检查某字段是否为空值

//C#
public User PopulateUser(IDataRecord dr) {
User user = new User();
user.UserId = Convert.ToInt32(dr["UserId"]);
//检查 NULL 的示例
if (dr["UserName"] != DBNull.Value){
user.UserName = Convert.ToString(dr["UserName"]);
}
user.Password = Convert.ToString(dr["Password"]);
return user;
}