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

clinqMySQLsql

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?

Best Answer

Just save the string to a temp variable and then use that in your expression:

var strItem = item.Key.ToString();

IQueryable<entity> pages = from p in context.pages
                           where  p.Serial == strItem
                           select p;

The problem arises because ToString() isn't really executed, it is turned into a MethodGroup and then parsed and translated to SQL. Since there is no ToString() equivalent, the expression fails.

Note:

Make sure you also check out Alex's answer regarding the SqlFunctions helper class that was added later. In many cases it can eliminate the need for the temporary variable.

Related Solutions

C# – Method cannot be translated into a store expression

Reason: By design, LINQ to Entities requires the whole LINQ query expression to be translated to a server query. Only a few uncorrelated subexpressions (expressions in the query that do not depend on the results from the server) are evaluated on the client before the query is translated. Arbitrary method invocations that do not have a known translation, like GetHomeFeatures() in this case, are not supported.
To be more specific, LINQ to Entities only support Parameterless constructors and Initializers.

Solution: Therefore, to get over this exception you need to merge your sub query into the main one for GetCommunityFeatures() and GetHomeFeatures() instead of directly invoking methods from within the LINQ query. Also, there is an issue on the lines that you were trying to instantiate a new instance of LazyList using its parameterized constructors, just as you might have been doing in LINQ to SQL. For that the solution would be to switch to client evaluation of LINQ queries (LINQ to Objects). This will require you to invoke the AsEnumerable method for your LINQ to Entities queries prior to calling the LazyList constructor.

Something like this should work:

public IQueryable<Models.Estate> GetEstates()
{
    return from e in entity.Estates.AsEnumerable()
       let AllCommFeat = from f in entity.CommunityFeatures
                         select new CommunityFeatures {
                             Name = f.CommunityFeature1,
                             CommunityFeatureId = f.CommunityFeatureId
                         },
       let AllHomeFeat = from f in entity.HomeFeatures
                         select new HomeFeatures() {
                             Name = f.HomeFeature1,
                             HomeFeatureId = f.HomeFeatureId
                         },
       select new Models.Estate {
            EstateId = e.EstateId,
            AllHomeFeatures = new LazyList<HomeFeatures>(AllHomeFeat),
            AllCommunityFeatures = new LazyList<CommunityFeatures>(AllCommFeat)
       };
}

More Info: Please take a look at LINQ to Entities, what is not supported? for more info. Also check out LINQ to Entities, Workarounds on what is not supported for a detailed discussion on the possible solutions. (Both links are the cached versions because the original website is down)

.net – LINQ to Entities does not recognize the method

As you've figured out, Entity Framework can't actually run your C# code as part of its query. It has to be able to convert the query to an actual SQL statement. In order for that to work, you will have to restructure your query expression into an expression that Entity Framework can handle.

public System.Linq.Expressions.Expression<Func<Charity, bool>> IsSatisfied()
{
    string name = this.charityName;
    string referenceNumber = this.referenceNumber;
    return p => 
        (string.IsNullOrEmpty(name) || 
            p.registeredName.ToLower().Contains(name.ToLower()) ||
            p.alias.ToLower().Contains(name.ToLower()) ||
            p.charityId.ToLower().Contains(name.ToLower())) &&
        (string.IsNullOrEmpty(referenceNumber) ||
            p.charityReference.ToLower().Contains(referenceNumber.ToLower()));
}

Best Answer

Note:

Related Solutions

C# – Method cannot be translated into a store expression

.net – LINQ to Entities does not recognize the method

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