.net - Why doesn't the C# compiler automatically infer the types in this code? -

why c# compiler not infer fact fooext.multiply() satisfies signature of functions.apply()? have specify separate delegate variable of type func<foo,int,int> code work ... seems type inference should handle this. wrong? and, if so, why?

edit: compilation error received is:

the type arguments method firstclassfunctions.functions.apply<t1,t2,tr>(t1, system.func<t1,t2,tr>, t2)' cannot inferred usage. try specifying type arguments explicitly

namespace firstclassfunctions  {     public class foo  {         public int value { get; set; }          public int multiply(int j) {             return value*j;         }     }      public static class fooext  {         public static int multiply(foo foo, int j) {             return foo.multiply(j);         }     }      public static class functions  {         public static func<tr> apply<t1,t2,tr>( t1 obj,                                                  func<t1,t2,tr> f, t2 val ) {             return () => f(obj, val);         }     }       public class main  {         public static void run()  {             var x = new foo {value = 10};             // line below won't compile ...             var result = x.apply(fooext.multiply, 5);             // will:             func<foo, int, int> f = fooext.multiply;             var result = x.apply(f, 5);         }     } 

i believe result of vs2008 c# compiler's inability correctly infer types involved when converting method group delegate. @eric lippert discusses behavior in post c# 3.0 return type inference not work on method groups.

if recall correctly, improvements made in new c# compiler that's part of vs2010 expands cases method group inference possible.

now, rules type inference quite complicated, , i'm far expert in subject. real knowledge can address question if mistaken.