Getting character values in Linq to Entities

Question

I have a table Assessment which has (among others) two columns, CurrentGrade and PreviousGrade , both of which are defined as char(1) . A grade is a symbol from A to Z.

Now I want to write a query to get the difference between the current grade and the previous grade of all records in Assessment , to be expressed as a number, eg if previous is A and current is D, then the difference is 3.

I would have done something like this:

var q = db.Assessments.Select(a => new { a.ID, Diff = a.CurrentGrade[0] - a.PreviousGrade[0] });

... except that this doesn't work, because there's no SQL translation for String.getChars .

And I don't want to do this in local memory, because in my real code, this is just part of a big "where" clause that involves other expressions that must be done in Linq to Entities.

Any recommendations how to do this?

Answer 1

In Linq-To-Entities , you can use SqlFunctions.Ascii :

using System.Data.Objects.SqlClient;

var result = db.Assessments.Select(a => new { a.id, 
    Diff = SqlFunctions.Ascii(a.CurrentGrade).Value - SqlFunctions.Ascii(a.PreviousGrade).Value });

Note that if CurrentGrade is higher (A) than PreviousGrade (D), you'll get an answer of -3 which is reverse of what you may want... So you may want to multiply by -1 to get the true direction of the grade difference.

var result = db.Assessments.Select(a => new { a.id, 
    Diff = (SqlFunctions.Ascii(a.CurrentGrade).Value - SqlFunctions.Ascii(a.PreviousGrade).Value) * -1 });