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    <p>The <a href="http://mef.codeplex.com/" rel="nofollow noreferrer">Managed Extensibility Framework</a> (available through codeplex for pre-.NET-4.0, or built-in .NET 4.0 in the <a href="http://msdn.microsoft.com/en-us/library/system.componentmodel.composition(VS.100).aspx" rel="nofollow noreferrer">System.ComponentModel.Composition</a> namespace) was built for this. Say you have a <em>service</em> that can ask a user to select a wizard and then create it. It uses a wizard <em>provider</em> to create the wizards, and needs to know the name and available spells (<em>metadata</em>) for the wizards that a provider creates. You might use interfaces like these:</p> <pre><code>namespace Wizardry { using System.Collections.Generic; public interface IWizardProvider { IWizard CreateWizard(); } public interface IWizard { IMagic GetMagic(string magicWord); } public interface IWizardProviderMetadata { string Name { get; } IEnumerable&lt;string&gt; Spells { get; } } } </code></pre> <p>The wizard creation service <em>imports</em> the available wizard providers, selects one through some mechanism (user feedback in your case), and uses the provider to create the wizard.</p> <pre><code>namespace Wizardry { using System; using System.Collections.Generic; using System.ComponentModel.Composition; using System.Linq; public class UserWizardCreationService { [Import] private IEnumerable&lt;Lazy&lt;IWizardProvider, IWizardProviderMetadata&gt;&gt; WizardProviders { get; set; } public IWizard CreateWizard() { IWizard wizard = null; Lazy&lt;IWizardProvider, IWizardProviderMetadata&gt; lazyWizardProvider = null; IWizardProvider wizardProvider = null; // example 1: get a provider that can create a "White Wizard" lazyWizardProvider = WizardProviders.FirstOrDefault(provider =&gt; provider.Metadata.Name == "White Wizard"); if (lazyWizardProvider != null) wizardProvider = lazyWizardProvider.Value; // example 2: get a provider that can create a wizard that can cast the "booblah" spell lazyWizardProvider = WizardProviders.FirstOrDefault(provider =&gt; provider.Metadata.Spells.Contains("booblah")); if (lazyWizardProvider != null) wizardProvider = lazyWizardProvider.Value; // finally, for whatever wizard provider we have, use it to create a wizard if (wizardProvider != null) wizard = wizardProvider.CreateWizard(); return wizard; } } } </code></pre> <p>You can then create and <em>export</em> an arbitrary number of wizard providers with spells, and the creation service will be able to find them:</p> <pre><code>namespace Wizardry { using System.ComponentModel.Composition; [Export(typeof(IWizardProvider))] [Name("White Wizard")] [Spells("booblah", "zoombar")] public class WhiteWizardProvider : IWizardProvider { public IWizard CreateWizard() { return new WhiteWizard(); } } [Export(typeof(IWizardProvider))] [Name("White Wizard")] [Spells("zoogle", "xclondon")] public class BlackWizardProvider : IWizardProvider { public IWizard CreateWizard() { return new BlackWizard(); } } } </code></pre> <p>Of course you'll need to implement the wizards as well.</p> <pre><code>namespace Wizardry { using System; public class WhiteWizard : IWizard { public IMagic GetMagic(string magicWord) { throw new NotImplementedException(); } } public class BlackWizard : IWizard { public IMagic GetMagic(string magicWord) { throw new NotImplementedException(); } } } </code></pre> <p>To keep things clean, this code uses a custom <code>NameAttribute</code> and <code>SpellsAttribute</code> as a much cleaner form of exporting metadata than <a href="http://msdn.microsoft.com/en-us/library/system.componentmodel.composition.exportmetadataattribute(VS.100).aspx" rel="nofollow noreferrer"><code>ExportMetadataAttribute</code></a>:</p> <pre><code>namespace Wizardry { using System; [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method | AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = true)] public abstract class MultipleBaseMetadataAttribute : Attribute { } [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method | AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)] public abstract class SingletonBaseMetadataAttribute : Attribute { } public sealed class NameAttribute : SingletonBaseMetadataAttribute { public NameAttribute(string value) { this.Name = value; } public string Name { get; private set; } } public sealed class SpellsAttribute : MultipleBaseMetadataAttribute { public SpellsAttribute(params string[] value) { this.Spells = value; } public string[] Spells { get; private set; } } } </code></pre>
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