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Using F# Indexers from C#
f#c#

Classes I have defined in F# use indexers with the C# 'standard' Item name, as follows,

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type Foo = class
  val elements: int[]
  member obj.Item
    with get (i) = obj.elements.[i]
    and set (i, v) = obj.elements.[i] <- v
  new () = { elements = Array.create 5 0}
end

and these work fine when exercised from F#. However, I find I cannot use my indexers from, C# like this,

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Foo f = new Foo();
f[1] = 5;

because I get the following error from the C# compiler:

<i>Error 1 Cannot apply indexing with [] to an expression of type<i> There is some reference to <code lang=csharp>System.Reflection.DefaultMemberAttribute </code> in a <A href="http://msmvps.com/blogs/jon.skeet/archive/2007/11/10/getting-started-with-f.aspx">conversation between Robert Pickering and Jon Skeet</A>, but I couldn't make sense of it. Foundations of F# has a note saying that, <i>"when working with indexers with a name other than Item, remember that it will be difficult for other .NET languages to use your classes"<i> but I suspect more is required. Many thanks, Rob P.S. I have emoticons disabled in my hubFS preferences, but I continue to be plagued by them. Any advice?

.

It appears that the F# compiler should be adding a DefaultMemberAttribute to the class to make the indexer useable from C#. It is not at the moment, so it is a bug in the F# compiler. However fortunately this is easy to fix, by adding the attribute your self:

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[<System.Reflection.DefaultMember("Item")>]
type Foo = class
  val elements: int[]
  member obj.Item
    with get (i) = obj.elements.[i]
    and set (i, v) = obj.elements.[i] <- v
  new () = { elements = Array.create 5 0}
end

I tested this code and it seemed work okay from C#.

However, I think this bug is somewhat forgivable from the F# point of view as the C# spec total fails to mention that the compiler does this!

By on 12/2/2007 8:08 AM ()

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