LINQ to Entities does not recognize the method 

Question

I'm migrating some stuff from one mysql server to a sql server but i can't figure out how to make this code work:

using (var context = new Context())
{
    ...

    foreach (var item in collection)
    {
        IQueryable<entity> pages = from p in context.pages
                                   where  p.Serial == item.Key.ToString()
                                   select p;
        foreach (var page in pages)
        {
            DataManager.AddPageToDocument(page, item.Value);
        }
    }

    Console.WriteLine("Done!");
    Console.Read();
}

When it enters into the second foreach (var page in pages) it throws an exception saying:

LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression.

Anyone know why this happens?

Answer 1

As others have answered, this breaks because .ToString fails to translate to relevant SQL on the way into the database.

However, Microsoft provides the SqlFunctions class that is a collection of methods that can be used in situations like this.

For this case, what you are looking for here is SqlFunctions.StringConvert:

from p in context.pages
where  p.Serial == SqlFunctions.StringConvert((double)item.Key.Id)
select p;

Good when the solution with temporary variables is not desirable for whatever reasons.

Similar to SqlFunctions you also have the EntityFunctions (with EF6 obsoleted by DbFunctions) that provides a different set of functions that also are data source agnostic (not limited to e.g. SQL).

Answer 2

Just turn the LINQ to Entity query into a LINQ to Objects query (e.g. call ToArray) anytime you need to use a method call in your LINQ query.

Answer 3

Had a similar problem. Solved it by calling ToList() on the entity collection and querying the list. If the collection is small this is an option.

IQueryable<entity> pages = context.pages.ToList().Where(p=>p.serial == item.Key.ToString())

Hope this helps.

Answer 4

The problem is that you are calling ToString in a LINQ to Entities query. That means the parser is trying to convert the ToString call into its equivalent SQL (which isn't possible...hence the exception).

All you have to do is move the ToString call to a separate line:

var keyString = item.Key.ToString();

var pages = from p in context.entities
            where p.Serial == keyString
            select p;

Answer 5

In MVC, assume you are searching record(s) based on your requirement or information. It is working properly.

[HttpPost]
[ActionName("Index")]
public ActionResult SearchRecord(FormCollection formcollection)
{       
    EmployeeContext employeeContext = new EmployeeContext();

    string searchby=formcollection["SearchBy"];
    string value=formcollection["Value"];

    if (formcollection["SearchBy"] == "Gender")
    {
        List<MvcApplication1.Models.Employee> emplist = employeeContext.Employees.Where(x => x.Gender == value).ToList();
        return View("Index", emplist);
    }
    else
    {
        List<MvcApplication1.Models.Employee> emplist = employeeContext.Employees.Where(x => x.Name == value).ToList();
        return View("Index", emplist);
    }         
}

Answer 6

Change it like this and it should work:

var key = item.Key.ToString();
IQueryable<entity> pages = from p in context.pages
                           where  p.Serial == key
                           select p;

The reason why the exception is not thrown in the line the LINQ query is declared but in the line of the foreach is the deferred execution feature, i.e. the LINQ query is not executed until you try to access the result. And this happens in the foreach and not earlier.