Normally, if I have a nullable type for an optional parameter, I would put null as the default value. This way I know that if the value is null, the caller doesn't want to specify any value for that one.
public void Foo(string text, string text2= null);
If the parameter is normally a positive integer, I can use a negative number
public void Foo(string text, int index=-1);
How about DateTime? It is not nullable, and (as far as I know) it doesn't have a meaningless number that cannot be a true input either (like -1 for positive integer). Or is there? What would you use in this situation?
I also know that I can use the nullable DateTime type, but this means that the method caller will have to use Nullable as well as opposed to just conveniently pass a DateTime.
You can make value types nullable using the ? operator in C#:
DateTime? myDate = null;
From this, you can make the parameter optional:
void Foo(DateTime? myDate = null)
{
}
Further reading on Nullable Types.
This is not the only way to skin the cat however, you can use default(DateTime), however you cannot use DateTime.MinValue, MaxValue, or Now in optional parameters because they are not compile time constants.
Of course, you don't need to use optional parameters, you can use overloaded methods if you wish to make use of Min, Max, or Now.
void Foo()
{
Foo(DateTime.MinValue);
}
void Foo(DateTime d)
{
}
If you want to go overkill (well, maybe not overkill, plenty of valid reasons to do this), then you could define a new date type that understands when it has a value:
class SmarterDateTime
{
public bool IsSet { get; set; }
// Wrapper around DateTime etc excluded.
}
As for what should be the default, you can choose to make any date represent a default if you wish, but for things like optional parameters you'll have limitations.
Personally, I tend to use DateTime.MinValue.
default (DateTime) - operator default is intended for It
At the question "what can be a default value for a DateTime" the response must be: you can only use default(DateTime). This because the default value must be const and both DateTime.MinValue and DateTime.MaxValue are only static readonly, but note that
default(DateTime) == DateTime.MinValue
down to the Kind.
If you want you can implement an overload with one less parameter (the DateTime) and from that overload call the "main" method passing the value you prefer.
But as written by others, the problem is that you wrote wrong premises.
No, DateTime (as nearly all the ValueTypes. Nearly all because Nullable<Nullable<int>> is illegal, even while Nullable<T> is a ValueType) is nullable. Nullable<DateTime> or DateTime? (same thing)
Even int are nullable, you know? int? :-)
DateTime.MinValue will be the default value.
check the dateTime default parameter , its value would be 1/1/0001 12:00:00 AM,
private void M(Int32 x = 9, String s = “A”, DateTimedt = default(DateTime), Guidguid = new Guid()) {
Console.WriteLine(“x={0}, s={1}, dt={2}, guid={3}”, x, s, dt, guid);
}
If you use the Nullable the callers of your function can just pass a regular DateTime to it, so they won't notice a thing :) There are implicit operators that will do this for you
If you want to set a default in your function you can do:
public void Foo(DateTime? value = null)
{
if ( value == null )
{
value = ... // default
}
}
Code Snippet
public DateTime method1()
{
if (condition)
return new DateTime(2007, 5, 30, 11, 32, 00);
else
return default(DateTime);
}
The default statement will initialise a value type to it's default value. In the case of a datetime this value is also exposed as a static property called DateTime.MinValue. If using C# 1.0 the statement "default(DateTime)" would be equivalent to "DateTime.MinValue". You could use this special value as a kind of "marker" value, meaning if it is returned it indicates an invalid datetime.
If using C# 2.0 again, it is also possible to use what is called a nullable type, and actually return NULL, as shown in the following example
Code Snippet
public DateTime? method2()
{
if (condition)
return new DateTime(2007, 5, 30, 11, 32, 00);
else
return null;
}
// This is the best way to null out the DateTime.
//
DateTime dateTime2 = DateTime.MinValue;
You might consider using the value DateTime.MinValue and use overloading.
Depends on your use-case.
Any that won't match real data will work, but that depends on your use of it (so in a way does -1 for integers, since it's a perfectly good integer only for your use of it being one were only positive integers make sense).
If you are sending a minimum date (interested in all foo that are later) then any date prior to the earliest sensible date will do, and code like .Where(f -> f.When > myDate) will work without even having to look for that special case.
Likewise with maximum dates in reverse (any date that would be after the latest sensible date).
Otherwise, just avoid the use of defaults entirely, and overload instead.
