如何使用具有可空日期的null-coalescing运算符

Question

My view model has a nullable date property such as...

[DataType(DataType.Date)]
public DateTime? SanctionExpires { get; set; }

But when I try to use the null-coalescing operator such as...

var test = model.SanctionExpires.Value.ToUniversalTime() ?? model.SanctionExpires;

I get the following error...

operator '??' cannot be applied to operands of type 'datetime' and 'datetime'

I thought this should work because I set my date property to nullable and this post suggests it should work too. What am I doing wrong?

UPDATE

To give more clarity I want to use null-coaslescing to assign a Dapper parameter which will update a DateTime field in my database. The field can accept null values or datetime values. Basically if a user specifies a date then assign the provided date to the parameter otherwise assign null...

p.Add("@SanctionExpires", model.SanctionExpires.Value.ToUniversalTime() ?? model.SanctionExpires);

Note that I have to do the UTC conversion because i'm using SQL Azure which uses UTC for dates otherwise my time is out by 11 hours (i'm in Australia) if i just pass a data from my local system. In sql i'm using a function to offset the date to my timezone before updating the table.

My current solution to get it working is below but not sure if this is the cleanest way. Would be nice to get in one line...

if (model.SanctionExpires == null)
    p.Add("@SanctionExpires", model.SanctionExpires);
else
    p.Add("@SanctionExpires", model.SanctionExpires.Value.ToUniversalTime());

Answer 1

.Value on a nullable assumes it can't be null. Hence is applying the null-coalescing operator on it useless and forbidden.

You could use the operator like this, but going on your code I am not sure if this is what you need:

DateTime test = (model.SanctionExpires ?? DateTime.Now).ToUniversalTime();

For your update: you can shortcut that evaluation like this:

DateTime? test = model.SanctionExpires?.ToUniversalTime();

Answer 2

You can also use the GetValueOrDefault method to return default value if your DateTime object is null. In case you want to use the actual date, you should use DateTime.Now .

For example following code snippet can help you:

var defaultDateTime = DateTime.Now;
var dateTime = model.SanctionExpires.GetValueOrDefault(defaultDateTime).ToUniversalTime();

If you call GetValueOrDefault without params it will use default(DateTime) if your DateTime is null

var dateTime = model.SanctionExpires.GetValueOrDefault().ToUniversalTime(); //dateTime == default(DateTime) in case !model.SanctionExpires.HasValue

Update

If you are using C#5.0 or below you can use following code snippet because NULL conditional operator is available not before C#6.0.

p.Add("@SanctionExpires", !model.SanctionExpires.HasValue ? null : (DateTime?)model.SanctionExpires.Value.ToUniversalTime());