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POAccessViolationException when calling C function from dll on C#
 

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  1. POAccessViolationException when calling C function from dll on C#
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    <p>I'm trying to use PInvoke in order to call an unmanaged function from a C dll. Due to the fact that the source of the dll can't be released to developers, some of them have called the function in Delphi using the following declaration. We use SAT.dll with a CDECL calling convention.</p> <pre class="lang-pascal prettyprint-override"><code>function AssociarAssinatura( numeroSessao : Longint; codigoDeAtivacao: PChar; CNPJvalue : PChar; assinaturaCNPJs : PChar ) : PChar ; cdecl; External 'SAT.DLL'; </code></pre> <p>Based on that structure, I made the following Console Application in C# in order to test the same function from the same DLL. I made some research and found out that the equivalent to Longint in delphi is int in C# and the equivalent of PChar is a pointer to a string (But I used C#'s string).</p> <pre class="lang-cs prettyprint-override"><code>class Program { [DllImport("SAT.dll", CallingConvention = CallingConvention.Cdecl)] private static extern string AssociarAssinatura(int numeroSessao, string codigoDeAtivacao, string CNPJvalue, string assinaturaCNPJs); static void Main(string[] args) { Console.WriteLine("Comienza"); int numeroSessao = 111111; string codigoDeAtivacao = "123123123"; string cnpJvalue = "2222222222222211111111111111"; string assinaturaCnpJs = "lrafwegmqcgvpzpbdmcmcgdvf"; string resposta = AssociarAssinatura(numeroSessao, codigoDeAtivacao, cnpJvalue, assinaturaCnpJs); Console.WriteLine(resposta); } } </code></pre> <p>When I call the function, an AccesViolationException is thrown. The code of AssociarAssinatura has some inner prints that show that the code from the function is indeed running well. Due to this I guess the problem is related when the function is returning it's value. My best guess is that somehow I'm having issues with the calling convention. Any thoughts?</p>
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    1. USLevrak
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    1. COIt crashes when the pinvoke marshaller tries to release the returned string. You'd have to declare the return type as IntPtr and marshal it yourself with Marshal.PtrToStringAnsi(). But you still have no good way to release the string, that will become a memory leak that will ultimately crash your program with OOM.
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    2. COIs there anything I can do about it?
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    3. COIt worked! Thanks! @HansPassant
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