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    <p>The problem is that Simple Injector will never be able to wrap an <code>ICommandHandler&lt;T&gt;</code> implementation with one of your decorators, because there is an unresolvable generic type <code>TCommand</code>. You would have noticed this if the <code>Handle</code> method of your decorators would call the <code>decorated</code> instance. For instance:</p> <pre><code>public class CreateValidFriendlyUrlCommandHandler&lt;TCommand&gt; : ICommandHandler&lt;CreateProductCommand&gt; { private readonly ICommandHandler&lt;TCommand&gt; decorated; public CreateValidFriendlyUrlCommandHandler( ICommandHandler&lt;TCommand&gt; decorated) { this.decorated = decorated; } public void Handle(CreateProductCommand command) { // This won't compile since CreateProductCommand and // TCommand are not related. this.decorated.Handle(command); } } </code></pre> <p>This code won't compile, since the decorator's <code>Handle</code> method takes an <code>CreateProductCommand</code> argument, while the <code>decorated</code> instance takes a <code>TCommand</code> argument, which isn't specified (and nowhere is stated that <code>CreateProductCommand</code> is a <code>TCommand</code>).</p> <p>In fact you didn't create a decorator at all. A decorator wraps an instance of the same interface that it implements. You wrap an <code>ICommandHandler&lt;TCommand&gt;</code> while you implement an <code>ICommandHandler&lt;CreateProductCommand&gt;</code>. The only way you would get this to work is when you explicitly specify the <code>TCommand</code> to be a <code>CreateProductCommand</code>, as follows:</p> <pre><code>ICommandHandler&lt;CreateProductCommand&gt; handler = new CreateValidFriendlyUrlCommandHandler&lt;CreateProductCommand&gt;( new CreateProductCommandHandler() ); </code></pre> <p>Still, there is no way for Simple Injector to 'guess' that this <code>TCommand</code> should be a <code>CreateProductCommand</code> and that's why your 'decorator' didn't get wrapped.</p> <p>Long story short: ditch the <code>TCommand</code>:</p> <pre><code>public class CreateValidFriendlyUrlCommandHandler : ICommandHandler&lt;CreateProductCommand&gt; { private ICommandHandler&lt;CreateProductCommand&gt; decorated; public CreateValidFriendlyUrlCommandHandler( ICommandHandler&lt;CreateProductCommand&gt; decorated) { this.decorated = decorated; } public void Handle(CreateProductCommand command) { // logic here } } </code></pre> <p>Or make it generic with a type constraint:</p> <pre><code> public class CreateValidFriendlyUrlCommandHandler&lt;TCommand&gt; : ICommandHandler&lt;TCommand&gt; where TCommand : CreateProductCommand { private ICommandHandler&lt;TCommand&gt; decorated; public CreateValidFriendlyUrlCommandHandler( ICommandHandler&lt;TCommand&gt; decorated) { this.decorated = decorated; } public void Handle(TCommand command) { // logic here } } </code></pre> <p>or remove the type constraint and allow handling any type of command, not only <code>CreateProductCommand</code>.</p> <p>Note that if you are defining many decorators that can only handle one specific type of command handler, you might want to reconsider your strategy. There might be a problem in your design.</p>
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    1. COOK that makes sense, some of what I have was me trying to get it working as I want. I have an open generic registration and want some decorators but I only want them applied when the command is a specific type. It seems Ive gone about it in the wrong way though. Thanks :)
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    2. CONow that I am awake I had another go at it, once I stopped using RegisterManyForOpenGeneric and ditched TCommand as suggested I now have it working :)
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    3. CO@DavidMcLean: There is no reason to stop using `RegisterManyForOpenGeneric` since this registers all implementations of an open generic interface in one go. There's no good alternative for that.
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