linq to sql custom method using Expression<Func<…>>

Question

Maybe I am making myself somewhat difficult in this issue. The goal is to use an object (here SearchForm ) to analyze the database records and returns if the record matches the search entry.

Since my website allows an advanced search form, the SearchForm object contains a lot properties (5 input fields, some dropdown lists, and a lot checkboxes). I am using

• C# NET 4.0 (not higher due of web host)
• LINQ-to-SQL
• ASP.NET MVC 4

Here is my snippet so far:

public static IEnumerable<CarDetails> SearchByForm(SearchForm form) {
    // get db instance
    _db = new MyDBDataContext();
    var q = from Cars_car c in _db.Cars_cars
    join Cars_equipment e in _db.Cars_equipments on c.equipmentId equals e.id
    where ValidData.Invoke(c, form)
    select new CarDetails {
         // filling CarDetails data
    };
    return q;
}

And the ValidData method is an Expression<Func<...> type.

private static Expression<Func<Cars_car, SearchForm, bool>> ValidData = (c, f) => MatchesSearchData(f, c);

where MatchesSearchData is the method behind to validate each records

private static bool MatchesSearchData(SearchForm f, Cars_car c) {
    // long check if true or false and returns it
}

But there is an error, the overloaded method has some invalid arguments at ValidData.Invoke(c, form) .

Am I making myself difficult, or is this the good way to accomplish my results? If so, can someone provide me a solution to the above error note? If not, some guidance to use other way to solve this?

There is a way to do this with stored procedures, true, but I rather prefer to do this at this layer.

Used sources;

• article about custom LINQ to SQL method
• blog post calling custom methods
• other blog post about same subject

EDIT 1

An answer from user Erti-Chris Eelmaa has solved the compiler error by using Compile() -method.

    where ValidData.Compile().Invoke(c, form)

But when running, it redirects me to the error page. Apparently, i am getting method has no supported translation to sql error.

In Controllers,

public ActionResult Results(SearchForm form) {
    ViewBag.cars = Cars_car.SearchByForm(form);
    return View("Results");
}

and on template (just quick testing)

@foreach (var car in ViewBag.cars)** {
<p>results : car.Make !</p>
}

** Here, an method Boolean Invoke(MVCproject2.Models.Cars_car, MVCproject2.Models.SearchForm) has no supported translation to sql error got thrown. That's weird, because that Expression method was added to provide a custom method to LINQ to SQL.

Or am i wrong ?

Answer 1

If you want method to be part of the SQL query, you need to create your method on SQL server, and then call it. Like so:

CREATE FUNCTION ReverseCustName(@string varchar(100))
RETURNS varchar(100)
AS
BEGIN
    DECLARE @custName varchar(100)
    -- Implementation left as exercise for users.
    RETURN @custName
END

// and C# part

[Function(Name = "dbo.ReverseCustName", IsComposable = true)]
[return: Parameter(DbType = "VarChar(100)")]
public string ReverseCustName([Parameter(Name = "string",
    DbType = "VarChar(100)")] string @string)
{
    return ((string)(this.ExecuteMethodCall(this,
        ((MethodInfo)(MethodInfo.GetCurrentMethod())),
        @string).ReturnValue));
}

You can now use ReverseCustName(string) in your "SQL" query.

Source: http://msdn.microsoft.com/en-us/library/vstudio/bb386973(v=vs.100).aspx

Alternatively you can download everything in to your C# client and execute Where there.

public static IEnumerable<CarDetails> SearchByForm(SearchForm form) {
    Func<RetailCar_car, SearchForm> compiledLambda = ValidData.Compile();
    // get db instance
    _db = new MyDBDataContext();
    var q = (from Cars_car c in _db.Cars_cars
    join Cars_equipment e in _db.Cars_equipments on c.equipmentId equals e.id)
    .ToList()
    .Where(x => compiledLambda(c, form))
    .Select(x => new CarDetails {});

    return q;
}

Let's get this straight:when you are using DbContext , you're using IQueryable . The class is meant to generate SQL queries. If you combine your C# method with IQueryable, LINQSQL doesn't know how to translate it to SQL query.

Those two solutions correspond to: either create new SQL method in SQL server, or download everything to C# side using ToList() , which means that IQueryable will be switched to IEnumerable .

Answer 2

Can you please confirm whether the following LINQ is correct

var q = from Cars_car c in _db.Cars_cars
    join Cars_equipment e in _db.Cars_equipments on c.equipmentId equals e.id 

I can't understand why you are using alias to alias here. According to me, this snippet should be like

var q = from c in _db.Cars_cars
        join e in _db.Cars_equipments on c.equipmentId equals e.id 

May be this is not the reason for the error, but I wanted to confirm if my understanding is correct.