How do I extend the Func delegate to contain more than the maximum four parameters?

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+2 Q:

How do I extend the Func delegate to contain more than the maximum four parameters?

I have been using LINQ with compiled queries, basically passing into the compiled query using Func but the problem is that it has a maximum of four parameters.

Is it good practice to extend this?

Is there any way to extend this or should I create my own delegate?

Sometimes I need to pass six params and others five and others four or less... so with four or less I could continue using the Func delegate.

Currently passing in data context and a mix of the params I need are dependent on each individual compiled query.

+3 A:

Just declare your own delegates - it's only a single line of code to do so. See my examples for declaring the .NET 3.5 delegates in .NET 2.0 for a sort of template, if you need it.

I don't know how well this will work with LINQ compiled queries - hopefully it won't be a problem, but your use case isn't quite clear enough to me to say for sure.

Note that in .NET 4.0 there's support for Func/Action with up to 8 parameters.

Jon Skeet 2009-11-09 10:06:50

Thanks jon, so i presume if .net 4.0 is going to have up to 8 params its not a good idea to create my own Func with 5 params.. i should name it something else OR AT LEAST put it in its own namespace?

mark smith 2009-11-09 11:20:27

@mark: It depends on what you're building, and how you're planning to approach .NET 4.0. If it's a library which should work with different versions of .NET, that's tricky. If it's an application which you can "upgrade" once by removing your delegate declarations, that's simpler.

Jon Skeet 2009-11-09 11:27:58

Thanks Jon, thats what i will do it just remove the delegate when .net 4.0 comes along

mark smith 2009-11-09 12:37:18

Jon, i have 1 question - maybe you can confirm, the new delegate is working but i need to pass it into CompileQuery of LINQ .. and that only supports up to 4 as well... I have looked at the code with reflector but it seems maybe the best way would be to pass the parameters in a parameter Object as per Ruben's suggestion.. But this means i going to have to create a new parameter object for every different collection of parameters? Do you know of a better way?

mark smith 2009-11-09 13:08:32

Not really - if you need to pass it into LINQ to SQL, you're going to have to use a delegate type that LINQ to SQL understands.

Jon Skeet 2009-11-09 13:16:21

Agreed, so i am going to create a parameter class and pass it into the stanadard FUNC and then into CompiledQuery ... Thank you for conifrming this!! - i learn something new every day!!!

mark smith 2009-11-09 13:20:47

+2 A:

What Jon said plus...

As you've suggested, generally you shouldnt end up with too many params - generally you should find some uniting concept jumping out telling you to Introduce Parameter Object. Having said that, 4 wouldnt be the place where I'd place a 'that's just crazy' line.

Ruben Bartelink 2009-11-09 10:11:49

Thanks Ruben for your comment, i will keep this for the future! Currently i need only a max of 6 i think - so i think its quite manageable

mark smith 2009-11-09 11:27:22

Hi Ruben, i am thinking of using your technique but in the example it creates a parameter object with types i.e. int,string etc etc.. Do you know if its possible to use a kind of generic version so that i can use this object for passing all different types inside. I could just specify it as Object i suppose but then i will need to Cast to the actual type?

mark smith 2009-11-09 13:10:10

Hi Mark, The reference to the example of the pattern was inserted by an editor (Thanks Groo!). If you're looking to use a predefined class, that's more like Tuples (which are in F# and .NET 4). The main point of IPO is that that you'd create a new class with a good name which represents the overall concept behind the set of partams you're passing, e.g., instead of "header, body, trailer" you have a `DocumentDefinition` with properties or methods wrapping it up. There's nothing stopping you using a Dictionary as a Bag to pass them as, but that would be a differnt approach.

Ruben Bartelink 2009-11-09 13:54:18

For info on tuples (Like Twitter, SO counts the letters of a URL as real letters!), see http://blogs.msdn.com/bclteam/archive/2009/07/07/building-tuple-matt-ellis.aspx

Ruben Bartelink 2009-11-09 13:55:43

Just reading your conversation w/ The Horse... This may or may not be applicable to your context - its just a general refactoring that applies when you're find yourself passing lots of parameters that are a class waiting to be identified.

Ruben Bartelink 2009-11-09 13:58:44

thanks, i have managed to get something going...thanks for your comments

mark smith 2009-11-10 07:57:27

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How do I extend the Func delegate to contain more than the maximum four parameters?

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