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POHow to properly downcast in C# with a SWIG generated interface?
 

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  1. POHow to properly downcast in C# with a SWIG generated interface?
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    <p>I've got a very large and mature C++ code base that I'm trying to use SWIG on to generate a C# interface for. I cannot change the actual C++ code itself but we can use whatever SWIG offers in the way of extending/updating it. I'm facing an issue where a C++ function that is written as below is causing issues in C#.</p> <pre><code>A* SomeClass::next(A*) </code></pre> <p>The caller might do something like:</p> <pre><code>A* acurr = 0; while( (acurr = sc-&gt;next(acurr)) != 0 ){ if( acurr isoftype B ){ B* b = (B*)a; ...do some stuff with b.. } elseif( acurr isoftype C ) ... } </code></pre> <p>Essentially, iterating through a container of elements that, depending on their true type, does something different. The SWIG generated C# layer for the "next" function unfortunately does the following:</p> <pre><code>return new A(); </code></pre> <p>So the calling code in C# cannot determine if the returned object is actually a derived class or not, it actually appears to always be the base class (which does make sense). I've come across several solutions:</p> <ol> <li>Use the %extend SWIG keyword to add a method on an object and ultimately call dynamic_cast. The downside to this approach, as I see it, is that this requires you to know the inheritance hierarchy. In my case it is rather huge and I see this is as a maintenance issue.</li> <li>Use the %factory keyword to supply the method and the derived types and have SWIG automatically generate the dynamic_cast code. This appears to be a better solution that the first, however upon a deeper look it still requires you to hunt down all the methods and all the possible derived types it could return. Again, a huge maintenance issue. I wish I had a doc link for this but I can't find one. I found out about this functionality by looking through the example code that comes with SWIG.</li> <li>Create a C# method to create an instance of the derived object and transfer the cPtr to the new instance. While I consider this clumsy, it does work. See an example below.</li> </ol> <pre> public static object castTo(object fromObj, Type toType) { object retval = null; BaseClass fromObj2 = fromObj as BaseClass; HandleRef hr = BaseClass.getCPtr(fromObj2); IntPtr cPtr = hr.Handle; object toObj = Activator.CreateInstance(toType, cPtr, false); // make sure it actually is what we think it is if (fromObj.GetType().IsInstanceOfType(toObj)) { return toObj; } return retval; } </pre> <p>Are these really the options? And if I'm not willing to dig through all the existing functions and class derivations, then I'm left with #3? Any help would be appreciated.</p>
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    1. COWhich solution did you go with, if I may ask? I am facing a very similar problem in another target language…
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