Related
if (File.Exists(settingsFile))
{
string[] lines = File.ReadAllLines(settingsFile);
if (lines.Length > 0)
{
trackBarHours.Value = Convert.ToInt32(optionsfile.GetKey("trackbarhours"));
trackBarMinutes.Value = Convert.ToInt32(optionsfile.GetKey("trackbarminutes"));
trackBarSeconds.Value = Convert.ToInt32(optionsfile.GetKey("trackbarseconds"));
savedMilliseconds = Convert.ToInt32(optionsfile.GetKey("milliseconds"));
dateTimePicker1.Value = Convert.ToDateTime(optionsfile.GetKey("timetargetvalue"));
richTextBox1.Text = optionsfile.GetKey("result");
}
}
because the key "timetargetvalue" is not yet created in the settingsFile because i didn't saved it yet for the first time the value of the key of "timetargetvalue" is '01/01/0001 00:00:00'
in that case that there is no yet the key hwo can i handle the datetime exception ?
dateTimePicker1 is a DateTimePicker control.
the exception is on the line :
dateTimePicker1.Value = Convert.ToDateTime(optionsfile.GetKey("timetargetvalue"));
System.ArgumentOutOfRangeException: 'Value of '01/01/0001 00:00:00' is not valid for 'Value'. 'Value' should be between 'MinDate' and 'MaxDate'.
Parameter name: Value'
what should i check against of so it will not throw the exception ?
DateTimePicker.Value must be above DateTimePicker.MinimumDateTime, which is 'January 1, 1753'.
When you haven't set the timetargetvalue, it will resolve to '01/01/0001 00:00:00', as you have seen, which is too early.
So you need to check the value before assigning it to DateTimePicker.Value.
You can do it like this:
DateTime tempDateTime = Convert.ToDateTime(optionsfile.GetKey("timetargetvalue");
dateTimePicker1.Value = tempDateTime >= DateTimePicker.MinimumDateTime ? tempDateTime : DateTimePicker.MinimumDateTime;
When dealing with a Struct such as DateTime that does not have any value we need to consider that this is not a class and can not be set to null. It must always have some value. (see https://learn.microsoft.com/en-us/dotnet/api/system.datetime?view=net-7.0)
The exception mentions in a round about way that the range of acceptable values is between dateTimePicker1.MinDate and dateTimePicker1.MaxDate so one option is to check if your value is within this range. But it's unlikely to be the best option. (see https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.datetimepicker.mindate?view=windowsdesktop-6.0)
I'm pretty sure that DateTime default value is equal to that of DateTime.Min but if you really wanted to check if the value is default then I would suggest comparing it to default(DateTime) would be better.
This pretty much covers the use of DateTime and value defaults when null is not an option. Which brings up a possibly more desirable option. Encapsulation.
We could instead encapsulate the DateTime struct into a Nullable class. The encapsulating class will be nullable and will also be able to present the encapsulated value through a property called Value. (see https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/nullable-value-types)
There are two ways to declare such a Nullable class, both of which compile to the same thing:
Nullable<DateTime> myNullableDate = null;
DateTime? anotherNullableDate = null;
Since the DateTime is encapsulated in a Nullable object we can start using a null reference check. We can also call a method on Nullable called HasValue which returns a bool (True if it has a value).
EDIT: I notice that you're not doing any checks before trying to parse the DateTime and then directly setting it into the DateTimePicker.Value which can accept a null value. (although setting null won't clear a previously set value).
As such perhaps what you might want to do is handle the scenario a bit better and then use a DateTime.TryParse() instead. (see https://learn.microsoft.com/en-us/dotnet/api/system.datetime.tryparse?view=net-7.0)
e.g. (not the most optimized code, but I think it's easier to follow along in a more verbose form)
private DateTime? LoadDateFromOptions(string key)
{
var rawValue = optionsfile.GetKey(key);
if (!string.IsNullOrEmpty(rawValue))
{
return null;
}
DateTime dateValue;
bool isSuccess = DateTime.TryParse(rawValue, out dateValue);
if (isSuccess)
{
return dateValue;
}
else
{
return null;
}
}
and then instead of having that exception you can load the value optionally a bit more like this:
var timeTarget = LoadDateFromOptions("timetargetvalue");
if (timeTarget != null)
{
dateTimePicker1.Value = timeTarget;
}
This question already has answers here:
Nullable DateTime used in property with expression returns unexpected default value
(2 answers)
Closed 1 year ago.
In the following example, I'm returning a DateTimeOffset? using the default value
var a = ConvertToDateTimeOffsetA(null); // 1/1/0001 12:00:00 AM +00:00
var b = ConvertToDateTimeOffsetB(null); // null
private static DateTimeOffset? ConvertToDateTimeOffsetA(DateTime? date)
{
return date != null
? new DateTimeOffset(date.Value, TimeSpan.Zero)
: default;
}
private static DateTimeOffset? ConvertToDateTimeOffsetB(DateTime? date)
{
if (date != null)
return new DateTimeOffset(date.Value, TimeSpan.Zero);
return default;
}
Why is there a difference between the returned output in the ternary compared to the if statement?
