I'm looking to validate a DateTime variable to ensure that it isn't blank on the UI. The string equivalent checking would be String.IsNullOrEmpty(), but how would I go about it with my DateTime variable?
DateTime is a value type, so it cannot be null. To check if a DateTime variable has the default (all 0) value you can compare it with new DateTime() or default(DateTime).
Another option would be to use DateTime? instead of DateTime for user input and check HasValue property.
To check if a DateTime is null in C#, you must first ensure the DateTime is nullable.
// DateTime? means it is nullable
var DateTime? date = null;
// .HasValue only exists if DateTime is nullable
if(date.HasValue)
Console.WriteLine("date has a value");
else
Console.WriteLine("date is null");
If your DateTime is not nullable, make it nullable (unless you are absolutely certain in will never be null). You don't want to go down the road of assigning "nullish" values to your DateTimes. In my experience, this creates confusing bugs / code.
Related
I need to convert a DateTimeOffset? to a DateTime. The value originally comes from a XAML CalendarDatePicker, but I need to change it to DateTime to store the value. I have found this description of how to convert DateTimeOffset, but I do not think that it answers my question because they don't use a nullable type.
Nullable types are useful, but can sometimes be confusing at first. The Nullable<T> is a struct where T is a struct as well. This struct wraps the value for the instance variable T and exposes three primary members.
HasValue // Returns bool indicating whether there is a value present.
Value // Returns the value of T if one is present, or throws.
GetValueOrDefault() // Gets the value or returns default(T).
You can make any struct nullable by adding a '?' after the declaration of the variable type, or by wrapping the variable type with Nullable< varible type here >. As depicted below:
Nullable<DateTimeOffset> a = null;
DateTimeOffset? b = null;
I believe that what you would want here is to check if there is in fact a value with .HasValue and then take the .Value from the offset and perform your standard conversion.
Example
var now = DateTime.Now;
DateTimeOffset? offset = now;
DateTime dateTime = offset.HasValue ? offset.Value.DateTime : DateTime.MaxValue;
Or if you want a DateTime? do this:
var now = DateTime.Now;
DateTimeOffset? offset = now;
DateTime? dateTime = offset.HasValue ? offset.Value.DateTime : (DateTime?)null;
I had used AddDays(x) method before on DateTime objects and it was working fine but now my object is defined like this:
public DateTime? To_Date { get; set; }
And looks like this one does not have a AddDays method. How can I call it then?
Because DateTime? can be in a state representing null, you need to consider what to do if it's null.
The most obvious thing to do is to stay with a value of null (the day after null is null).
DateTime? later = ToDate.HasValue
? To_Date.Value.AddDays(numberOfDays)
: (DateTime?)null;
It maybe that there's some meaningful default date you can use, in which case:
DateTime later = (ToDate ?? defaultDate).AddDays(numberOfDays);
This will use defaultDate when ToDate has no value, and the value of ToDate otherwise.
You need to do To_Date.Value.AddDays(1);
To_Date refers to the wrapper which only has a couple of properties. One is a flag to indicate whether or not it's null, the other is the value itself. First check that it's not null, try to use AddDays on the value.
Try this:
To_Date.Value.AddDays(x);
I would declare an empty String variable like this:
string myString = string.Empty;
Is there an equivalent for a 'DateTime' variable ?
Update :
The problem is I use this 'DateTime' as a parameter for a 'StoredProcedure' in SQL.
E.g:
DateTime? someDate = null;
myCommand.Parameters.AddWithValue("#SurgeryDate", someDate);
When I run this code an exception is catched telling me the 'StoredProcedure' expected a '#SurgeryDate' parameter.
But i provided it.
Any idea why?
Since DateTime is a value type you cannot assign null to it, but exactly for these cases (absence of a value) Nullable<T> was introduced - use a nullable DateTime instead:
DateTime? myTime = null;
No. You have 2 options:
DateTime date = DateTime.MinValue;
This works when you need to do something every X amount of time (since you will always be over MinValue) but can actually cause subtle errors (such as using some operators w/o first checking if you are not MinValue) if you are not careful.
