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linq select clause incorrect error

On this linq query

return (from q in db.foo.Where(f => f.ID >= id).OrderBy(f => f.ID).Skip(1).Take(1);

Why do I get the error


The type of the expression in the select clause is incorrect.  Type inference failed in the call to 'Select'.      
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ToString1
Asked:
ToString1
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5 Solutions
 
kris_perCommented:

You need to add 'select' to the query like:

return (from q in db.foo.Where(f => f.ID >= id).OrderBy(f => f.ID).Skip(1).Take(1) select q);

OR use it like:

return db.foo.Where(f => f.ID >= id).OrderBy(f => f.ID).Skip(1).Take(1);
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naspinskiCommented:
Just replace Take(1) with .First()

Take returns a collection, even though in this case it us just a collection of 1
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naspinskiCommented:
Or you can just append a .First() to the end
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ToString1Author Commented:
Ok thanks folks; I'm on the move but will award point tomorrow mornng
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ToString1Author Commented:
Ok thanks folks; I'm on the move but will award point tomorrow mornng
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Fernando SotoRetiredCommented:
Hi ToString1;

In you last post on this question you gave the example as follows :

FOO first = dataContext.Foos.First();
FOO last = dataContext.FOOs.Last();

And wanted something like :

FOO nextFoo = dataContext.FOOs  //get the next foo object

In Linq there are two forms of a query, (1) Method syntax as your examples show above. So I gave you a solution in the same query type, Method syntax. (2) Query syntax which starts with, "from XX in DataContext.Table" and MUST end with a "select or group clause".

You can intermix the two types in one query but you need to be careful to make sure that all Query syntax queries end in a select or group clause.

Now may I suggest that you use this query

return dataContext.Foos.Where(f => f.ID > id).OrderBy(f => f.ID).FirstOrDefault();

Which will sent a query to the server something like this:

SELECT TOP (1) [t0].[ID], ... "All other columns"
FROM [dbo].[Foos] AS [t0]
WHERE [t0].[ID] > @p0
ORDER BY [t0].[ID]
-- @p0: Input Int (Size = -1; Prec = 0; Scale = 0) [4]

Where a query like this one :

return dataContext.Foos.Where(f => f.ID >= id).OrderBy(f => f.ID).Skip(1).First();

Which will sent a query to the server something like this:

SELECT TOP (1) [t0].[ID], ... "All other columns"
FROM (
    SELECT TOP (1) [t0].[ID], ... "All other columns"
    FROM (
        SELECT ROW_NUMBER() OVER (ORDER BY [t0].[ID]) AS [ROW_NUMBER], [t0].[ID],
          ... "All other columns"
        FROM [dbo].[Foos] AS [t0]
        WHERE [t0].[ID] >= @p0
        ) AS [t1]
    WHERE [t1].[ROW_NUMBER] > @p1
    ) AS [t2]
ORDER BY [t2].[ROW_NUMBER]
-- @p0: Input Int (Size = -1; Prec = 0; Scale = 0) [4]
-- @p1: Input Int (Size = -1; Prec = 0; Scale = 0) [1]

Also the query I have suggested uses the FirstOrDefault() which if no record is found will return a null and will not throw an exception because of it.

Fernando
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ToString1Author Commented:
Thanks guys;  I will update later today with points
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ToString1Author Commented:
Thanks folks - very interesting.

I am still unsure about the skip/take versus FirstOrDefault  ?
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ToString1Author Commented:
Hi folks If I put

return dataContext.Foos.Where(f => f.ID > id).OrderBy(f => f.ID).FirstOrDefault();


Could not find an implementation of the query pattern for source type .....  'Select' not found
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Fernando SotoRetiredCommented:
Hi ToString1;

To your question, "I am still unsure about the skip/take versus FirstOrDefault  ?", The Skip(X) where X is an integer skips X records from the current position in the collection. The Take(X) where X is an integer takes / returns X records from the current position in the collection. FirstOrDefault returns the first record in the collection and if the query does not produce any results it returns the default value for the data type to be returned, in this case a null value.

Fernando
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Fernando SotoRetiredCommented:
To your question about this statement :

return dataContext.Foos.Where(f => f.ID > id).OrderBy(f => f.ID).FirstOrDefault();

The dataContext is a DBML model class that maps to the database table? Because the error states that the source does implement a Select does not make sense seeming no Select is being used.
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