My guess would be the ternary first coerces the type to DateTimeOffset and then inline converts back to Nullable<DateTimeOffset>, but I'm not quite sure why?
In the ternary version, it is interpreting your default as default(DateTimeOffset), the other output expression type in the conditional. Then it is interpreting the ternary as a whole as a nullable, which will never be null.
In the second case, your return is using default(DateTimeOffset?), from the declared return type.
In this case, you may want to use an explicit type in the default expression, or: just use the second form and add a comment (and ideally a unit test), so nobody "fixes" it in the future.
We are trying to override the DateTime.MinValue in our application, but by doing it we noticed that our Web services are timing-out, following is a sample code. Not sure what is wrong/what we are missing.
public MainWindow()
{
//Introducing this.. Causes timeout of the webservice call...
typeof(DateTime).GetField("MinValue").SetValue(typeof(DateTime),new DateTime(1900, 1, 1));
var yesitworks= DateTime.MinValue;
InitializeComponent();
ServiceReference1.Service1Client client = new ServiceReference1.Service1Client();
//Below call will timeout...
var value =client.GetData(10);
}
PS: This might not be the best solution for what we are trying resolve but now its more of curiosity as to why it is not working? how is it related.
DateTime.MinValue is a static readonly field. That means that library authors will not expect it to change, and may write code that depends on it having the expected value.
Hence, you should not change the value of DateTime.MinValue.
For example, a library may use it as the default value for a variable:
private mostRecentDate= DateTime.MinValue;
foreach (var date in myDates)
{
if (date > mostRecentDate)
{
mostRecentDate= date;
}
}
// Do something with the most recent date in myDates...
In this example, if myDates only contained dates earlier than your new value for DateTime.MinValue, then this code would set mostRecentDate to DateTime.MinValue rather than the latest date in myDates.
While this rather contrived example may not be good programming practise (for example, you could use Nullable instead), it is valid code, whose behaviour would be changed if you changed the value of DateTime.MinValue.
The point is that libraries you are using could also be dependant on the value on DateTime.MinValue, so changing it could break them. You are llucky in so far as you found out that this introduced a bug early. If you are unlucky, you would not see a problem until your software had gone live and some corner case was hit.
I had a similar problem recently.
You didn't tell why you wanted to override DateTime.MinValue, but I guess the reason is similar to mine:
I have a server written in .NET, which has .NET clients and (via COM-Interop) MS Access clients.
The clients pass DateTime values, and the server needs to check whether they passed a "real" value or DateTime.MinValue.
My problem was:
.NET's DateTime.MinValue is January 1st of the year 1
The smallest possible value for VBA's Date type is January 1st of the year 100
⇒ Checking for DateTime.MinValue didn't work when the data was coming from MS Access, because Date variables in Access can't hold a date as small as .NET's DateTime.MinValue.
At that point I tried to override DateTime.MinValue too, and found out it doesn't work.
My solution was to write an extension method for DateTime:
public static class DateTimeExtensions
{
public static bool MinValue(this DateTime input)
{
// check the min values of .NET *and* VBA
if (input == DateTime.MinValue || input == new DateTime(100, 1, 1))
{
return true;
}
return false;
}
}
For the code in your question, it would need to look like this:
public static class DateTimeExtensions
{
public static bool MinValue(this DateTime input)
{
if (input == new DateTime(1900, 1, 1))
{
return true;
}
return false;
}
}
Usage:
DateTime theDate = DateTime.Now;
// vanilla .NET
bool isMin1 = (theDate == DateTime.MinValue);
// with the extension method
bool isMin2 = theDate.MinValue();
I don't think you will be able to change the DateTime MinValue as it is read only, But if you can DON'T
DateTime:
public struct DateTime : IComparable, IFormattable, IConvertible, ISerializable, IComparable<DateTime>, IEquatable<DateTime>
{
public static readonly DateTime MaxValue
public static readonly DateTime MinValue
....
I have DAL where I convert database null value to their equivalent representation in C#. For example:
NULL for Numeric = 0
NULL for String = String.Empty
NULL for DateTime = "1/1/0001" (i.e. DateTime.MinValue)
The problem, for date, lies in the presentation layer, especially in GridViews. You cannot show 1/1/01 to users.
What I used to do is check if myDate.Year=1 or myDate.Year < AcceptedDate and display empty string, but seems to be extra effort unlike other types
Please am open to better approach. Thanks.
Use Nullable datatype to store null value.
