为什么我不能在linq查询中调用函数？

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.

Answer 2

Your linq query expression is translated into sql at runtime, and the framework doesn't know how to convert the prova method call into an appropriate sql. Instead, you can do something like this:

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

This will pull the pages into memory first via the toList, where the prova method can then be evaluated.

Answer 3

With linq to sql the Expression tree get parsed to SQL query instead of just running it like it is with linq to objects .

Your ORM provider doesn't know hot to convert the method to SQL.

You shouldn't use methods in linq to SQL LINQ queries.