Accessing array in struct of C DLL from C#

Question

I am trying to access and make changes to elements in a struct that are in a DLL. I have followed this example on how to use struct from a DLL: http://nawatt.com/index.php/corporate/blog/78-using-c-dlls-in-c

I have been successful in making changes to non-array variables, but whenever I try to make changes to an array I get a Runtime Error.

This is an example of my C DLL code:

    //lib_qrs.dll

    #ifdef __cplusplus
    extern "C" {
    #endif    

    typedef struct TEST_STRUCT
    {   
        unsigned short check[5];

    } test_struct;

    __declspec(dllexport)  void __stdcall test(test_struct *test, unsigned short val){

          // This is an example of what DOES NOT WORK
          test->check[0]=val+1;
          test->check[1]=val+2;
          test->check[2]=val+3;
          test->check[3]=val+4;
          test->check[4]=val+5; 
    }
    #ifdef __cplusplus
    }
    #endif

This is an example of my C# code:

    [StructLayout(LayoutKind.Sequential)]
    public struct TEST_STRUCT
    {   
        public UInt16[] check;

    }

    public class Program
    {
       [DllImport("lib_qrs.dll", EntryPoint="test", CallingConvention = CallingConvention.StdCall)]
        public static extern void test(ref TEST_STRUCT test, int val);
        public TEST_STRUCT testStruct = new TEST_STRUCT();

        static void Main(string[] args)
        {
            testStruct.check=new UInt16[5];
            // WHERE ERROR OCCURS
            test(ref testStruct, 5);
        }
     }

The error that I get is: *"An unhandled exception of type 'System.AccessViolationException' occurred in Test.exe Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt."

I understand that I have to be very careful with memory allocation when I replicate my structure in C#, but I don't know what I am doing wrong or how I can fix this array issue. Does anyone have an idea of how I could get around this?

Hans Passant · Accepted Answer · 2014-03-05 17:56:33Z

3

The default marshaling for that array is unsigned short*, not unsigned short[]. You'll need to apply the [MarshalAs] attribute to tell the pinvoke marshaller about it. Fix:

    [StructLayout(LayoutKind.Sequential)]
    public struct TEST_STRUCT {
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 5)]
        public UInt16[] check;
    }

And since you are returning values in the array, you also have to tell the pinvoke marshaller that it needs to copy the array values back. That requires the [Out] attribute:

    [DllImport("lib_qrs.dll")]
    public static extern void test([Out] ref TEST_STRUCT test, int val);

Do note that neither is necessary at all if the you just declare the argument as ushort[], assuming that the structure doesn't contain any additional fields.

answered Mar 5, 2014 at 17:56

Hans Passant

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1 Comment

gtdevel Over a year ago

Thanks, that worked great. The only difference is that if I used the [Out] attribute I got a build error that told me I either need to use ref or [Out]. So when I left it just as ref, it worked.

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