understanding func<T, TResult> C#

I am trying to refactor some of my code to use Func I have created a sample to try to understand how it works but I am getting compilation error. In the e.g. below when I am trying to access the parameter attributes or return the return value the compiler complain.

Clarifications please.

using System;

namespace chsarp
{
    class Program
    {
        static void Main(string[] args)
        {
            ParamInput input = new ParamInput();
            input.ReservationID = 开发者_StackOverflow中文版10;
            Test testMethod = new Test();
            Func<ParamInput, ReservationResult> methodToInvoke = testMethod.GetStatus;
            ReservationResult result = TestFn(methodToInvoke, input);
        }

        static Result TestFn<Param, Result>(Func<Param, Result> methodToInvoke, Param parameter) where Result : new()
        {
            Result result = new Result();
            try
            {
                result = methodToInvoke(parameter);
            }
            catch (Exception exception)
            {
                result.status = string.Format("Error-{0} during reserving {1}",
                                                parameter.ReservationID,
                                                exception.Message);
            }
            return result;
        }
    }

    class Test
    {
        public ReservationResult GetStatus(ParamInput msg)
        {
            ReservationResult result = new ReservationResult();
            result.status = string.Format("The ReservationID-{0}", msg.ReservationID);
            return result;

        }
    }

    class ParamInput
    {
        public int ReservationID { get; set; }
    }

    class ReservationResult
    {
        public string status { get; set; }
    }

}

---

Annotated compile errors:

error CS1061: 'Result' does not contain a definition for 'status'
error CS1061: 'Param' does not contain a definition for 'ReservationID'

Code using generic type arguments can only compile if the compiler knows the member of the generic type. It doesn't know that the Result type argument has a "status" member. It certainly doesn't if you invoke TestFn<object, object>().

You'll need to use a constraint to make this work. You already do, but new() isn't strong enough. Something like "where Result : IResult" for example, where IResult is an interface type that has a status property. Or a base class. The compiler can now be 100% sure that any allowed concrete instance of the generic method will execute without causing runtime errors. Since it can only ever be compiled when it was invoked with a type that has a "status" property.

All the same applies to the Param type argument. The usefulness of a generic method here rapidly disappears when you do this, it is not the proper usage. Unless you can leverage IResult, like you can IEnumerable.

---

Compiles...

You ought to descibe what you aim is in your question.

See Hans' answer for details that might help you understand the error.

Your problem is that you're saying the function should take a Result that is instantiable, but you are not saying it has to have a property called "status".

namespace chsarp
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<int, string> m = Test.GetStatus;
            var t = TestFn(m, 10);
        }

        static string TestFn<TParam>(Func<TParam, string> m, TParam p)
        {
            try { return m(p); }
            catch (Exception exception)
            {
                return string.Format("Reserving \"{0}\" failure exception: {1}", p, exception);
            }
        }
    }

    static class Test
    {
        public static string GetStatus(int inp)
        {
            return string.Format("The ReservationID-{0}", inp);
        }
    }
}