CASE Statement Equivalent in LINQ

Last Friday while having a discussion on various LINQ standard query operators the topic of expressing T-SQL CASE statement in LINQ came up.

The most obvious solution to this is using the ternary operator as shown below:

using (SchoolEntities sc = new SchoolEntities())
            {
                var query = from grade in sc.StudentGrade
                            join student in sc.Person on grade.Person.PersonID
                                          equals student.PersonID
                            select new
                            {
                                FirstName = student.FirstName,
                                LastName = student.LastName,
                                Grade = grade.Grade.Value >= 4 ? "A" :
                                            grade.Grade.Value >= 3 ? "B" :
                                            grade.Grade.Value >= 2 ? "C" :
                                            grade.Grade.Value != null ? "D" : "-"
                            }; 

                foreach (var q in query)
                {
                    Console.WriteLine("{0} {1} has {2} grade", q.FirstName, q.LastName, q.Grade);
                }
                Console.Read();
            } 

This works fine and the a SQL is fired in the background with T-SQL CASE statement as shown below:

SELECT 
1 AS [C1], 
[Extent2].[FirstName] AS [FirstName], 
[Extent2].[LastName] AS [LastName], 
CASE WHEN ([Extent1].[Grade] >= cast(4 as decimal(18))) THEN N'A' ELSE CASE WHEN ([Extent1].[Grade] >= cast(3 as decimal(18))) THEN N'B' ELSE 
CASE WHEN ([Extent1].[Grade] >= cast(2 as decimal(18))) THEN N'C' ELSE CASE WHEN ([Extent1].[Grade] IS NOT NULL) THEN N'D' ELSE N'-' END END 
END END AS [C2]
FROM  [dbo].[StudentGrade] AS [Extent1]
INNER JOIN [dbo].[Person] AS [Extent2] ON ([Extent1].[StudentID] = [Extent2].[PersonID]) OR (([Extent1].[StudentID] IS NULL) AND 
([Extent2].[PersonID] IS NULL)) 

I though if this CASE statement becomes more complicated we need to extract it into a separate method as shown and call the method from LINQ query.

static void Main(string[] args)
        {
            using (SchoolEntities sc = new SchoolEntities())
            {
                 var query = from grade in sc.StudentGrade
                            join student in sc.Person on grade.Person.PersonID
                                          equals student.PersonID
                            select new
                            {
                                FirstName = student.FirstName,
                                LastName = student.LastName,
                                Grade = GetGrade(grade.Grade.Value)
                            }; 

                foreach (var q in query)
                {
                    Console.WriteLine("{0} {1} has {2} grade", q.FirstName, q.LastName, q.Grade);
                }
                Console.Read();
            }
        }
        public static string GetGrade(decimal value)
        {
            string grade; 

                if(value >=4 ) grade = "A";
                if( value >=3) grade = "B";
                if( value >=2) grade = "C";
                if( value >=0)
                {
                    grade = "D";
                }
                else
                {
                    grade = "-";
                } 

            return grade;
        } 

But this results in the exception:

LINQ to Entities does not recognize the method ‘System.String GetGrade(System.Decimal)’ method, and this method cannot be translated into a store expression.

This query compiles but fails during execution as Entity Framework is not able to convert this custom method to a SQL equivalent function or statement.This issue will be in Entity Framework or LINQ to SQL but will work well LINQ to Objects where there is no backing database.