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POPassing a C# class object in and out of a C++ DLL class
 

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  1. POPassing a C# class object in and out of a C++ DLL class
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    <p>I've been working on a prototype code application that runs in C# and uses classes and functions from older C++ code (in the form of an imported DLL). The code requirement is to pass in a class object to the unmanaged C++ DLL (from C#) and have it be stored/modified for retrieval later by the C# application. Here's the code I have so far...</p> <p>Simple C++ DLL Class:</p> <pre><code>class CClass : public CObject { public: int intTest1 }; </code></pre> <p>C++ DLL Functions:</p> <pre><code>CClass *Holder = new CClass; extern "C" { // obj always comes in with a 0 value. __declspec(dllexport) void SetDLLObj(CClass* obj) { Holder = obj; } // obj should leave with value of Holder (from SetDLLObj). __declspec(dllexport) void GetDLLObj(__out CClass* &amp;obj) { obj = Holder; } } </code></pre> <p>C# Class and Wrapper:</p> <pre><code>[StructureLayout(LayoutKind.Sequential)] public class CSObject { public int intTest2; } class LibWrapper { [DLLImport("CPPDLL.dll")] public static extern void SetDLLObj([MarshalAs(UnmanagedType.LPStruct)] CSObject csObj); public static extern void GetDLLObj([MarshalAs(UnmanagedType.LPStruct)] ref CSObject csObj); } </code></pre> <p>C# Function Call to DLL:</p> <pre><code>class TestCall { public static void CallDLL() { ... CSObject objIn = new CSObject(); objIn.intTest2 = 1234; // Just so it contains something. LibWrapper.SetDLLObj(objIn); CSObject objOut = new CSObject(); LibWrapper.GetDLLObj(ref objOut); MessageBox.Show(objOut.intTest2.ToString()); // This only outputs "0". ... } } </code></pre> <p>Nothing but junk values appear to be available within the DLL (coming from the passed in C# object). I believe I am missing something with the class marshalling or a memory/pointer issue. What am I missing?</p> <p>Edit: I changed the above code to reflect changes to the method/function definitions, in C#/C++, suggested by Bond. The value (1234) being passed in is retrieved by the C# code correctly now. This has exposed another issue in the C++ DLL. The 1234 value is not available to the C++ code. Instead the object has a value of 0 inside the DLL. I would like to use predefined C++ functions to edit the object from within the DLL. Any more help is greatly appreciated. Thanks!</p>
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    1. CO__declspec(dllexport) void SetDLLObj(CClass obj) { Holder = &obj; } will sore pointer of local CClass so you may get junk couse of that.
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    2. COthe Holder does appear to be storing the value coming into the C++ code for retrieval later. I can take a look at it, but my main concern right now is pulling a value from the object passed into the DLL.
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