Writing F# queries in LINQPad

Question

I'm trying to use F# to construct a query against a database, but I can't get even the simplest of queries to compile.

I can construct a C# query like this:

from c in Categories
select c

Everything works fine. However, when I try to do what should be the same thing in F#:

query { for c in Categories do
        select c }

I get the following error:

Invalid use of a type name and/or object constructor. If necessary use 'new' and apply the constructor to its arguments, e.g. 'new Type(args)'. The required signature is: Categories() : unit.

LINQPad comes bundled with a number of samples, but none of the F# samples actually show how to use it to query against a database. I've looked around the internet, also, but I can't find any examples using F# to query a database in LINQPad. What am I missing here?

Basle

Answer 1

Out of curiosity I decided to try and get something working with F# in LinqPad. I was able to get it working by selecting "F# Program" instead of F# expression. I'm sure it's possible using just an expression, but I am 100% unfamiliar with F# so I just did enough to get a working example. Selecting this option gave me a single line of:

let dc = new TypedDataContext()

From there, I followed this example page to get this query working/executing:

let dc = new TypedDataContext()

let query1 = query { for c in dc.MyTable do
                     select c }

query1
|> Seq.iter (fun x -> printfn "SomeField: %s" x.SomeField)

Poodle

I figured there was probably some sort of magic word I had to use; turns out dc was it. In 'F# Expression' mode, query { for c in dc.Categories do select c } works fine. Thanks for pointing me in the right direction. – Basle

No problem. You'll probably also notice the warning you get using dc.Categories about not using the this keyword. Looks like similar to C#, the DataContext is actually scoped to this, so you can do this.Catagories and get rid of the warning. – Poodle

Answer 2

You just need to add "this." in front of the mapped table if you want to run it as a F# Expression in LINQPad.

query { for c in this.Categories do
        select c }

Loverly