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C interop in .Net RRS feed

  • Question

  • Hello, I am getting "Method's type signature is not PInvoke compatible." when GetStruct() is called.

    The c part of the code is written using Dev C++ editor and .Net part is using VS 2005 can any one let me know where I am going wrong.

    Following is .Net part

    class Program
        {
            [StructLayout(LayoutKind.Sequential, Size = 38, Pack = 1)]
            public struct Simple
            {
                public Int32 x;
                [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)]
                public string str;
                public Int16 y;
                public float p;
                public double q;
    
                public override string ToString()
                {
                    return String.Format("X={0}, STR={1}, Y={2}, P={3}, Q={4}", this.x, this.str, this.y, this.p, this.q);
                }
            }
    
            
            [DllImport("CInteropDotnet.dll", EntryPoint = "GetStruct", CallingConvention = CallingConvention.Cdecl)]
            [return: MarshalAs(UnmanagedType.Struct)]
            public static extern Simple GetStruct();
    
            static void Main(string[] args)
            {            
                Simple s = GetStruct();
                Console.WriteLine(s);
                Console.ReadKey();
            }
        }

    Following is c part of code

    #define DLLEXPORT __declspec(dllexport)
    
    struct Simple
    {
           int x;
           char *str;
           short y;
           float p;
           double q;
    };
    
    DLLEXPORT struct Simple GetStruct()
    {      
    Simple z;           
    return z;
    }
    


    - Rajiv

    Monday, March 26, 2012 7:52 AM

Answers

  • Hello Rajiv,

    1. User-defined structs can only be returned via an "out" parameter, e.g. :

    [DllImport("CInteropDotnet.dll", EntryPoint = "GetStruct2", CallingConvention = CallingConvention.Cdecl)]
    public static extern void GetStruct2(out Simple simple_receiver);

    2. Furthermore, the "str" member, if it is to be an inline character array, must be declared using "UnmanagedType.ByValTStr" together with the "SizeConst" parameter :

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
    public struct Simple
    {
        public Int32 x;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
        public string str;
        public Int16 y;
        public float p;
        public double q;
        public override string ToString()
        {
            return String.Format("X={0}, STR={1}, Y={2}, P={3}, Q={4}", this.x, this.str, this.y, this.p, this.q);
        }
    }

    Note also that the "CharSet" argument (for the StructLayoutAttribute) must also be used.

    3. The corresponding C structure must be declared as :

    struct Simple
    {
           int x;
           char str[10];
           short y;
           float p;
           double q;
    };

    4. Here is a sample implementation for GetStruct2() :

    extern "C" DLLEXPORT void GetStruct2(/*[out]*/ struct Simple* pSimpleReceiver)
    {
    	memset (pSimpleReceiver, 0, sizeof(Simple));
    	strcpy(pSimpleReceiver -> str, "abc");
    }

    - Bio.


    Please visit my blog : http://limbioliong.wordpress.com/

    Monday, March 26, 2012 8:41 AM
  • Hello Rajiv,

    >> But now I guess if we pass the object as out/ref parameter we also have to pin the struct variable to memory...

    1. No pinning to memory will be required in either case (out/ref).

    2. The CLR component that takes charge of the low-level invoking of the external DLL API (e.g. GetStruct()) is the Interop Marshaler.

    3. Let's take the situation where the struct is declared as an "out" parameter, e.g. :

    static void DoTest2()
    {
        Simple s;
        GetStruct2(out s);
        Console.WriteLine(s);
        Console.ReadKey();
    }

    what happens under the covers is that the interop marshaler will allocate memory for an unmanaged representation of the "Simple" structure.

    4. This is done with the help of Marshal.AllocHGlobal(). The size is determined via Marshal.SizeOf() and it requires the specifications contained in the StructLayoutAttribute and various MarshalAsAttribute’s. In short, these attributes must be set such that the interop marshaler will know that the unmanaged representation of "Simple" is :

    #pragma pack(push, 1)
    struct Simple
    {
           int x;
           char str[10];
           short y;
           float p;
           double q;
    };
    #pragma pack(pop)

    5. It is this unmanaged "Simple" structure that gets passed to the GetStruct() function as a pointer. Now after GetStruct() has assigned values to the fields of the structure and returns, the interop marshaler will then create a managed "Simple" structure and assign field values according to the corresponding field values of the unmanaged structure. This is done via Marshal.PtrToStructure().

    6. After this is done, the unmanaged structure is freed via Marshal.FreeHGlobal().

    7. I will post again with explanations on how things are done when the structure is passed by reference.

    - Bio.


