DllImport call unmanaged code

[Killer3]

INPUT : VC++ dll (abcd.dll) has funtion named DecryptString. Requirement : i need to create a dotnet project which uses this function in unmanaged dll. Steps taken public class CryptoAPI { [DllImport("abcd.dll", EntryPoint="#392")] public static extern string DecryptString(string key,string text); } When i access the function in DOTNET string EncryptedText = CryptoAPI.DecryptString(Key,PlainText); It works fine couple of times.Some times it throws up an error "object reference not set to instance of an object". Can u please help me on this? Can u tell me if i call the method - 5 times is it loaded 5 times during the runtime? or only once when i do it for the first time. for thendral

[alexey N]

Maybe you pass to your function null or empty string ? Try to check this before call. I use this function: string.IsNullOrEmpty(string)

[Killer3]

I have validated the function before. The gets passed the unmanaged code as well the unmanaged code returns a value but the result set is not recieved back in the DOTNET string. getting me? string EncryptedText = CryptoAPI.EncryptCString (Key,PlainText); VC++ implementation() { char * EncryptCString (Key,PlainText); { AfxMessageBox(Key); AfxMessageBox(PlainText); ///Code to do manipulatione AfxMessageBox(encryptedvalue); return encryptedvalue } } the above code snippet ensure that the values of the parameter are passed correctly to the unmanaged code and it process and return the value correctly so the interop is the problem i suppose? ie) getting it from vc++ to DOTNET ? for thendral

[alexey N]

Try to set charset in DllImport attribute: [DllImport("abcd.dll", CharSet=CharSet.Ansi, EntryPoint="#392")] public static extern string DecryptString(string key,string text); And try unmanaged type conversions: [DllImport("abcd.dll", EntryPoint="#392")] public static extern [MarshalAs(UnmanagedType.LPStr)] string DecryptString(string key,string text); I don't know exactly wich type is char *. I hope you'll find it changing UnmanagedType.LPStr to other string types from UnmanagedType enum.

[shezh]

I had a similar function that returned a string from a C++ dll. Try declaring a global char array in your dll and then copy the string into the array, as below: // global char array char szReturn[MAX_PATH]; BOOL APIENTRY DllMain( HANDLE hModule, DWORD ul_reason_for_call, LPVOID lpReserved ) { return TRUE; } __declspec(dllexport) LPCTSTR Test() { lstrcpy(szReturn,"success!"); return szReturn; } Hope that helps ;)