LINQ to Entities for subtracting 2 dates

Question

I am trying to determine the number of days between 2 dates using LINQ with Entity Framework. It is telling me that it does not recognize Subtract on the System.TimeSpan class

Here is my where portion of the LINQ query.

where ((DateTime.Now.Subtract(vid.CreatedDate).TotalDays < maxAgeInDays))

Here is the error I receive in the VS.NET debugger

{"LINQ to Entities does not recognize the method 'System.TimeSpan Subtract(System.DateTime)' method, and this method cannot be translated into a store expression."}

Am I doing something wrong or is there a better way to get the number of days between 2 DateTimes in the entity framework?

thanks Michael

Answer 1

The accepted answer is better in this case, but for reference you can use the EntityFunctions class to perform operations on dates, among other things.

where (vid.CreatedDate >= EntityFunctions.AddDays(DateTime.Now, -maxAgeInDay))

Answer 2

Here is how I got it to work

I defined a datetime variable that represents the oldest date

DateTime oldestDate = DateTime.Now.Subtract(new TimeSpan(maxAgeInDays, 0, 0, 0, 0));
...

then I modified the where portion of the LINQ query

where (vid.CreatedDate >= oldestDate )

worked like a charm - thanks Micah for getting me to think about the expression tree

Answer 3

You can also use System.Data.Objects.EntityFucntions:

currentDate = DateTime.Now;

...
where  EntityFunctions.DiffDays(currentDate, vid.CreatedDate) < maxAgeIdDays 

All functions from EntityFunctions are only for Linq-to-entities and are mapped to SQL functions.

Answer 4

You run into these kind of isses because the predicate needs to be translated to an expression tree. And translation process doesn't recognize the DateTime.Now.Subtract method.

Answer 5

The fact is that by design, LINQ to Entities needs to translate the whole query to SQL statements. That's where it cannot recognize Subtract method. It will occur whenever you try to use a C#/VB method inside a query. In these cases you have to figure out a way to bring out that part from the query. This post explains a bit more: http://mosesofegypt.net/post/LINQ-to-Entities-what-is-not-supported.aspx

Answer 6

You may define new property in your model:

    public DateTime StartDate{ get; set; }
    public DateTime EndDate{ get; set; }
    public TimeSpan CalculateTime{
        get
        {
            return EndDate.Subtract(StartDate);
        }
    }

Now, you may use something like that:

var query = from temp in db.Table
select new MyModel {
    Id = temp.Id,
    Variable1 = temp.Variable1,
    ...
    EndDate = temp.EndDate,
    StartDate = temp.StartDate
}

When you have look at result, you may use return such as:

return query

Now, in query, we have CalculateTime (subtract between EndDate and Startdate).