    Please visit my blog : http://limbioliong.wordpress.com/

    Tuesday, March 27, 2012 7:06 AM
  • Hello Rajiv,

    1. In the case where the structure is passed by reference (ref), a very similar situation occurs albeit with some important differences. The following is a sample C++ API named GetStruct3() which takes a pointer to a "Simple" structure and assigns values to its fields :

    extern "C" DLLEXPORT void GetStruct3(/*[ref]*/ struct Simple* pSimpleReceiver)
    {
    	pSimpleReceiver -> x += 1;
    	pSimpleReceiver -> y += 1;
    	pSimpleReceiver -> p += 1;
    	pSimpleReceiver -> q += 1;
    	strcpy(pSimpleReceiver -> str, "abc");
    }

    2. The following is a sample C# client code that calls GetStruct3() :

    [DllImport("CInteropDotnet.dll", EntryPoint = "GetStruct3", CallingConvention = CallingConvention.Cdecl)]
    public static extern void GetStruct3(ref Simple simple_receiver);
    static void DoTest3()
    {
        Simple s = new Simple();
        s.x = 100;
        s.y = 100;
        s.p = 100;
        s.q = 100;
        s.str = "";
        GetStruct3(ref s);
        Console.WriteLine(s);
        Console.ReadKey();
    }

    3. This time, the interop marshaler will still create in memory an unmanaged representation of the "Simple" structure.

    4. The interop marshaler then copies the actual data of the members of the managed Simple struct instance "s" to this unmanaged memory. The interop marshaler uses the Marshal.StructureToPtr() method to perform this.

    5. The "x", "y", "p" and "q" members are blitted to the unmanaged buffer. The copying of the "str" member is done with specifications provided by the MarshalAsAttribute which was applied to it.

    6. The unmanaged API is then invoked passing along a pointer to the unmanaged "Simple" structure.

    7. After the GetStruct3() call, because it must be assumed that it has modified the structure, the returned data in the unmanaged structure must be copied back to the managed "Simple" structure "s". The blittable members are transferred directly to their managed counterparts.

    8. The "str" member is copied back to its managed counterpart with information provided by the MarshalAsAtttibute information contained in the member.

    9. This reverse copying from unmanaged memory to the managed struct is performed via Marshal.PtrToStructure().

    10. After the copying is done, the temporary unmanaged "Simple" structure is freed.

    11. I hope that you have benefitted from my last two posts and come away realizing that the StructLayoutAttribute and the MarshalAsAttributes are very important and must be applied correctly in order that marshaler be done perfectly.

    12. For more information on passing structures between managed and unmanaged code, pls refer to :

    "Passing Structures between Managed and Unmanaged Code"

    http://limbioliong.wordpress.com/2011/06/03/passing-structures-between-managed-and-unmanaged-code/

    - Bio.


    Please visit my blog : http://limbioliong.wordpress.com/

    Tuesday, March 27, 2012 7:34 AM

All replies

  • Hello Rajiv,

    1. User-defined structs can only be returned via an "out" parameter, e.g. :

    [DllImport("CInteropDotnet.dll", EntryPoint = "GetStruct2", CallingConvention = CallingConvention.Cdecl)]
    public static extern void GetStruct2(out Simple simple_receiver);

    2. Furthermore, the "str" member, if it is to be an inline character array, must be declared using "UnmanagedType.ByValTStr" together with the "SizeConst" parameter :

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
    public struct Simple
    {
        public Int32 x;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
        public string str;
        public Int16 y;
        public float p;
        public double q;
        public override string ToString()
        {
            return String.Format("X={0}, STR={1}, Y={2}, P={3}, Q={4}", this.x, this.str, this.y, this.p, this.q);
        }
    }

    Note also that the "CharSet" argument (for the StructLayoutAttribute) must also be used.

    3. The corresponding C structure must be declared as :

    struct Simple
    {
           int x;
           char str[10];
           short y;
           float p;
           double q;
    };

    4. Here is a sample implementation for GetStruct2() :

    extern "C" DLLEXPORT void GetStruct2(/*[out]*/ struct Simple* pSimpleReceiver)
    {
    	memset (pSimpleReceiver, 0, sizeof(Simple));
    	strcpy(pSimpleReceiver -> str, "abc");
    }

    - Bio.


    Please visit my blog : http://limbioliong.wordpress.com/

    Monday, March 26, 2012 8:41 AM
  • Thanks Bio, your reply was helpful.

