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C# ternary use results of method (with datetime variable)

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().

TryParse in one line: accepted challenge? [duplicate]

This question already has answers here:
TryParse in if condition
(7 answers)
Closed 9 years ago.
Just a challenge I guess, but I hope to use TryParse in just one line :) My code:
DateTime tempDate;
user.DataNascita = DateTime.TryParse(txtDataDiNascita.Text, out tempDate) ? tempDate : (DateTime?)null;
user.DataNascita is DateTime?, and I want to return the data if TryParse is correct, null otherwise. But I need the out one (so, new line). Can't I have all in one line?
Just curious...

I'm usually using this extension method in LINQ queries:
public static DateTime? TryGetDate(this string dateString, string format = null)
{
DateTime dt;
bool success = format == null ? DateTime.TryParse(dateString, out dt) : DateTime.TryParseExact(dateString, format, null, DateTimeStyles.None, out dt);
return success ? dt : (DateTime?)null;
}
You use it in this way:
user.DataNascita = txtDataDiNascita.Text.TryGetDate();
Here's another overload with multiple formats and an IFormatProvider(different cultures):
public static DateTime? TryGetDate(this string dateString, IFormatProvider provider, params string[] formats)
{
DateTime dt;
var success = DateTime.TryParseExact(dateString, formats, provider, DateTimeStyles.None, out dt);
return success ? dt : (DateTime?)null;
}

You'd need a helper method, basically. For example:
public static DateTime? TryParseDateTime(string text)
{
DateTime validDate;
return DateTime.TryParse(text, out validDate) ? validDate : (DateTime?) null;
}
Then you can just use:
user.DataNascita = ParseHelpers.TryParseDateTime(txtDataDiNascita.Text);
You'd probably want overloads corresponding with the overloads of DateTime.TryParse and DateTime.TryParseExact, too. I wouldn't personally make this an extension method as per Tim's answer, but that's a matter of personal preference.

yea it's easy I didn't find this much of a challenge
DateTime temp; if (DateTime.TryParse(txtDataDiNascita.Text, out temp)) user.DataNascita = temp;

DateTime tempDate; user.DataNascita= DateTime.TryParse(txtDataDiNascita.Text, out tempDate) ? tempDate : (DateTime?)null;
You could do it in a single line as above. but creating your helper method is good approach.

Set an empty DateTime variable

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;
}
}

Convert a string to datetime [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
How can I convert a string into datetime in .NET?
I have a string in the following format "15/03/2046". how can convert this string to a DateTime object?
My problem is when I do Convert.ToDateTime("15/03/2046") I get an exception.
when I do Convert.ToDateTime("03/03/2046") every thing works fine.
so I guess that I have to specify the format somehow while converting....

DateTime.Parse or its sister method DateTime.ParseExact.

Use DateTime.ParseExact to specify the format of the input string:
DateTime d = DateTime.ParseExact(
"15/03/2046",
"dd/MM/YYYY",
CultureInfo.InvariantCulture
);

More generic code, using extension method, and default value in case if it can't parse date
void Main()
{
var dt = "15/03/2046";
dt.ToDateTime("fr-FR", DateTime.Now).Dump();
}
public static class Extensions
{
public static DateTime ToDateTime(this string dateTime, string culture, DateTime defaultValue)
{
DateTime dt;
if (DateTime.TryParse(dateTime, System.Globalization.CultureInfo.CreateSpecificCulture(culture), System.Globalization.DateTimeStyles.None, out dt))
return dt;
else
return defaultValue;
}
}

C# newbie with DateTime variable I want to set to null

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");