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Lambda query to return Max() of Date or else null in C# [closed]

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I need Lambda query to filter dates from list and return Max Date. If the list does not satisfy the filter then should return a null value. This is my code, need a shorter version ->
var a = Dates.Where(x => x >= DateTime.Now).Select(x => x);
if(a.Count > 0)
{
return a.Max();
}
else return null;
The value returned to is in DateTime? format which shows error "Embedded statement cannot be a declaration or labeled statement".

var Dates = new List<DateTime>();
DateTime? max = Dates.Where(x => x >= DateTime.Now)
.Select(x => (DateTime?) x)
.DefaultIfEmpty(null)
.Max();
Console.WriteLine(max == null ? "null" : "not null");
You need to cast the result to Nullable<DateTime> one way or another, if your list is composed of DateTime.
Edit : as #RogerStewart mentioned in comments, to perform the cast you could use Cast<DateTime?>() instead of the Select statement.

DateTime substracting days doesn't work

I am using the following code in order to substract a day of the DateTime until I am getting Monday:
DateTime currentWeek = new DateTime(beginDate.Year, beginDate.Month, beginDate.Day);
while (currentWeek.DayOfWeek.ToString() != "Monday")
{
currentWeek.AddDays(-1);
MessageBox.Show(currentWeek.Day.ToString());
MessageBox.Show(currentWeek.DayOfWeek.ToString());
}
beginDate is in the first run set to the current Date of DateTime.Now.
For me this loops forever, and the day of currentWeek always stays the same (29) even though I am substracting 1 everytime I am looping through.
I am already using another function that takes a DateTime and a bool Parameter, which does pretty much the same and works:
private void ErstenTagDerWocheAuswaehlen(DateTime date, bool anfangDerWoche = true)
{
string wochentagName;
int incrementor;
if(anfangDerWoche == true)
{
wochentagName = "Monday";
incrementor = -1;
}
else
{
wochentagName = "Friday";
incrementor = 1;
}
while(date.DayOfWeek.ToString() != wochentagName)
{
date = date.AddDays(incrementor);
}
}
Can someone explain to me why the upper code doesn't work whilst the lower one does?

You have to assign the resulting value, DateTime is immutable.
currentWeek = currentWeek.AddDays(-1);
About your 2nd question:
Use the enum for day of the week, do not try to convert a day of the week to a string for a comparison. The type is DayOfWeek.
Again, a DateTime is not mutable so you have to return a DateTime instance as you can't mutate the one that was passed in (without passing it as ref)
Code change
private DateTime ErstenTagDerWocheAuswaehlen(DateTime date, bool anfangDerWoche = true)
{
System.DayOfWeek wochentagName;
int incrementor;
if(anfangDerWoche == true)
{
wochentagName = System.DayOfWeek.Monday;
incrementor = -1;
}
else
{
wochentagName = System.DayOfWeek.Friday;
incrementor = 1;
}
while(date.DayOfWeek != wochentagName)
{
date = date.AddDays(incrementor);
}
return date;
}

DateTime is an immutable struct, so you need to store the value returned from AddDays():
var t2 = currentWeek.AddDays(-1);
Then use t2. The call to AddDays() doesn't actually change currentWeek.

As DateTime is immutable, when using the AddDays it returns a new DateTime structure with the new information and does not change the given one.
Method documentation states:
Returns a new System.DateTime that adds the specified number of days to the value of this instance.
You must assign it to a variable:
currentWeek = currentWeek.AddDays(-1);

day of week and hour of day in a if statement c#

I want to do something if the day, and time of the day equal true in a if statement. I have the day part down, just can't figure out the time part out. Let say I wan the time to be 9AM.
Here is what I have so far
var dt_check_monday = DateTime.Now.DayOfWeek;
if (dt_check_monday == DayOfWeek.Monday && time_now = DateTime.Now.Hour==9)
{
//do something
}
I can't use this I get an error:
Operator '&&' cannot be applied to operands of type 'bool' and 'System.TimeSpan'
Thanks for any help in advance.

