C# in Depth

Changes to type inference and overload resolution247If you remember, the type inference rules of C# 2 were applied to each argument individually,with no way of using the types inferred from one argument to another. In ourcase, these rules would have stopped us from finding the types of TInput and TOutputfor the second argument, so the code in listing 9.11 would have failed to compile.Our eventual goal is to understand what makes listing 9.11 compile in C# 3 (and itdoes, I promise you), but we’ll start with something a bit more modest.9.4.2 Inferred return types of anonymous functionsListing 9.12 shows an example of some code that looks like it should compile butdoesn’t under the type inference rules of C# 2.Listing 9.12Attempting to infer the return type of an anonymous methoddelegate T MyFunc();static void WriteResult (MyFunc function){Console.WriteLine(function()); Declares generic}method with...delegate parameterWriteResult(delegate { return 5; });Compiling listing 9.12 under C# 2 gives an errorDeclares Functhat isn’t in .NET 2.0Requires typeinference for Terror CS0411: The type arguments for method'Snippet.WriteResult(Snippet.MyFunc)' cannot be inferred from theusage. Try specifying the type arguments explicitly.We can fix the error in two ways—either specify the type argument explicitly (as suggestedby the compiler) or cast the anonymous method to a concrete delegate type:WriteResult(delegate { return 5; });WriteResult((MyFunc)delegate { return 5; });Both of these work, but they’re slightly ugly. We’d like the compiler to perform thesame kind of type inference as for nondelegate types, using the type of the returnedexpression to infer the type of T. That’s exactly what C# 3 does for both anonymousmethods and lambda expressions—but there’s one catch. Although in many casesonly one return statement is involved, there can sometimes be more. Listing 9.13 is aslightly modified version of listing 9.12 where the anonymous method sometimesreturns an integer and sometimes returns an object.Listing 9.13Code returning an integer or an object depending on the time of daydelegate T MyFunc();static void WriteResult (MyFunc function){Console.WriteLine(function());}...Licensed to Rhona Hadida