I have an issue that I'd like to work around
I have declared a nullable DateTime and I'm trying to put either a date in it or a null value using a ternary operator.
This does not work:
DateTime? date;
date = getDate() == DateTime.MinValue ? null : getDate()
My own code is a bit more elaborate but basically what I'd like to use is
date = getDate() == DateTime.MinValue ? null : resultofgetDate()withoutactuallyrunningitagain
I don't want to do the function twice, but in this case as a bonus, since it's datetimes, it also gives an error in the else section saying
There is no implicit conversion between 'null' and 'System.DateTime' in my first example.
I'm not sure in what direction to look. I seem to need the opposite of the null coalescing operator (??).
There is no such operator. You can write something like this:
DateTime? date = getDate();
date = date == DateTime.MinValue ? null : date;
It seems as if getDate() is returning a DateTime and not a DateTime? (aka Nullable<DateTime>). The values used in ternary expressions have to be of the same type which is why you are getting your error.
Your first example should work with
date = getDate() == DateTime.MinValue ? null : (DateTime?)getDate()`
as pointed out in MaKCbIMKo's answer.
I'm assuming you return DateTime.MinValue as some sort of error handling / validation. If you change the method signature so it returns a DateTime? you can return null instead, then your statement becomes date = getDate()
Have you tried something like:
DateTime? date;
var dt = getDate();
date = dt == DateTime.MinValue ? (DateTime?)null : dt;
Hope it will help.
Please, do it as Rob described, but here is my take on an extension method:
public static T? Test<T>(this T? value, Predicate<T?> test, T? ifEquals) where T : struct
{
if (test(value))
{
return ifEquals;
}
return value;
}
Use it like this:
DateTime? d = GetDate().Test(t => t == DateTime.MinValue, null);
You can also create this extension method (one of the overloads calls the other one):
public static T? NullIfHasDefaultValue<T>(this T v) where T : struct
{
return EqualityComparer<T>.Default.Equals(v, default(T)) ? (T?)null : v;
}
public static T? NullIfHasDefaultValue<T>(this T? n) where T : struct
{
return n.GetValueOrDefault().NullIfHasDefaultValue();
}
Of course use it as getDate().NullIfHasDefaultValue().
Related
I have this line of code here:
command.Parameters["#DateCompleted"].Value = items[i].DateCompleted.Equals("01/01/0001 12:00:00 AM") ? null : items[i].DateCompleted;
but I got this error:
Type of conditional expression cannot be determined because there is no implicit conversion between '<null>' and 'System.DateTime'
What I am trying to do is not use the 01/01/0001 Date and use null because the item is null.
Additional Code:
command.Parameters.Add("#DateCompleted", System.Data.SqlDbType.DateTime);
Simply cast the null to a DateTime?. Also, assuming DateCompleted is a DateTime (it should be), then don't compare against a string, but against DateTime.MinValue instead.
command.Parameters["#DateCompleted"].Value =
items[i].DateCompleted.Equals(DateTime.MinValue)
? (DateTime?) null
: items[i].DateCompleted;
Use DateTime?
command.Parameters["#DateCompleted"].Value =
items[i].DateCompleted.Equals("01/01/0001 12:00:00 AM") ? (DateTime?)null :
items[i].DateCompleted;
To use the ternary operator where one of the return types is null, the other return type has to be a nullable type, which DateTime is not, being a struct. What you could do is replace the null with default(DateTime):
DateTime value = someCondition ? default(DateTime) : items[i].DateCompleted;
I want to check passed value is null or datetime value using ternary operator in c#?
I tried like this
fromDate == null ? null : Convert.ToDateTime(fromDate)
getting error:
type of conditional expression cannot be determined
I want to check whether variable fromDate is null or having date time value ?
variable fromDate is coming from Querystring and type of string.
From ?: Operator:
Either the type of first_expression and second_expression must be the same, or an implicit conversion must exist from one type to the other.
condition ? first_expression : second_expression;
Convert.ToDateTime returns DateTime and there is no implicit conversion between null and DateTime. And the conditional operator is an expression and that needs a return type.
One option seems logical to me using DateTime.TryParse (which returns boolean) as a first expression and use another boolean (true or false) as a second expression. Damiths' answer seems logical.
Or instead you can use nullable DateTime like DateTime?
DateTime? foo;
if(foo.HasValue)
{
//Nullable DateTime has a value..
}
if you have string value for fromDate do as below
DateTime dt;
bool isValidDate = String.IsNullOrEmpty(fromDate) ? false : DateTime.TryParse(fromDate, out dt);
if you know the datetime format/formats which your input having you better use DateTime.TryParseExact method
if fromDate is DateTime, then simple you can check as below
fromDate.HasValue
you don't need ?: Operator
The problem is that Convert.ToDateTime(fromDate) is of type DateTime, which cannot accept a null value, that is why this code won't work in this form. You have two choices to make. Either change it to regular if:
if(fromDate != null)
{
Convert.ToDateTime(fromDate)
}
Or cast DateTime to nullable DateTime:
fromDate == null ? null : (DateTime?)Convert.ToDateTime(fromDate)
Ternary operator in C# needs both values to be of the same type and it is explained here.
romDate == null ? null : Convert.ToDateTime(fromDate)
null and Convert.ToDateTime(fromDate) haven't common type. Compiler must be able to cast both expressions in canditional operator to same type.
I hope it will help you
string format = "ddd dd MMM h:mm tt yyyy";
DateTime dateTime;
fromDate=(DateTime.TryParseExact(fromDate, format, CultureInfo.InvariantCulture,
DateTimeStyles.None, out dateTime))?dateTime:null;
you must use String.IsNullOrEmpty to check if fromDate is null like this:
DateTime? date = String.IsNullOrEmpty(fromDte) ? null : (DateTime?)Convert.ToDateTime(fromDate)
It looks like the main problem is that you are trying to assign null to a DateTime
DateTime is a structure and not a reference type so this can't be done.
