String extension method in Linq query

Question

How to use string extension method in linq query:

public NewsType GetNewsType(string name)
{
   var newsType = db.NewsTypes.FirstOrDefault(x => x.Name.ToFriendlyUrl() ==  
                                              name.ToFriendlyUrl());
   return newsType;
}

Above query x.Name.ToFriendlyUrl() is not allowed at the minute. Is anyone know how to achieve with it.

Answer 1

Extension methods are indeed allowed in LINQ queries, moreover the LINQ methods themselves are implemented as extension methods.

It's quite another issue however, to use extension methods (or most other methods) in LINQ-to-SQL or LINQ-to-Entities queries. Those queries are not actually run in the C# code, but they are treated like expressions that are translated to SQL. I.e.

db.News.Where(x => x.Published).Select(x => x.Name)

is translated to the SQL Statement

Select Name 
From News
Where Published = 1

and it's results are returned to the C# code.

Since there is not way to transfer the ToFriendlyUrl() method to SQL, your code throws an error.

You have basically, two solutions/workarounds. One is to transform the call to a form could be translated into SQL, e.g. if the ToFriendlyUrl() method was just:

public static string ToFriendlyURL(this string value)
{
    return value.ToLower();
}

you can inline that code in the LINQ call, and that would work. If however, the methods is more complex, than your only solution is to just fetch the data from the base and then process it on the C# side:

var newsTypeQuery = db.NewsTypes.Where(x => // other conditions, if any);
var newsTypes = newsTypes.ToList(); //forces execution of the query
                                    // the result is now a C# list
var newsType = newsTypes.FirstOrDefault(x => 
                                 x.Name.ToFriendlyUrl() == name.ToFriendlyUrl());