DateTime? value = null;
int? myNullableInt = 1;
value = DateTime.Now;
How to check whether variable has value or null
if (value!=null)
String value can store null, so there is no diffrent datatype for string to store null.
string var;
if (var == null)
or
if (String.IsNullOrEmpty(var))
You can also use DateTime.MinValue constant.
http://msdn.microsoft.com/en-us/library/system.datetime.minvalue.aspx
Your conditions would be:
if (myDate == DateTime.MinValue)
You can use Nullable DateTime, so you will return DateTime? instead of DateTime from your DAL. This way you can check if returned value is null.
DateTime? dateTime = null;
As the others mention, you could use a System::Nullable<DateTime>.
The other approach I've seen is to use a standard DateTime and just use a special value such as DateTime.MinValue. This is useful if you need to honor an existing interface's types and can't change the DateTime to a Nullable<DateTime>.
You can either use a Nullable DateTime as the others suggested, or use this trick:
(To prevent non valid defaults.)
// If dateTime has not been initialize, initialize to Now
// (or to any other legal inital values)
dateTime = ((dateTime != new DateTime()) ? dateTime : DateTime.Now);
This trick is useful if you have to use a non-nullable DateTime and want to provide a default if none. (E.g. you have a non-nullable DateTime column in a DB and want to set the value only if row is new.)
I don't think you have much choice but to make the check like you have been and display accordingly. A nullable type might make things easier for you. Depending on your data, even the numeric should be treated this way. DBNull != 0.
I'm trying to reuse the same code I've always used but now it is encountering an error.
I'm looping through various user tables, and in there I do this:
DateTime dcdt = (DateTime)u.DateCreated;
DateTime lldt = (DateTime)u.LastLogon;
userRow["DateCreated"] = dcdt.ToShortDateString();
inside the loop. I get the error:
System.InvalidOperationException: Nullable object must have a value.
The error highlights "lldt" line, instead of "dcdt" which comes first. That is strange in and of itself. Both these fields in the database "allow nulls" is checked. And they both could be null or neither might be null.
The two values are both listed as DateTime? types through intellisense.
I don't understand why ASP.NET refuses to allow me to output blank for null dates. If it is empty/null, then logic would suggest that ASP.NET should just print nothing.
How else am I suppose to output null dates? I tried adding if statements to prevent trying to cast null DateTimes, but it doesn't help, it makes no sense.
As you've said, the data type of u.LastLogon is DateTime?. This means that it may or may not have a value. By casting to DateTime, you are requiring it to have a value. In this case, it does not.
Depending on what you're trying to do with it, you may want to check the HasValue property:
userRow["LastLogon"] = u.LastLogin.HasValue ?
(object) u.LastLogin.ToShortDateString() : DBNull.Value;
If your database LastLogon column is of DateTime type, then you should be able to do:
userRow["LastLogon"] = u.LastLogin.HasValue ?
(object) u.LastLogin.Value : DBNull.Value;
You need to do something like the following in your data access code:
DataTable dt = ExecuteSomeQuery() ;
object value = dt.Rows[0]["nullable_datetime_column"] ;
DateTime? instance = value != null && value is DateTime ? (DateTime?)value : (DateTime?)null ) ;
If the column returned is NULL, it will be returned as a System.DBNull, otherwise it will be returned as an instance of DateTime (or whatever the appropriate mapped type is — int, string, etc). Consequently, you need to check the type of object returned from the query before trying to cast it.
Looks like you are trying to call a method (dcdt.ToShortDateString()) on a DateTime? which doesn't have a value (it is, indeed, null). Try this:
dcdt.HasValue ? dcdt.ToShortDateString() : String.Empty;
EDIT (Just re-read the question): Also, don't try to convert to DateTime. Preserve the nullable.
EDIT #2 (based on comments):
Try this:
if (dcdt.HasValue)
{ userRow["DateCreated"] = dcdt.ToShortDateString(); }
else
{ userRow = DbNull.Value }
I saw that Dexter asked how he should go about it. Well, I would create an extension.
static class DateTimeExtensions
{
public static string ToString(this DateTime? dateTime, string format)
{
return dateTime.HasValue ? dateTime.Value.ToString(format) : String.Empty;
}
}
And then you can do:
DateTime? dt = null;
DateTime? dt2 = DateTime.Now;
Console.WriteLine(dt.ToString("dd-MM-yy"));
Console.WriteLine(dt2.ToString("dd-MM-yy"));
Note that I can call extension method on a nullable type if the object is null.
The problem is .NET null is not the same as SQL NULL. SQL Null is System.DBNull. So it is a [non-null] value in .NET.
Short answer
DateTime? dateTime = u.LastLogon?.ToShortDateString()