What's the best way to add custom functionality to existing code in C#?

Question

Say, for example, I have many web applications using the same class library compiled into a dll in C# and I want to update a specific method of a class for a particular application. I don't want to change the existing code because it will affect everyone, and I don't want to inherit from a class to create a new one. I just want to change an existing method of a class for an individual application.

One way to achieve this would be to have the compiled classes as 'base' classes and have a separate file containing all the override classes inheriting the base classes. Then the applications will use the inherited classes. Then to change a class for an individual application you could just update the inherited classes. The only problem I see with this method is that every application would need 2 files, one of base classes and one of inherited (probably not compiled).

Can anyone recommend a better way of doing this?

Answer 1

Maybe extension methods would work?

Answer 2

I think extension methods is the best option. For more information go here.

Answer 3

You can't change the implementation of a compiled method; polymorphism is one option; the other is encapsulation, perhaps via a decorator pattern over an interface, using a factory to create the instance - i.e.

interface IFoo { void Bar();}
class Foo : IFoo {public void Bar() { /* imp 1 */ } }
class FooWrapper : IFoo {
    IFoo parent;
    public FooWrapper(IFoo parent) {this. parent = parent;}
    public void Bar() { /* imp 2, perhaps using "parent" }
}

(I won't bother with an example of polymorphism)

Polymorphism and decoration achieve a similar end, but have different pros/cons - for example, decorators can be assembled in flexible chains for different scenarios.

With any other approach (such as extension methods), you end up calling a different method; if that suits, then fine.

Answer 4

It seems strange that you can't override an existing method without using an inherited class. This is very straightforward in languages like javascript. The above suggestions have given me food for thought but nether really improve on my initial idea. Extension methods don't allow you to modify the existing method. With encapsulation I would still need to use a new class.

Say I had 10 applications all using the class User with method login(). I don't want to change where this method is called (ie. changing classname or method parameters). I just want to change the method itself for that application so additional checks can be made before returning.

Answer 5

Is it possible to make the method in the class library Virtual? This would allow you to override it where you want to but keep it "as is" otherwise.

Answer 6

If you simply want to add stuffs in the methods, and not modify its contents, you can use Aspect Oriented Programming (AOP). In C#, this can be done using PostSharp or AspectDNG.

AOP focuses on the separation of concerns: you write methods containing only business logic, and every other aspects of the application (security, logging, and so on) are encapsulated in their own modules. At compile time, these aspects' code are injected into the business code in particular locations you specified.

You can also have a look at this question. Even though the question aims specifically at runtime modification of methods, some answers might give you some hints.

Answer 7

Polymorphism seems to be the best solution for this. This way base classes can be compiled and left alone and inherited classes can be changed for individual applications without changing the way the original is used (unless necessary).

BaseClasses.cs

public class BaseUser {

    public BaseUser() {}

    public bool login() {

        return false;

    }

}

Classes.cs

public class User : BaseUser {}

Application

User u = new User();
u.login();

Answer 8

Extension methods. This should be added to a static class. You should visit: http://weblogs.asp.net/scottgu/archive/2007/03/13/new-orcas-language-feature-extension-methods.aspx

Example of an extension method for the DateTime class to make it return the week number:

public static int WeekNumber(this DateTime dtPassed)
    {
        CultureInfo ciCurr = CultureInfo.CurrentCulture;
        int weekNum = 0;
        try
        {
            weekNum = ciCurr.Calendar.GetWeekOfYear(dtPassed,
           CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Sunday);
        }
        catch (Exception ex)
        {
            //TODO: Add error handling code
        }

        return weekNum;
    }