LINQ to Entities does not recognize the method 

问题:

I have this linq query:

private void GetReceivedInvoiceTasks(User user, List<Task> tasks)
{
    var areaIds = user.Areas.Select(x => x.AreaId).ToArray();

    var taskList = from i in _db.Invoices
                   join a in _db.Areas on i.AreaId equals a.AreaId
                   where i.Status == InvoiceStatuses.Received && areaIds.Contains(a.AreaId)
                   select new Task {
                       LinkText = string.Format(Invoice {0} has been received from {1}, i.InvoiceNumber, i.Organisation.Name),
                       Link = Views.Edit
                   };
}

It has issues though. I'm trying to create tasks. For each new task when I set the link text to a constant string like "Hello" it is fine. However above I'm trying to build the property linktext using properties of the invoice.

I get this error:

base {System.SystemException} = {"LINQ to Entities does not recognize the method 'System.String Format(System.String, System.Object, System.Object)' method, and this method cannot be translated into a store expression."}

Anyone know why? Anyone know an alternative way of doing this to make it work?

回答1:

Entity Framework is trying to execute your projection on the SQL side, where there is no equivalent to string.Format. Use AsEnumerable() to force evaluation of that part with Linq to Objects.

Based on the previous answer I have given you I would restructure your query like this:

int statusReceived = (int)InvoiceStatuses.Received;
var areaIds = user.Areas.Select(x=> x.AreaId).ToArray();

var taskList = (from i in _db.Invoices
               where i.Status == statusReceived && areaIds.Contains(i.AreaId)
               select i)
               .AsEnumerable()
               .Select( x => new Task()
               {
                  LinkText = string.Format("Invoice {0} has been received from {1}", x.InvoiceNumber, x.Organisation.Name),
                  Link = Views.Edit
                });

Also I see you use related entities in the query (Organisation.Name) make sure you add the proper Include to your query, or specifically materialize those properties for later use, i.e.:

var taskList = (from i in _db.Invoices
               where i.Status == statusReceived && areaIds.Contains(i.AreaId)
               select new { i.InvoiceNumber, OrganisationName = i.Organisation.Name})
               .AsEnumerable()
               .Select( x => new Task()
               {
                  LinkText = string.Format("Invoice {0} has been received from {1}", x.InvoiceNumber, x.OrganisationName),
                  Link = Views.Edit
                });

回答2:

IQueriable derives from IEnumerable, the main resemblance is that when you make your query it is posted to the database engine in it's language, the thin moment is where you tell C# to handle the data on the server(not client side) or to tell SQL to handle data.

So basically when you say IEnumerable.ToString(), C# gets the data collection and calls ToString() on the object. But when you say IQueriable.ToString() C# tells SQL to call ToString() on the object but there is no such method in SQL.

The drawback is that when you handle data in C# the whole collection that you are looking trough must be built up in memory before C# applies the filters.

Most efficient way to do it is to make the query as IQueriable with all the filters that you can apply.

And then build it up in memory and make the data formatting in C#.

IQueryable<Customer> dataQuery = Customers.Where(c => c.ID < 100 && c.ZIP == 12345 && c.Name == "John Doe");

 var inMemCollection = dataQuery.AsEnumerable().Select(c => new
                                                  {
                                                     c.ID
                                                     c.Name,
                                                     c.ZIP,
                                                     c.DateRegisterred.ToString("dd,MMM,yyyy")
                                                   });