And you can use Nullable:
DateTime? date = null;
Which is nice and avoids most issues while introducing only 1 or 2.
It really depends on what you are trying to achieve.
You can set a DateTime variable to be '1/1/0001 00:00:00' but the variable itself cannot be null. To get this MinTime use:
DateTime variableName = DateTime.MinValue;
You may want to use a nullable datetime. Datetime? someDate = null;
You may find instances of people using DateTime.Max or DateTime.Min in such instances, but I highly doubt you want to do that. It leads to bugs with edge cases, code that's harder to read, etc.
The method you used (AddWithValue) doesn't convert null values to database nulls. You should use DBNull.Value instead:
myCommand.Parameters.AddWithValue(
"#SurgeryDate",
someDate == null ? DBNull.Value : (object)someDate
);
This will pass the someDate value if it is not null, or DBNull.Value otherwise. In this case correct value will be passed to the database.
Either:
DateTime dt = new DateTime();
or
DateTime dt = default(DateTime);
If you set the date to
DateTime dNewDate = new DateTime();
The value is set to {1/1/0001 12:00:00 AM}
Option 1: Use a nullable DateTime?
Option 2: Use DateTime.MinValue
Personally, I'd prefer option 1.
A string is a sequence of characters. So it makes sense to have an empty string, which is just an empty sequence of characters.
But DateTime is just a single value, so it's doesn't make sense to talk about an “empty” DateTime.
If you want to represent the concept of “no value”, that's represented as null in .Net. And if you want to use that with value types, you need to explicitly make them nullable. That means either using Nullable<DateTime>, or the equivalent DateTime?.
DateTime (just like all value types) also has a default value, that's assigned to uninitialized fields and you can also get it by new DateTime() or default(DateTime). But you probably don't want to use it, since it represents valid date: 1.1.0001 0:00:00.
There's no such thing as an empty date per se, do you mean something like:
DateTime? myDateTime = null;
The .addwithvalue needs dbnull.
You could do something like this:
DateTime? someDate = null;
//...
if (someDate == null)
myCommand.Parameters.AddWithValue("#SurgeryDate", DBnull.value);
or use a method extension...
public static class Extensions
{
public static SqlParameter AddWithNullValue(this SqlParameterCollection collection, string parameterName, object value)
{
if (value == null)
return collection.AddWithValue(parameterName, DBNull.Value);
else
return collection.AddWithValue(parameterName, value);
}
}
This will work for null able dateTime parameter
. .
SearchUsingDate(DateTime? StartDate, DateTime? EndDate){
DateTime LastDate;
if (EndDate != null)
{
LastDate = (DateTime)EndDate;
LastDate = LastDate.AddDays(1);
EndDate = LastDate;
}
}
Am just trying to understand why this exception throws.
Cannot implicitly convert type 'System.DateTime?' to 'System.DateTime'. An explicit conversion exists (are you missing a cast?)
This is what am trying to do
story.BroadcastOn is a date time value am getting from database (eg: 23/03/2012 1:56 Pm).
Am trying to convert the time from 12 to 24 hrs format,this is what i was trying to do
DateTime testconverttime = story.BroadcastOn;`//this is where it throws exception
so i have to use the parse to get rid of this like below to solve my issue but it doesn't make sense to me
if (!string.IsNullOrEmpty(story.BroadcastOn.ToString()))
{
DateTime localTime = story.BroadcastOn.Value;//Thanks all for the great suggestion.
converttime = localTime.ToString("dd/MMM/yyyy HH:mm ", CultureInfo.CurrentCulture);
}
already i converted my 12 hrs to 24hrs but trying to understand the exception,some one will give me an explanation please.
DateTime testconverttime = story.BroadcastOn.Value;
It's a nullable type (can have the null state also)
A nullable value type (DateTime is a value type), has the concept of the null value (no value). So if for example a column of datetime in database has nulls, then you can use Nullable<DateTime> or in short DateTime? to store the values which comes from that column.
About DateTime.ToString() and String.ToDateTime(): this is called yo-yo programming. You probably saw with Debuger that there is a representation of valid DateTime, which was giving by calling ToString(), but, in future, don't try to cast a type to another type via this yo-yo technique.
