C# Calling C++ DLL Function which returns a struct

Question

I have an C++ dll which defines a struct and an dll call like this:

typedef const char* FString;

typedef struct {
    FString version;
    FString build_no;
    FString build_type;
    FString build_date;
    FString build_info;
    FString comment;
} FVersionInfo;

extern "C" FAPI_EXPORT FVersionInfo CALLINGCONV fGetVersion(void);

On the c# side i using an dynamic loading:

    [DllImport("kernel32.dll", EntryPoint = "LoadLibrary")]
    static extern int LoadLibrary(
        [MarshalAs(UnmanagedType.LPStr)] string lpLibFileName);

    [DllImport("kernel32.dll", EntryPoint = "GetProcAddress")]
    static extern IntPtr GetProcAddress(int hModule,
        [MarshalAs(UnmanagedType.LPStr)] string lpProcName);

    [DllImport("kernel32.dll", EntryPoint = "FreeLibrary")]
    static extern bool FreeLibrary(int hModule);

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    public struct FVersionInfo
    {
        public string Version;
        public string Build_No;
        public string Build_Type;
        public string Build_Date;
        public string Build_Info;
        public string Comment;
    }

    [UnmanagedFunctionPointer(CallingConvention.StdCall, CharSet = CharSet.Ansi)]
    public delegate FVersionInfo fGetVersion();

    public fGetVersion GetVersion;

    FHandle = LoadLibrary(@pName);
    IntPtr intPtr;
    intPtr = GetProcAddress(FHandle, "fGetVersion");
    GetVersion = (fGetVersion)Marshal.GetDelegateForFunctionPointer(intPtr, typeof(fGetVersion));

The calling code should be:

FVersionInfo version = new FVersionInfo();
version = GetVersion();

My first problem is, that i become an "System.Runtime.InteropServices.MarshalDirectiveException" on the call of Marshal.GetDelegateForFunctionPointer in the c# loading part.

I have then tested using IntPtr as struct return parameter like that:

[UnmanagedFunctionPointer(CallingConvention.StdCall, CharSet = CharSet.Ansi)]
public delegate IntPtr fGetVersion();

So i got the Marshal.GetDelegateForFunctionPointer to work, but later i've got the same problems with marshaling:

IntPtr DllValue = new IntPtr();
FVersionInfo version = new FVersionInfo();
DllValue = fGetVersion();
Marshal.PtrToStructure(DllValue, FVersionInfo);

Here it crashes at the fGetVersion() call with "Managed Debugging Assistant 'PInvokeStackImbalance'". I think it means, that the stack is corrupted (imbalanced).

I have tested with many variants of the structure definition, but no result.

Any ideas or suggestions would be welcome!

Answer 1

Thanks for the direction, but i found an working solution:

1. I changed the declaration of the struct to

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)] public struct FVersionInfo { public IntPtr Version; public IntPtr Build_No; public IntPtr Build_Type; public IntPtr Build_Date; public IntPtr Build_Info; public IntPtr Comment; }

2. So i passed the Marshal.GetDelegateForFunctionPointer without any problems.

3. I changed my using code to:

 GF.FVersionInfo vi = new GF.FVersionInfo(); vi = gf.GetVersion();

4. After that, i could access the strings for example with

string MyVersion = Marshal.PtrToStringAnsi(VersionInfos.Version);

Answer 2

An example when using a struct with const char * members. This can be a scenario where you have your own C++ DLL structure to pass back and forth.

// CPP Source code
// ----------------------------------

// Struct definition
struct MyStruct_t {
    const char * sStr;
    size_t       iInt;
};
// ----------------------------------


// C# Source code
// ----------------------------------

// Struct definition
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]       
public struct MyStruct_t {
    public string sStr;
    public int    iInt;
};

// DLL prototype
[DllImport("Cpp.dll", CallingConvention = CallingConvention.StdCall)]
public static extern void MyFunctionCall(IntPtr MyStruct_t);

// Prepare a struct pointer to pass to DLL
MyStruct_t pMyStruct_t = new MyStruct_t();
IntPtr pMyStructPTR    = Marshal.AllocHGlobal(Marshal.SizeOf(pMyStruct_t));
Marshal.StructureToPtr(pMyStruct_t, pMyStructPTR, false);

// Call C++ DLL
MyFunctionCall(pMyStructPTR);

// The DLL updated struct, get the struct
MyStruct_t pMyStruct = (MyStruct_t)Marshal.PtrToStructure(pMyStructPTR, typeof(MyStruct_t));

// Show data
MessageBox.Show(pMyStruct.sStr);

// clean up
if (pMyStructPTR != IntPtr.Zero) { Marshal.FreeHGlobal(pMyStructPTR); pMyStructPTR = IntPtr.Zero; }
// ----------------------------------