Why can't these type arguments be inferred?

Question

Possible Duplicate:
C# 3.0 generic type inference - passing a delegate as a function parameter

Why can't the type arguments of the following code sample in the call in Main be inferred?

using System;

class Program
{
    static void Main(string[] args)
    {
        Method(Action);
    }

    static void Action(int arg)
    {
        // ...
    }

    static void Method<T>(Action<T> action)
    {
        // ...
    }
}

This gives the following error message:

error CS0411: The type arguments for method Program.Method<T>(System.Action<T>) cannot be inferred from the usage. Try specifying the type arguments explicitly.

@Pieter: The post mentioned by Darin has the answer, and has some good links to more info. — James Michael Hare, Sep 08 '11 at 18:53
Read 10 posts or something, but the above one wasn't among them. — Pieter van Ginkel, Sep 08 '11 at 18:56
@Pieter: first hit here: http://www.google.com/#sclient=psy&hl=en&source=hp&q=c%23+infer+delegate+generic&pbx=1&oq=c%23+infer+delegate+generic&aq=f&aqi=&aql=&gs_sm=e&gs_upl=3066l4016l1l4115l8l7l0l0l0l0l214l696l0.3.1l4l0&bav=on.2,or.r_gc.r_pw.&fp=b7e6040383bebbf&biw=1440&bih=795 — Darin Dimitrov, Sep 08 '11 at 18:59
Yep, you're right. I however searched through the suggestions that were presented when asking the question. May'be I'll put a bit more effort in searching the next time :). — Pieter van Ginkel, Sep 08 '11 at 19:00
more relevant link http://stackoverflow.com/questions/6229131/why-cant-c-sharp-infer-type-from-this-seemingly-simple-obvious-case — nawfal, Oct 10 '12 at 10:16

score 2 · Answer 1 · answered Sep 08 '11 at 18:59

The problem is that Action (aside from already being a type; please rename it) is actually a method group that is convertible to the delegate type Action<int>. The type-inference engine can't infer the type because method group expressions are typeless. If you actually cast the method group to Action<int>, then type-inference succeeds:

Method((Action<int>)Action); // Legal

TheCodeKing · Answer 2 · 2011-09-08T19:10:47.940

0

Just put this is a compiler I see what you mean.

I think it's because Action is used as a method group, but it's not of type Action<int>.

If you cast it to this type it works:

Method((Action<int>)Action);

edited Sep 08 '11 at 19:10

answered Sep 08 '11 at 18:51

TheCodeKing

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I would expect it to be int. And I guess the OP already knows what the compiler is telling him. – Darin Dimitrov Sep 08 '11 at 18:51
Yep until I put it in a compiler I didn't release the context of the problem. Not immediately obvious the Action refers to the method. – TheCodeKing Sep 08 '11 at 19:12

score 0 · Answer 3 · answered Sep 08 '11 at 18:53

0

This works:

    static void Main(string[] args)
    {
        Method<int>(Action);
    }

    static void Action(int arg)
    {
        // ...
    }

    static void Method<T>(Action<T> action)
    {
        // ...
    }

answered Sep 08 '11 at 18:53

Samich

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Yes, but that's not what's being asked here. What is asked is why what is shown by the OP **doesn't** work. – Darin Dimitrov Sep 08 '11 at 18:54

Why can't these type arguments be inferred?

3 Answers3