Question

This is my linq query (Linq to Sql) :

pagina = (from Pagine page in kDataBase.Pagines
          where prova(page.title_it) == urlSuddivisa[i].ToLower()
          select page).FirstOrDefault();

and this is the function I call (inside the same class) :

private string prova(string example)
{
    return example;
}             

it says :

there are not supported conversion to sql for the method ‘System.String prova(System.String).

Where am I wrong? And how can I fix it?

Answer 1

Since the linq query is converted into a SQL query that runs against your dB it is not able to translate your custom function into a SQL query.

Alternative ways of appraoaching the problem

• Use a stored procedure and read about using stored procedure in linq

• retrieve your data from SQL to memory and then filter it based on your function. The disadvantage obviously here is you will be retrieving a lot many number of rows from the database than what is required

  var paginas=(from Pagine page in kDataBase.Pagines).ToList().Where(p =>prova(p.title_it) == urlSuddivisa[i].ToLower()).FirstOrDefault();

  is what you want.

On a side note I am guessing your method prova does more than just returning the string otherwise the function is outright useless and you may as well get rid of the function. Also I believe you are firing multiple queries to the database cos of the array that you are using with a counter i you could always use the IN query and get around it, remember the restriction of the number of element in an IN query.