Thursday, July 08, 2010

Type inference in generic methods

Did you know that in .NET generic methods have type inference? It can also be named as implicit typing.

Let's see how type inference looks in code. In the sample below there is a class with generic methods

class NonGenericType
    {
        public static int GenericMethod1<TValue>(TValue p1, int p2)
        {
            Console.WriteLine(p1.GetType().Name);
            return default(int);
        }

        public static TValue GenericMethod2<TValue>(int p1, TValue p2)
        {
            Console.WriteLine(p2.GetType().Name);
            return default(TValue);
        }

        public static TReturn GenericMethod3<TValue, TReturn>(int p1, TValue p2)
        {
            Console.WriteLine(p2.GetType().Name);
            Console.WriteLine(typeof(TReturn).Name);
            return default(TReturn);
        }
    }
Here's the traditional way of using the above defined methods:
NonGenericType.GenericMethod1<string>("test", 5);
NonGenericType.GenericMethod2<double>(1, 0.5);
Nothing fancy here, we specify what type to place instead of TValue type parameter.
Type inference gives us the possibility to omit directly specifying type parameters. Instead we just use methods as if they're non generic.
NonGenericType.GenericMethod1("test", 5);
NonGenericType.GenericMethod2(1, 0.5);
Type inference can become handy as it reduces typing, but in my opinion it makes code less readable.
Also type inference cannot "guess" the return type of the method:
NonGenericType.GenericMethod3(1, 0.5);
 //error CS0411: The type arguments for method 'TypeInference.NonGenericType.GenericMethod3<TValue,TReturn>(int, TValue)' cannot be inferred from the usage. Try specifying the type arguments explicitly.
Nice explanation why inference does not work in the scenario above was given by Eric Lippert
Happy coding :)

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