Try this, assuming the nullable type has a value:
DateTime testconverttime = story.BroadcastOn.Value;
DateTime? and DateTime are not the same type. DateTime? is the nullable version of the DateTime type.
You should check whether it's null, and then either cast or retrieve the value of the nullable:
if (story.BroadcastOn.HasValue) {
var broadcastOn = story.BroadcastOn.Value;
// do stuff with broadcastOn
} else {
// handle null BroadcastOn
}
or
if (story.BroadcastOn != null) {
var broadcastOn = (DateTime) story.BroadcastOn;
// do stuff with broadcastOn
} else {
// handle null BroadcastOn
}
Using .HasValue or comparing with null, and using .Value and casting to the non-nullable type should be equivalent. You can also use the ?? operator.
Try casting it as a DateTime, like so:
DateTime testconverttime = (DateTime)story.BroadcastOn
System.DateTime? and System.DateTime are 2 different types. You need to use story.BroadcastOn.Value if you are sure if it is not null.
DateTime? and DateTime are not the same type so you can not implicitly assign DateTime? to DateTime
You need to either explicitly cast it or assign the value via the Value property of DateTime. However, if BroadcastOn is null, either method will throw an exception.
If you don't know BroadcastOn is not null then you best option to is use the null-coalescing operator:
DateTime dt = story.BroadcastOn ?? default(DateTime);
When I do the following I get:
inv.RSV = pid.RSVDate
I get the following: cannot implicitly convert type System.DateTime? to System.DateTime.
In this case, inv.RSV is DateTime and pid.RSVDate is DateTime?
I tried the following but was not successful:
if (pid.RSVDate != null)
{
inv.RSV = pid.RSVDate != null ? pid.RSVDate : (DateTime?)null;
}
If pid.RSVDate is null, I like to not assign inv.RSV anything in which case it will be null.
DateTime can't be null. It's default is DateTime.MinValue.
What you want to do is the following:
if (pid.RSVDate.HasValue)
{
inv.RSV = pid.RSVDate.Value;
}
Or, more succinctly:
inv.RSV = pid.RSVDate ?? DateTime.MinValue;
You need to make the RSV property nullable too, or choose a default value for the case where RSVDate is null.
inv.RSV = pid.RSVDate ?? DateTime.MinValue;
Because inv.RSV is not a nullable field, it can't be NULL. When you initialize your object, it is a defaulting inv.RSV to an empty DateTime, the same as you would get if you said
inv.RSV = new DateTime()
So, if you want to set inv.RSV to pid.RSV if it's not NULL, or the default DateTime value is it is null, do this:
inv.RSV = pid.RSVDate.GetValueOrDefault()
if the one being assigned into as a DateTime and the one being assigned from is DateTime?, you could use
int.RSV = pid.RSVDate.GetValueOrDefault();
This supports an overload that allows you to specify the default value, if the default for DateTime is not ideal.
If pid.RSVDate is null, I like to not assign inv.RSV anything in which
case it will be null.
int.RSV will not be null, as you have already said it's DateTime, not a nullable type. If it is never assigned by you, it will have the default value of it's type, which is DateTime.MinValue, or January 1, 0001.
inv.RSV was null to begin with. How do I say do not update it there is no value for pid.RSVDate
Again, this simply cannot be, given your description of the property. However, if generally speaking you do not want to update inv.RSV if pid.RSVDate is null (and you're just getting mixed up in your words), then you would simply write an if check around the assignment.
if (pid.RSVDate != null)
{
inv.RSV = pid.RSVDate.Value;
}
pid.RSVDate has the possibility of being null, whereas inv.RSV does not, so what would happen if RSVDate is null?
You need to check if the value is null before -
if(pid.RSVDate.HasValue)
inv.RSV = pid.RSVDate.Value;
But what would the value of inv.RSV be if RSVDate is null? Is there always going to be a date in this property? If so, you could use the ?? operator to assign a default if you want to.
pid.RSV = pid.RSVDate ?? myDefaultDateTime;