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Is it possible to make a Func delegate an extension method? For example, just like you could create the function

bool isSet(this string x) {return x.Length > 0;}

I'd like to be able to write something like

Func<string, bool> isSet = (this x => x.Length > 0);

Of course, the above is not syntactically correct. Is there anything that is? If not, is that a limitation in syntax or compilation?

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    Maybe if you explain why you want something like that, we can offer a suitable alternative / suggestion. Commented Jun 25, 2012 at 16:32
  • I would imagine that the reason its not possible as simply as that is because extension methods work at compile time whereas what you have above is only going to be run at run time. You might be able to do something with reflection on this to add a new Method (not sure) but even if you did it would only be usable in runtime code (probably via reflection). I'm no expert though so this might be wrong but its what my gut tells me. :) Commented Jun 25, 2012 at 16:33
  • @vcsjones Short version: as a macro for that specific method. Long answer: Suppose (hypothetically) isSet would need to be called 35 times in one function, but nowhere else across an entire solution. Then I would want the syntactic sugar within one method, but without it spilling over into Intellisense for the others (or complicating the class schema). Definitely not a big deal. Commented Jun 25, 2012 at 16:44
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    @Arithmomaniac: I would argue that if you've got 35 calls to the same method within one method, your method is too long... Commented Jun 25, 2012 at 16:46
  • @Arithmomaniac: I've added an answer that should be a suitable workaround. In essence you can call a func in the same way as any other method. You'd be passing your string into it rather than calling it as an extension method but it seems to suit the spirit of what you are asking. Commented Jun 25, 2012 at 16:53

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Short answer: no, thats not possible.

Extension methods are syntactic sugar and can only be defined under certain circumstances (static method inside a static class). There is no equivalent of this with lambda functions.

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Is it possible to make a Func delegate an extension method?

No. Extension methods have to be declared as normal static methods in top-level (non-nested) non-generic static classes.

It looks like you would be trying to create an extension method only for the scope of the method - there's no concept like that in C#.

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Yeah, I was faster than Jon Skeet :). However, you get the upvotes anyways... :(
@vcsjones: Have added that in brackets for clarity.
@PhilipDaubmeier: In all fairness you do have more upvotes. :) I'm not going to +1 Jon Skeet just because he gets quite enough of that already. ;-)
@Chris: at the time I wrote the comment, Jon had +4 and my answer still 0 ;)
@PhilipDaubmeier: that may have been a result of StackOverflows partial page refreshes. If you just clicked on the new answer banner it won't update your votes but will show all the current votes on the new answer.
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To answer the question in comments on why this is wanted you coudl define the isSet func normally and just use that as a method call which will have the same effect as your extension method but with different syntax.

The syntax difference in use is purely that you'll be passing the string in as a parameter rather than calling it as a method on that string.

A working example:

public void Method() { Func<string, bool> isSet = (x => x.Length > 0); List<string> testlist = new List<string>() {"", "fasfas", "","asdalsdkjasdl", "asdasd"}; foreach (string val in testlist) { string text = String.Format("Value is {0}, Is Longer than 0 length: {1}", val, isSet(val)); Console.WriteLine(text); } }

This method defines isSet as you have above (but without the this syntax). It then defines a list of test values and iterates over them generating some output, part of which is just calling isSet(val). Funcs can be used like this quite happily and should do what you want I'd think.

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Ah, ok. I wouldn't have thought this was any different in most practical situations than an extension method. I figured maybe you just didn't know that you could do more than pass them around as arguments to linq and stuff like that. I suspect this is the best you'll get though so no eating your cake.

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