Important pattern in C#

While programming, one sometimes needs static methods - methods that cannot meaningfully be associated with an object (a class instance). One example that comes to mind is Math.Abs, but Factory.CreateCustomer might be another. On the other hand, static methods make the methods that use them hard to test - what do you do if you try to test a method that calls Factory.CreateCustomer()? You don't want it to do that, you might want to return a fake customer, or null, or a NullCustomer - any of a number of choices.

One solution - the most recommended one, in fact, as far as I can tell - is to extract the method in an interface (IFactory.CreateCustomer) and pass an instance of that interface to either the constructor of the class under test, or directly to the tested method. Which is nice, except sometimes you can't alter the constructor of a class (like Pages in ASP.NET and Entities when working with Linq to Sql), and passing the interface to each method looks somewhat ugly.

A few days ago I found another method (I think it was on Ayende's blog, but I'm not sure):

  1. public static class Factory  
  2. {  
  3.   public static Func<int?, Customer> FindCustomer = customerID => GetDB().FindCustomer(CustomerID);  
  4. }  


This declares FindCustomer as a delegate to a function taking a nullable int (the customer ID) and returning a Customer, and initializes this delegate to point to a specific function. However, and here is the great part, nothing prevents one from overwriting this function in testing to return a mock customer or whatever.

It's too early for me to tell if this is a great idea or a bad one, so I might change my mind later; but so far I'm pleased with it.

Comments

Post a Comment

Popular posts from this blog

Posting dynamic Master / Detail forms with Knockout

Image
I have the following dynamic form in my GitHub Inventory project : I created the form with KnockoutJS , using some ideas from this article . The detail view looks like this: <table id="acquisition_items" class="jtable"> <thead> <tr> <th>Product Name</th> <th>Quantity</th> <th>Price</th> <th>Value</th> <th><button data-bind="click: addItem">[+]</button></th> </tr> </thead> <tbody data-bind="foreach: items"> <tr> <td> <input data-bind="value: ProductName" type="text" value="" /> </td> <td> <input data-bind="value: Quantity" type="text" value="" /> </td> <td>
Read more

Comparing Excel files, take two

Image
I already wrote this project once, on GitHub , but I can't say that I like the result. I wrote that code without TDD, mostly to see if I can still work in the "normal" fashion. I can, but I had a bug that took me a while to even discover and then was difficult to write tests for. I also hate the temporal dependency in that design (if you don't call the SortBy method first, the ExcludeRecords method will return incorrect results). Anyway; here's attempt number two, writing that code using TDD. Structure I start by creating my usual structure for projects of this type - a library for the logic, a console application for the main program and a test project for tests. I also use the Enable NuGet Package Restore option (right-click on the solution node) to add the three NuGet files that will allow someone to automatically download all the packages I'm referencing without wasting space in the source code repository. (Right now I've only added the Moq pa
Read more

EF Code First: seeding with foreign keys

This is just a quick tip, more for future reference for myself than anything else. When using Entity Framework Code First, one of the things you might want to do while developing the application is to seed your database with some test data. This can be something as easy as: protected override void Seed(InventoryDB context) { context.Products.AddOrUpdate( p => p.Name, new Product { Name = "Hammer", SalePrice = 11.99m, }, new Product { Name = "Nail Pack x100", SalePrice = 0.05m, }, new Product { Name = "Saw", SalePrice = 19.99m, }, new Product { Name = "Toolkit", SalePrice = 39.99m, } ); //... } (I am using the AddOrUpdate method so the code detects whether those products already exist and doesn't add them again if they do.) The Product class is quite simple and – most importantly – doesn't depend on any other: public class Product { public int Id { g
Read more