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POChaining layers with IoC, setting lower callback to upper and avoid circular reference
 

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    <p>I have a scenario where I need a lower layer to be controlled by an upper layer much like a puppet master pulling on strings.</p> <p>The lower layer also will call back to the upper layer as some internal events are generated from time to time.</p> <p>I am using SimpleInjector, I inject the ILower in to the Upper constructor. I cannot inject the Upper in to the Lower as it would cause a circular reference.</p> <p>Instead I have a register callback function to link the two layers. However, I have to scatter my code with null checks.</p> <p>Are there any nicer ways or different architectures to achieve this linking of objects?</p> <pre><code>// an interface that transport can callback from transport to client public interface ILowerToUpperCallback { void ReplyA(); void ReplyB(); } // transport interface that client calls public interface ILower { void Test1(); void Test2(); void RegisterCallback(ILowerToUpperCallback callback); } public class Upper : ILowerToUpperCallback { private readonly ILower lower; public Upper(ILower lower) { this.lower = lower; this.lower.RegisterCallback(this); } void ReplyA() { } void ReplyB() { } } public class Lower : ILower { private ILowerToUpperCallback callback; /* this is not possible, would cause a circular reference public Lower(ILowerToUpperCallback callback) { this.callback = callback; } */ // set by different method instead, what happens if this is never set?! void RegisterCallback(ILowerToUpperCallback callback) { this.callback = callback; } void OnTimer() { // some timer function if(this.callback != null) // these null checks are everywhere :( this.callback.ReplyA(); } } </code></pre>
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