Entityframework query error on compilation.

Question

I have a query:

               AmazonMWSTestEntities amazonMWSDb = new AmazonMWSTestEntities();
               return from f in amazonMWSDb.FeedSubmissions
                      select f.ID.ToString();

This is in a function that is returning IEnumerable<string> and when I try to compile this I get the error:

Cannot convert lambda expression to type 'string' because it is not a delegate type

But in other places in the project I have a similar query:

                AmazonMWSTestEntities amazonMWSDb = new AmazonMWSTestEntities();
                return from r in amazonMWSDb.Reports where !r.Acknowledged select r.ID.ToString();

that yields no such error. What am I doing wrong?

I tried surrounding the Guid that I am converting to a string with a parenthesis as the error seemed to indicate that the "ToString" was being applied to the lambda expression but this didn't help and result in the same error.

Thank you.

---

Kevin Burton

Answer 1

Hi Kevin,

I think you can try modify your code as following:

AmazonMWSTestEntities amazonMWSDb = new AmazonMWSTestEntities();
               return (from f in amazonMWSDb.FeedSubmissions
                      select f.ID).ToList().Select(t=>t.ToString()); 

For your second query, amazonMWSDB.Reports is IEnmerable type, so you can use ToString() method on it.

AmazonMWSTestEntities amazonMWSDb = new AmazonMWSTestEntities();
               return (from f in amazonMWSDb.FeedSubmissions.ToList()
                      select f.ID.ToString());

This query should work too, but it will query all the records in database, the performance is bad.

Have a nice day.

---

Alan Chen[MSFT]
MSDN Community Support | Feedback to us
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Answer 2

I will try your solution but I will have to choke it up to something I don't understand if it works. You see when I create the database context from the database it creates the same property type for both of the tables that I wanted to include. In other words the properties Reports and FeedSubmissins return the same type, namely ObjectContext<T>. In the case of Reports the 'T' is a Report type and FeedSumissions it is a FeedSubmission type. If the query was something like:

from r in amazonMWSDb.Reports where !r.Acknowledged select r.ID

then I would get an IEnumerable<Guid>. So why if I change the query to

from r in amazonMWSDb.Reports where !r.Acknowledged select r.ID.ToString()

can't I expect to get an IEnumerable<string>?

---

Kevin Burton

Answer 3

Hi Kenvin,

This the difference between IQueryable and IEnmerable:

http://social.msdn.microsoft.com/Forums/nl-NL/adodotnetentityframework/thread/35b74da4-3dd0-4c96-83df-17bc496e4b8e

You can use SQL Profiler to watch the "T-SQLs", Entity Framework provider couldn't recognise(Translate) the "ToString()"method, so your exception is here. The way to convert the type is make the collection to be IEnumerable<> first, then convert it in memory.

Have a nice day.

---

Alan Chen[MSFT]
MSDN Community Support | Feedback to us
Get or Request Code Sample from Microsoft
Please remember to mark the replies as answers if they help and unmark them if they provide no help.