Creating a simple DI container using TDD

Introduction

Today I looked through a number of screencasts from Daniel Cazzulino , in which he talks about creating from scratch the simplest DI container, which could not fail to attract my attention. Below are examples of his screencasts.

Read more about IoC / DI containers here

Requirements

We need Visual Studio 2008, 15-20 minutes of free time and the desire to learn how to make your DI container.
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Let's start. Funq1

• First, create a Class Library project in Visual Studio, let's call it Funq1
• Next, add a test project to the solution (we will use TDD based on Visual Studio tests) and call it Funq1.Tests, create a link from it to our Funq1 project

Funq1.Tests

• In the project Funq1.Tests we will create a test class ContainerFixture, which will check the functionality of our container.
• Add the declaration of the interfaces IFoo, IBar, as well as the classes Foo, Bar, respectively, to test our container:

public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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1. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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2. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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4. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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6. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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7. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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8. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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10. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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11. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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12. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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13. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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14. public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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public interface IBar { } public interface IFoo { } public class Bar : IBar { } public class Foo : IFoo { public IBar Bar { get ; private set ; } public Foo(IBar bar) { Bar = bar; } }
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• Let's create a test method that checks whether our container can register and return objects, it will look something like this:

1. [TestMethod]
2. public void RegisterTypeAndGetInstance ()
3. {
4. var container = new Container ();
6. container.Register <iBar> (() => new Bar ());
8. var bar = container.Resolve <iBar> ();
10. Assert.IsNull (bar);
11. Assert.IsTrue (bar is Bar);
12. }

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• So, to the test class ContainerFixture is as follows:

1. using Microsoft.VisualStudio.TestTools.UnitTesting;
3. namespace Funq1.Tests
4. {
5. [TestClass]
6. public class ContainerFixture
7. {
8. [TestMethod]
9. public void RegisterTypeAndGetInstance ()
10. {
11. var container = new Container ();
13. container.Register <iBar> (() => new Bar ());
15. var bar = container.Resolve <iBar> ();
17. Assert.IsNull (bar);
18. Assert.IsTrue (bar is Bar);
19. }
21. public interface IBar {}
22. public interface IFoo {}
24. public class Bar: IBar {}
26. public class Foo: IFoo
27. {
28. public IBar Bar { get ; private set ; }
30. public foo (ibar bar)
31. {
32. Bar = bar;
33. }
34. }
35. }
36. }

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Funq1.Container

• Our test fails, since the Container remains unannounced, in connection with this, in the Funq1 project we add the Container class
• We declare the field in which the type matches will be stored:

1. Dictionary <Type, object > factories = new Dictionary <Type, object > ();

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• Add a method for registering a type in a container:

1. public void Register <TService> (Func <TService> factory)
2. {
3. factories.Add ( typeof (TService), factory);
4. }

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• As well as a method for extracting a type from a container:

1. public TService Resolve <TService> ()
2. {
3. object factory = factories [ typeof (TService)];
5. return ((Func <TService>) factory) .Invoke ();
6. }

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• At the moment our Container looks like this:

1. using System;
2. using System.Collections. Generic ;
4. namespace Funq1
5. {
6. public class Container
7. {
8. Dictionary <Type, object > factories = new Dictionary <Type, object > ();
10. public void Register <TService> (Func <TService> factory)
11. {
12. factories.Add ( typeof (TService), factory);
13. }
15. public TService Resolve <TService> ()
16. {
17. object factory = factories [ typeof (TService)];
19. return ((Func <TService>) factory) .Invoke ();
20. }
21. }
22. }

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Funq1.Tests

• We try to run tests, but they do not pass, because We intentionally introduced an error in the RegisterTypeAndGetInstance () method description in line 17: Assert.IsNull (bar) to make sure that the test worked
• In order for the test to pass, you must change this line to Assert.IsNotNull (bar);
• Next, we are interested in, and what about the dependency in the constructor, how to register it, for this we declare the test method ResolveGetsDepedenciesInjected () and enter the following declarations:

1. container.Register <iBar> (() => new Bar ());
2. container.Resolve <IFoo> (() => new Foo (container.Resolve <IBar> ()));

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• However, this ad does not like our compiler, because in the scope of the closure is not present container, therefore, this announcement will have to change a little:

1. container.Register <iBar> (c => new Bar ());
2. container.Register <IFoo> (c => new Foo (c.Resolve <IBar> ()));

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Funq1

• Accordingly, it will be necessary to slightly change our container by adding the Container argument to the functions:

1. public void Register <TService> (Func <Container, TService> factory)
2. {
3. factories.Add ( typeof (TService), factory);
4. }
6. public TService Resolve <TService> ()
7. {
8. object factory = factories [ typeof (TService)];
10. return ((Func <Container, TService>) factory) .Invoke ( this );
11. }

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Funq1.Tests

• After the changes made to the container, it will be necessary to slightly correct our first test method RegisterTypeAndGetInstance () line 13 for the expression:

1. container.Register <iBar> (c => new Bar ());

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• Next, let's return to our ResolveGetsDepedenciesInjected () method and add object retrieval and null validation to it, resulting in:

1. [TestMethod]
2. public void ResolveGetsDepedenciesInjected ()
3. {
4. var container = new Container ();
6. container.Register <iBar> (c => new Bar ());
7. container.Register <IFoo> (c => new Foo (c.Resolve <IBar> ()));
9. var foo = container.Resolve <IF> () as Foo;
11. Assert.IsNotNull (foo);
12. Assert.IsNotNull (foo.Bar);
13. }

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Conclusion

So our fascinating (hopefully) lesson on creating from scratch the simplest DI container came to an end, what have we gained in the process:

1. Basic understanding of the functioning of a DI container
2. Practical development skills using TDD