方法名称（int，DateTime）不支持SQL转换

Question

I have a function, Name(int, DateTime) , that I'd like to use in a linq2sql query.

Its a simple function that computes the difference between the passed in DateTime object and DateTime.Now using a TimeSpan . I then do some basic math stuff with .TotalHours and the passed in int. The most complicated function is Math.Pow(double,double).

public static double Name(int num, DateTime Timestamp)
{
        DateTime now = DateTime.Now;
        DateTime then = Timestamp;

        TimeSpan elapsedSpan = new TimeSpan(now.Ticks - posted.Ticks);
        double t = elapsedSpan.TotalHours;

        return System.Math.Pow(t, num);
}

I get this error:

Method 'Double Name(Int32, System.DateTime)' has no supported translation to SQL. 

When running a query like this:

var q = from k in db.Stuff
        select new {Name=Name(k.num, k.Timestamp)};

I understand that the function cannot be converted to SQL as it currently is written. Is there a way to convert what I have into something that is SQL useable?

I should note that a ToList() or ToArray() method is not useable because the datatable is large.

Answer 1

As your conversion is only use when projecting, I suggest that you simply do that bit in .NET instead of in SQL:

var q = db.Stuff.Select(k => new { k.num, k.Timestamp })
                .AsEnumerable() // Do further processing in .NET
                .Select(k => Name(k.num, k.Timestamp));

Note that I've taken the result out of an anonymous type too, so the type of q will just be an IEnumerable<double> .

If you really need the processing to be done in the database, I suggest you write a user-defined function in SQL yourself and tell LINQ to SQL about that (or make it part of a view that you select from).

Answer 2

LINQ-to-SQL can't translate arbitrary code methods, as they are IL (it expects "expression trees", which is what lambdas and LINQ query syntax a generate). If you only need this logic in your .NET code, switch to LINQ-to-Objects before calling, Ie

var qry = some complex LINQ query

var data = from row in qry.AsEnumerable()
            select new {
               row.Id,
               Name = YourType.Name(row.Foo, row.Bar)
            };

The AsEnumerable() breaks the LINQ composition in two, allowing your managed code to run as normal.

If you need to do something similar at the database , then write the code in TSQL as a UDF, then map the UDF into LINQ-to-SQL by dragging it onto the designer. You can then access it as a new method on the data-context, Ie

 select db.Name(row.Foo, row.Bar)