Passing C# structure with string array to c++ function which accepts void * for c# structure and char** for c# string array

Question

I want to send a C# structure with string array to a C++ function which accepts void * for the c# structure and char** for the c# structure string array member.

I was able to send the structure to c++ function,but the issue is , not able to access the string array data member of c# structure from c++ function. When sending the string array separately,i was able to access the array elements.

Sample code is-

C# Code:

[StructLayout(LayoutKind.Sequential)]
public struct TestInfo
{
    public int TestId;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
    public String[] Parameters;
}

[DllImport("TestAPI.dll", CallingConvention = CallingConvention.StdCall, EntryPoint    "TestAPI")]
private static extern void TestAPI(ref TestInfo data);

static unsafe void Main(string[] args)
{
TestInfo  testinfoObj = new TestInfo();
testinfoObj.TestId = 1;
List<string> names = new List<string>();
names.Add("first");
names.Add("second");
names.Add("third");
testinfoObj.Parameters=names.ToArray();
TestAPI(ref testinfoObj);
}



VC++ Code:

/*Structure with details for TestInfo*/
typedef struct TestInfo
{
int  TestId;
char **Parameters;
}TestInfo_t;

//c++ function
__declspec(dllexport) int TestAPI(void *data)
{
TestInfo *cmd_data_ptr= NULL;
cmd_data_ptr = (TestInfo) data;
printf("ID is %d \r\n",cmd_data_ptr->TestId);//Working fine

for(i = 0; i < 3; i++)
printf("value: %s \r\n",((char *)cmd_data_ptr->Parameters)[i]);/*Error-Additional     information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt*/
}

When analyzing the memory stack, it is observed that,when i print ((char *)cmd_data_ptr->Parameters), the first array element("first") is getting printed, but using ((char *)cmd_data_ptr->Parameters)[i], not able access elements and above mentioned exception is coming.

The structure memory address contains address of all the structure elements,but while accessing the data from c++,it is accessing only the first element of the string array.

David Heffernan · Accepted Answer · 2013-07-15 14:04:02Z

[MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
public String[] Parameters;

is an inline array. The C++ declaration that matches is:

char* Parameters[2];

But you are trying to match it to:

char** Parameters;

and that's completely different.

You will need to marshal this by hand. In the C# struct declare Parameters to be IntPtr. Then allocate native memory with Marshal.AllocHGlobal to hold an array of pointers. And then populate those pointers with pointers to your strings.

[StructLayout(LayoutKind.Sequential)]
public struct TestInfo
{
    public int TestId;
    public IntPtr Parameters;
}

static void Main(string[] args) // no need for unsafe
{
    TestInfo testInfo;
    testInfo.TestId = 1;
    testInfo.Parameters = Marshal.AllocHGlobal(2*Marshal.SizeOf(typeof(IntPtr)));
    IntPtr ptr = testInfo.Parameters;
    Marshal.WriteIntPtr(ptr, Marshal.StringToHGlobalAnsi("foo"));
    ptr += Marshal.SizeOf(typeof(IntPtr));
    Marshal.WriteIntPtr(ptr, Marshal.StringToHGlobalAnsi("bar"));
    TestAPI(ref testinfoObj);
    // now you want to call FreeHGlobal, I'll leave that code to you
}

An alternative would be to use a pinned IntPtr[] and put that in testInfo.Parameters.

JerKimball · Accepted Answer · 2013-07-15 16:26:08Z

This is really more of an expansion/extension to David's answer, but here's one way to wrap up the custom marshalling:

public struct LocalTestInfo
{
    public int TestId;
    public IEnumerable<string> Parameters;

    public static explicit operator TestInfo(LocalTestInfo info)
    {
        var marshalled = new TestInfo
            {
                TestId = info.TestId, 
            };
        var paramsArray = info.Parameters
            .Select(Marshal.StringToHGlobalAnsi)
            .ToArray();
        marshalled.pinnedHandle = GCHandle.Alloc(
            paramsArray, 
            GCHandleType.Pinned);
        marshalled.Parameters = 
            marshalled.pinnedHandle.AddrOfPinnedObject();
        return marshalled;
    }
}

[StructLayout(LayoutKind.Sequential)]
public struct TestInfo : IDisposable
{
    public int TestId;
    public IntPtr Parameters;

    [NonSerialized]
    public GCHandle pinnedHandle;

    public void Dispose()
    {
        if (pinnedHandle.IsAllocated)
        {
            Console.WriteLine("Freeing pinned handle");
            var paramsArray = (IntPtr[])this.pinnedHandle.Target;
            foreach (IntPtr ptr in paramsArray)
            {
                Console.WriteLine("Freeing @ " + ptr);
                Marshal.FreeHGlobal(ptr);
            }
            pinnedHandle.Free();
        }
    }
}

Note for my test I swapped over to CDecl:

[DllImport(@"Test.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int TestAPI(ref TestInfo info);

Also I think you had a typo in the C++ side:

extern "C" 
__declspec(dllexport) int TestAPI(void *data)
{
    TestInfo *cmd_data_ptr= NULL;
    cmd_data_ptr = (TestInfo*) data;
    printf("ID is %d \r\n",cmd_data_ptr->TestId);

    // char**, not char*
    char** paramsArray = ((char **)cmd_data_ptr->Parameters);
    for(int i = 0; i < 3; i++)
    {
        printf("value: %s \r\n",paramsArray[i]);
    }
    return 0;
}

And a test rig:

static void Main(string[] args)
{
    var localInfo = new LocalTestInfo()
    {
        TestId = 1,
        Parameters = new[]
        {
            "Foo", 
            "Bar",
            "Baz"
        }
    };
    TestInfo forMarshalling;
    using (forMarshalling = (TestInfo)localInfo)
    {
        TestAPI(ref forMarshalling);                
    }
}

The reverse marshalling operator is left as an exercise to the reader, but should basically look like the inverse of the explicit TestInfo operator.

@DavidHeffernan Meh, it's a mess, but thank'ye nonetheless - I've been bitten numerous times for not cleaning up after myself wrt Pinvokes, so I've got a number of go-to patterns for automagical cleanup.

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