Question

Can someone explain dependency injection with a basic .NET example and provide a few links to .NET resources to extend on the subject?

This is not a duplicate of What is dependency injection? because I am asking about specific .NET examples and resources.

Answer 1

Here’s a common example. You need to log in your application. But, at design time, you’re not sure if the client wants to log to a database, files, or the event log.

So, you want to use DI to defer that choice to one that can be configured by the client.

This is some pseudocode (roughly based on Unity):

You create a logging interface:

public interface ILog {   void Log(string text); } 

then use this interface in your classes

public class SomeClass {   [Dependency]   public ILog Log {get;set;} } 

inject those dependencies at runtime

public class SomeClassFactory {   public SomeClass Create()   {     var result = new SomeClass();     DependencyInjector.Inject(result);     return result;   } } 

and the instance is configured in app.config:

<?xml version='1.0' encoding='utf-8'?> <configuration>   <configSections>     <section name ='unity'              type='Microsoft.Practices.Unity.Configuration.UnityConfigurationSection,               Microsoft.Practices.Unity.Configuration'/>   </configSections>   <unity>     <typeAliases>       <typeAlias alias='singleton'                  type='Microsoft.Practices.Unity.ContainerControlledLifetimeManager,Microsoft.Practices.Unity' />     </typeAliases>     <containers>       <container>         <types>           <type type='MyAssembly.ILog,MyAssembly'                 mapTo='MyImplementations.SqlLog, MyImplementations'>             <lifetime type='singleton'/>           </type>         </types>       </container>     </containers>   </unity> </configuration> 

Now if you want to change the type of logger, you just go into the configuration and specify another type.