Monday, April 17, 2006

A tale of two implementations

Doing dynamic programming is all the rage these days. Some of the biggest proponents seem to be those that practice TDD. To that style of development, everything is fair game and dependency injection is used to solve many issues. Those injections are usually based on some reference to a provider of some sort. Others, caught in the declarative world of modern databinding are using named properties of objects.

Both worlds have need of the ability to link up the desired behavior, and they usually accomplish it through string constants that point at the desired provider or property. This is where one of my oldest code-smells kicks in; you can't dependency-check a string constant very well. The TDDers will tell you, "No problem, Mon! The tests will catch that." Old hackers like me don't want to wait that long (and that's one of the reasons type-less languages have so little appeal to me). We want something that will catch the errors for us immediately. Using things like Visual Assist's red-squiggle, the compiler's error messages, or even the IDE's cool automatic scoped rename ability keeps me from having to even run the test to catch the stupids.

So back to the string constants. How do we get the compiler/IDE to understand the connection? By making it a code link, of course. We want to replace those string literals with actual references, in some form, to the actual property or provider. This isn't a new problem, so it should come as no surprise that it's been tackled a couple of times recently for .Net development.

Let us study a couple examples and be us I mean you, I've finished my homework.

The wrong way: Get rid of nasty "string"

The right way: Static Reflection

Okay, are you back now? I'll wait, really...

What I don't like about the first idea is that it reads HORRIBLY. The code's intent is completely non-obvious, it creates all kinds of proxies where none are needed. It's ugly. Ayende's solution works without making any significant objects at runtime. The whole thing is just compiled away into a single delegate which will likely get optimized out of existance anyway.

What's the moral? If the code you are replacing is much easier to understand and maintain than the code you are using instead... you are not done refactoring.

Update: Of course this breaks down at pointing to properties because C# is overprotective, read the comments for the best-until-they-lighten-up-at-C#-land.


Ayende Rahien said...

Actually, I can see his point, you can't use my approach for this stuff.
The compiler just won't let you do it.

I gave it a try, and you can't get a delegate instance from a property, since the compiler thinks that you are trying to get the _value_ of the property, and not the getter method.

IDisposable said...

Yes, delegates to the property accessors cannot be generated in C# 2.0 (I hope it is something they fix for C# 3.0). In the meantime, I use another wrapper function like GetAge() which merely returns this.Age; To get the name of the method, you need something like this in the StaticReflection.cs file:
  public static String MethodName<TRet>(Func<TRet> func0)
    return func0.Method.Name;
  public static String MethodName<TRet, A0>(Func<TRet, A0> func1)
    return func1.Method.Name;

IDisposable said...

This is the best I can do:
using System;
using System.Collections.Generic;
using System.Text;
using System.Reflection;
namespace Whee
    class Person
        public int _age;
        public int Age
            get { return _age; }
            set { _age = value; }
        // this method only exists to wrap the property get
        public int GetAge()
            return _age;
        public Person()
            _age = 42;
    class Program
        public delegate TRet Func<TRet>();
        public static MethodInfo MethodInfo<TRet>(Func<TRet> func0)
            return func0.Method;
        public static string MethodName<TRet>(Func<TRet> func0)
            return func0.Method.Name;
        delegate int GetAge();
        static void Main(string[] args)
            Person me = new Person();
            //GetAge getter = me.Age;     // cannot implicitly converty 'int' to 'GetAge'
            //GetAge getter = me.get_Age; // cannot explicitly call operator or accessor
            GetAge getter = me.GetAge;
            Console.WriteLine(getter.ToString() + " returns: " + getter.Invoke());
            MethodInfo getterInfo = MethodInfo<int>(me.GetAge);
            Console.WriteLine(getterInfo.ToString() + " returns: " + getterInfo.Invoke(me, null));
            String name = MethodName<int>(me.GetAge);

Alois Kraus said...

If you want to pass properties by reference then you should switch to Visual Basic. There are some VB features which C# does still lack like passing properties by reference:

Module Module1

Public Class PropertyByRef
Public Delegate Function RetValDelegate() As Integer

Public Sub New()
Value = 10
Func(Value) ' pass property by reference print it and change the value to 500
Func(Value) ' pass it againt print the changed value

End Sub

Public Sub Func(ByRef value As Integer)
System.Console.WriteLine("Got value " & value)
value = 500
End Sub

Dim myValue As Integer
Property Value() As Integer
Return myValue
End Get
Set(ByVal value As Integer)
myValue = value
End Set
End Property

End Class

Sub Main()

Dim inst = New PropertyByRef()
End Sub

End Module


Alois Kraus

IDisposable said...

Ahhhhh, my eyes! VB code!!!

:) Seriously, this is just a stupid C# restriction that should be removed ASAP.