriteLine(Convert.IsDBNull(result));
//
writer.WriteLine(result.GetType() == typeof(DBNull));
writer.WriteLine(result is DBNull);
}