Either use a nullable DateTime (DateTime?) or a specific value to indicate null, such as DateTime.MinValue
Have a look here for an example: http://www.dotnetperls.com/nullable-datetime
DateTime? testDate = (DateTime?)arrayOfObjects[dateObject];
Does that code look ok? I attempted to use the as operator but I got the 'non-nullable' error. What I'm trying to say is that the object I'm choosing from the array is either DateTime or a null DateTime but either can be assigned to testDate.
Doesn't feel right doing it this way, I think I'm missing something obvious.
EDIT: I suppose it's the same as the way I could've adapted the as in the following way:
DateTime? testDate = arrayOfObjects[dateObject] as DateTime?;
Is either line of code the best way of handling potential nulls?
Is either line of code the best way of handling potential nulls?
The second form will silently result in null when the array contains something other than a DateTime. That seems a good reason to use the first.
To the basic question:
am I missing something or is this the typical (or at least an acceptable) approach
It is acceptable but a little obscure maybe, because it is 'hiding' an unboxing operation.
You could use:
DateTime? testDate = null;
if (arrayOfObjects[dateObject] != null)
testDate = (DateTime) arrayOfObjects[dateObject]; // no '?'
But that's verbose. And this particular problem doesn't lend itself well to the conditional operator (?:)
So I would stick with your first version.
DateTime? is a shorter form for another struct
Nullable<DateTime> {
bool HasValue;
DateTime Value;
}
You will never get this type from your DB, so the first line will never cast correctly. The database will provide you with a DateTime value stored in an object variable. Or a null (untyped).
DateTime is a struct, so "as" operator won't work for it. So, simply check for null as follows:
DateTime? testDate = arrayOfObjects[dateObject] == null ? (DateTime?) null : (DateTime)arrayOfObjects[dateObject];
A better approach would be to do something like:
if(arrayOfObjects[dateObject] != null && arrayOfObjects[dateObject] is DateTime)
{
DateTime testDate = (DateTime)arrayOfObjects[dateObject];
// logic here
}
or something like:
DateTime? testDate = null;
if(arrayOfObjects[dateObject] is DateTime)
{
testDate = (DateTime)arrayOfObjects[dateObject];
}
else if (arrayOfObjects[dateObject] is Nullable<DateTime>)
{
testDate = (Nullable<DateTime>)arrayOfObjects[dateObject];
}
Try using Array.ConvertAll method. Below is roughly how it is implemented:
DateTime?[] dates = Array.ConvertAll<object, DateTime?>arrayOfObjects,DateTime);
Note: This is just a rough idea. you can correct it to suit your self.
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;
}
}
I have an output data class with a DateTime variable. I want to clear that to a null value in a loader class but the compiler complains with:
Cannot convert null to 'System.Data.Time' because it is a non-nullable value type.
I understand that, but if I change the type to DateTime? creating the nullable type wrapper I get:
No overload for method 'ToString' takes '1' arguments
I have an output line that reads.
ACCOUNT_ESTABLISHED_DATE.ToString("yyyy-MM-dd")
So the question is, when I set the DateTime as nullable, how do I get around the fact that is no longer behaves like a DateTime that has the formatted ToString available?
Use its Value property, like so:
DateTime? dt = DateTime.Now; // or whatever
MessageBox.Show(dt.Value.ToString(...));
try
ACCOUNT_ESTABLISHED_DATE.Value.ToString("yyyy-MM-dd")
You need to access the actual value using the 'Value' property of the nullable type.
You should make sure 'Value' contains something first testing the ACCOUNT_ESTABLISHED_DATE.HasValue property.
HTH
Whenever you wrap something Nullable<> (which is what you're doing with DateTime?), you need to do obj.Value.ToString().
You should write:
ACCOUNT_ESTABLISHED_DATE.Value.ToString("yyyy-MM-dd")
.NET doesn't have a method out of the box for this. You'd need to have a helper method like:
public string Format(DateTime? date, string format)
{
if (date == null)
return string.Empty;
return date.Value.ToString(format);
}
Or even better, an extension method for DateTime?:
public static class DateTimeExtensionMethods
{
public static string ToString(this DateTime? date, string format)
{
if (date == null)
return string.Empty;
return date.Value.ToString(format);
}
}
Then to use your extension method, just use the code you have in your question and make sure the namespace of the DateTimeExtensionMethods is imported into your class.
are you looking for
DateTime? dt = new DateTime();
or
Nullable<DateTime> dt = new DateTime();
ACCOUNT_ESTABLISHED_DATE.Value.ToString("yyyy-MM-dd");
You would have to use
dt.HasValue ? dt.Value.ToString("...") : dt.ToString();
This is because Nullable<T> is a proper type in its own right whose ToString() method is already nicely done, as it handles the null case well. But to get to the underlying non-nullable object you have to use the Value property. But then you'll have to check for null (or HasValue) yourself.
Have you looked at setting the DateTime to DataTime.MinValue?
Suggested here http://dotnetperls.com/datetime-null-minvalue
DateTime? date = getSomeDate();
if (date != null) {
date.Value.ToString("yyyy-MM-dd");
}
string strDate = string.Empty;
if(ACCOUNT_ESTABLISHED_DATE != null)
{
strDate = ACCOUNT_ESTABLISHED_DATE.Value.ToString("yyyy-MM-dd");
}
or you can use null collacing operator
DateTime newDate = ACCOUNT_ESTABLISHED_DATE ?? new Date();
newDate.ToString("yyyy-MM-dd");