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

[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.

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.

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
        marshalled.pinnedHandle = GCHandle.Alloc(
        marshalled.Parameters = 
        return marshalled;

public struct TestInfo : IDisposable
    public int TestId;
    public IntPtr Parameters;

    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);

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[]
    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.


  1. Evans

    • 2016/10/24

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  2. Melvin

    • 2018/3/8

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  3. Carl

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  4. Martin

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  5. Harvey

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    . Passing a pointer is also known as passing a value by reference.

  6. Xavier

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  7. Gael

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    . You cannot pass function to another function as parameter. But, you can pass function reference to another function using function pointers.

  8. Caiden

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  9. Salvador

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  10. Emmanuel

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  11. Flores

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  12. Hunter

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  14. Grasso

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  15. Allen

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  16. Foster

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  17. Caden

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