Nullable DateTimes and the AddDays() extension

Question

I have a DateTime variable that can either be null or a Datetime. I figured a nullable DateTime type would work, but I am getting an error telling me that said

Answer 1

If you look at the documentation for the Nullable<T> type, you'll see it only has a few members. The Value member will get you the T inside, if the Nullable<T> container has a value. The Nullable<T> doesn't borrow any members from the type T it wraps.

Answer 2

You need to go through the "Value" property:

lastInvite.Value.AddDays(7)

Note that this will throw an exception if the DateTime is actually null. Luckily, there is another property you can use to test for this, "HasValue".

if (lastInvite.HasValue){ /* code */ }

Answer 3

There are multiple ways to resolve this error.

1. Use Convert.ToDateTime() method which handles null datetime values.

   Convert.ToDateTime(lastInvite).AddDays(7);

2. Use Value property

   lastInvite.Value.AddDays(7)

3. Use GetValueOrDefault() method

   lastInvite.GetValueOrDefault().AddDays(7)

Answer 4

You should consider trying to make your logic more readable:

var inviteNudgeFlag = bool.Parse((string) Session["InviteNudgeFlag"]);

if(!inviteNudgeFlag)
{
    return;
}

var lastInvite = (DateTime?) Session["LastInviteSent"];   
var inviteCount = (int) Session["InviteCount"];
var numOfDays = 7;
var now = DateTime.Now;

var weekSinceLastInvite = lastInvite.HasValue
                            ? now >= lastInvite.Value.AddDays(numOfDays)
                            : now >= AcctCreation.AddDays(numOfDays);

var hasInvites = !lastInvite.HasValue || inviteCount > 0;
var canInvite = hasInvites && weekSinceLastInvite;

if(!canInvite)
{
    return;
}