C# 如果不是 NULL，则仅分配日期时间值

Question

We have a custom object that i'm trying to assign and populate from a dataset. It works fine unless there is a NULL value in the datetime fields.

here is the object example:

public class Test
{

    public DateTime Date1{ get; set; }
    public DateTime Date2{ get; set; }
 }

We are wondering how best to byparse assigning any value if the value is null:

var convertedList = (from rw in ds.Tables[0].AsEnumerable()
    select new Test()
       {
          Date1 = Convert.ToDateTime(rw?["StartDate"]),
          Date2 = Convert.ToDateTime(rw?["EndDate"])
        }).ToList();

The following throws an exception (as it should) when null. Just wondering the best practice to handle this? Note we cannot use a nullable datetime (datetime?) due to an external app.

Idealy, we would not want to assign the datetime if null

Answer 1

You may want to Assign some default value like:

Date1 = rw?["StartDate"]==null?DateTime.MinValue:Convert.ToDateTime(rw?["StartDate"]);

Or just ignore those with null values:

var convertedList = (from rw in dt2.AsEnumerable() where rw["StartDate"] != null && rw["EnDate"] != null
    select new Test()
    {
        Date1 = (rw["StartDate"] == null ? Convert.ToDateTime(rw["StartDate"]) : new DateTime()),
        Date2 = (rw["EndDate"] == null ? Convert.ToDateTime(rw["EndDate"]) : new DateTime())
    }).ToList();

Answer 2

You can check for NULL and replace with a date you want.

var convertedList = (from rw in dt2.AsEnumerable()
    select new Test()
    {
        Date1 = (rw["StartDate"] == null ? Convert.ToDateTime(rw["StartDate"]) : new DateTime()),
        Date2 = (rw["EndDate"] == null ? Convert.ToDateTime(rw["EndDate"]) : new DateTime())
    }).ToList();

Answer 3

你能不能不使用 DateTime.TryParse，它会尝试将字符串转换为 dateTime，但如果不能，它将使用 dateTime.minValue？

Answer 4

Are you going for sanity checks on inputted values? If so, what I call "Passive Error Reporting" might be the way.

INotifyDataErrorInfo is all about telling the user about issues - and allowing you to check of there are no errors before comitting - without "resorting" to Exceptions.

You use it often in WPF where the ViewModel might be required to be accept nullable values or even strings for Numbers (for entirely view reason), but the Model can not accept Null values or anything but parsed Integers.