= is an assignment. == is the 'equals'
Your second = should be a ==

You should just do this:
if (DateTime.Now.DayOfWeek == DayOfWeek.Monday && DateTime.Now.Hour == 9)
{
}
Your code has an assignment to an undeclared variable time_now and you're doing an assignment time_now = which is what's causing it to fail.
You should also consider revising how you name your variables, dt_check_monday means absolutely nothing if the value inside it is DayOfWeek.Wednesday, consider changing it to something like dt_currentDayOfWeek but that already exists in the form of DateTime.Now.DayOfWeek which is why I dropped the variable from my example.

I think time_now is having TimeSpan datatype. So you can try this
if (dt_check_monday == DayOfWeek.Monday && time_now.Hours == 9)
{
//do something
}

If you want to keep time_now for later use, you have to encase the assignment in the if-statement with paratheses.
var dt_check_monday = DateTime.Now.DayOfWeek;
if (dt_check_monday == DayOfWeek.Monday && (time_now = DateTime.Now.Hour) == 9)
{
//do something
}

skipping over potential nulls c#

I have this block of code that eventually get serialized to JSON, for use in the Jquery FullCalender plugin. The ToUnixTimeSpan method taskes in a DateTime object and returns the number of seconds since 1970.
DateEnd could be null. In this block of code how do i test for the null and skip the end = ToUnixTimespan(e.DateEnd), if DateEnd is null? is there a C# equivalent to the groovy safe operator?
var listEvents = from e in eventRepository.GetAllEvents()
select new
{
id = e.EventID,
title = e.EventTitle,
start = ToUnixTimespan(e.DateStart),
end = ToUnixTimespan(e.DateEnd),
url = "/Events/Details/" + e.EventID
};
Further info about the ToUnixTimespanMethod:
private long ToUnixTimespan(DateTime date)
{
TimeSpan tspan = date.ToUniversalTime().Subtract(new DateTime(1970, 1, 1, 0, 0, 0));
return (long)Math.Truncate(tspan.TotalSeconds);
}

Well, how about:
end = e.DateEnd == null ? (long?) null : ToUnixTimespan(e.DateEnd)
It's hard to say for sure as we don't know the type returned by ToUnixTimespan.

Wait-a-minute... Why am I wasting this on a comment, when I could be leveraging Jon's work for some rep.. ;-)
end = e.DateEnd == null ? (long?) null : ToUnixTimespan(e.DateEnd.Value)
That should solve the "cannot convert from 'System.DateTime?' to 'System.DateTime'." error.

comparing dates in c# taking null value into account

I am new to c#. I am comparing two dates where one is entered by user and the other one is sytem date time. i have the code working as it stands where the obstacle has occured is how to cater for null values. the basic code I have is:
if (mydate.ToShortDateString() != TodaysDate.ToShortDateString())
{
//Error Messaage
}
else
{
//do some code
}
Any feedback will be appreciated

Why are you converting them to strings? Why not just compare the date portions of them as in date1.Date != date2.Date.

You can declare mydate as DateTime?, then it can hold null values.
As to how to handle the error, it depends on whether having a null value for mydate is considered an error or not. If it's an error, you could do:
if (mydate == null || mydate.ToShortDateString() != TodaysDate.ToShortDateString()) {
// error
}
If it's not an error condition, you could do:
if (mydate != null && mydate.ToShortDateString() != TodaysDate.ToShortDateString()) {
// error
}
If you don't declare mydate as DateTime? but instead just declare it as DateTime, then you can check for DateTime.MinValue, like this (DateTime.MinValue is the default value for a DateTime variable):
if (mydate == DateTime.MinValue || mydate.ToShortDateString() != TodaysDate.ToShortDateString()) {
// error
}

Use the ?? operator:
if ((mydate??DateTime.MinValue).ToShortDateString() != TodaysDate.ToShortDateString())
{
//Error Messaage
}
else
{
//do some code
}