Comparing nullable DateTime?

Question

Looking for a better way to compare a nullable date time than the following:

Any suggestions?

// myobject.ExpireDatetime is of DateTime?
//
if (!myob         


        

Answer 1

I would recommend you to use the following:

int y = Nullable.Compare<DateTime>(DateTime.UtcNow, x); 

// x is the nullable date time object.
// y will be '-1' if x is greater than DateTime.UtcNow

Answer 2

If ExpireDateTime is a Nullable<DateTime>i would do following instead:

if (ExpireDateTime.HasValue && ExpireDateTime < DateTime.Now.ToUniversalTime())
{ 
}

Answer 3

The compiler lifts variables and generates code to check for nulls.

> new DateTime?()
null
> DateTime.Now > null
false
> DateTime.Now < null
false

> new int?()
null
> 10 >= null
false
> 10 =< null
false
> 10 == null
false
> 10 != null
true

Knowing this you can write simple code.

// d is a DateTime? 
DateTime now = DateTime.Now;

bool after = d > now;
bool before = d < now;
bool today = d?.Date == now.Date;

If d is null everything will be false, else it will work like normal DateTime comparison.

Answer 4

Use the Value property of the nullable:

objet.ExpireDateTime.Value

---

if (!object.ExpireDateTime.IsNull() 
    && DateTime.Compare(objet.ExpireDateTime.Value, 
                        DateTime.Now.ToUniversalTime()) < 0)
{ 
}    

Answer 5

Your question is not quite clear to me, but if we have

DateTime? ExpireDateTime;  // could be a variable or a property

it's OK to say just

if (ExpireDateTime < DateTime.UtcNow)
{
  ...
}

This will be OK if ExpireDateTime is null (HasValue is false). Some inexperienced developers will struggle to understand lifted operators, so to make it more clear, you could write

if (ExpireDateTime < (DateTime?)DateTime.UtcNow)
{
  ...
}

It's the same, but easier to read and understand.

Never write .Value if the nullable might be null, of course. You will get an InvalidOperationException "Nullable object must have a value" if you do so.