    But now I guess if we pass the object as out/ref parameter we also have to pin the struct variable to memory. The call to GetStruct() will work here as the GC operation doesnt happen in background. Now consider the worst case of Garbage collection happening. If possible can you let me know how the out/ref parameter object be pinned to memory.


    - Rajiv

    Tuesday, March 27, 2012 6:29 AM
  • Hello Rajiv,

    >> But now I guess if we pass the object as out/ref parameter we also have to pin the struct variable to memory...

    1. No pinning to memory will be required in either case (out/ref).

    2. The CLR component that takes charge of the low-level invoking of the external DLL API (e.g. GetStruct()) is the Interop Marshaler.

    3. Let's take the situation where the struct is declared as an "out" parameter, e.g. :

    static void DoTest2()
    {
        Simple s;
        GetStruct2(out s);
        Console.WriteLine(s);
        Console.ReadKey();
    }

    what happens under the covers is that the interop marshaler will allocate memory for an unmanaged representation of the "Simple" structure.

    4. This is done with the help of Marshal.AllocHGlobal(). The size is determined via Marshal.SizeOf() and it requires the specifications contained in the StructLayoutAttribute and various MarshalAsAttribute’s. In short, these attributes must be set such that the interop marshaler will know that the unmanaged representation of "Simple" is :

    #pragma pack(push, 1)
    struct Simple
    {
           int x;
           char str[10];
           short y;
           float p;
           double q;
    };
    #pragma pack(pop)

    5. It is this unmanaged "Simple" structure that gets passed to the GetStruct() function as a pointer. Now after GetStruct() has assigned values to the fields of the structure and returns, the interop marshaler will then create a managed "Simple" structure and assign field values according to the corresponding field values of the unmanaged structure. This is done via Marshal.PtrToStructure().

    6. After this is done, the unmanaged structure is freed via Marshal.FreeHGlobal().

    7. I will post again with explanations on how things are done when the structure is passed by reference.

    - Bio.


    Please visit my blog : http://limbioliong.wordpress.com/

    Tuesday, March 27, 2012 7:06 AM
  • Hello Rajiv,

    1. In the case where the structure is passed by reference (ref), a very similar situation occurs albeit with some important differences. The following is a sample C++ API named GetStruct3() which takes a pointer to a "Simple" structure and assigns values to its fields :

    extern "C" DLLEXPORT void GetStruct3(/*[ref]*/ struct Simple* pSimpleReceiver)
    {
    	pSimpleReceiver -> x += 1;
    	pSimpleReceiver -> y += 1;
    	pSimpleReceiver -> p += 1;
    	pSimpleReceiver -> q += 1;
    	strcpy(pSimpleReceiver -> str, "abc");
    }

    2. The following is a sample C# client code that calls GetStruct3() :

    [DllImport("CInteropDotnet.dll", EntryPoint = "GetStruct3", CallingConvention = CallingConvention.Cdecl)]
    public static extern void GetStruct3(ref Simple simple_receiver);
    static void DoTest3()
    {
        Simple s = new Simple();
        s.x = 100;
        s.y = 100;
        s.p = 100;
        s.q = 100;
        s.str = "";
        GetStruct3(ref s);
        Console.WriteLine(s);
        Console.ReadKey();
    }

    3. This time, the interop marshaler will still create in memory an unmanaged representation of the "Simple" structure.

    4. The interop marshaler then copies the actual data of the members of the managed Simple struct instance "s" to this unmanaged memory. The interop marshaler uses the Marshal.StructureToPtr() method to perform this.

    5. The "x", "y", "p" and "q" members are blitted to the unmanaged buffer. The copying of the "str" member is done with specifications provided by the MarshalAsAttribute which was applied to it.

    6. The unmanaged API is then invoked passing along a pointer to the unmanaged "Simple" structure.

    7. After the GetStruct3() call, because it must be assumed that it has modified the structure, the returned data in the unmanaged structure must be copied back to the managed "Simple" structure "s". The blittable members are transferred directly to their managed counterparts.

    8. The "str" member is copied back to its managed counterpart with information provided by the MarshalAsAtttibute information contained in the member.

    9. This reverse copying from unmanaged memory to the managed struct is performed via Marshal.PtrToStructure().

    10. After the copying is done, the temporary unmanaged "Simple" structure is freed.

    11. I hope that you have benefitted from my last two posts and come away realizing that the StructLayoutAttribute and the MarshalAsAttributes are very important and must be applied correctly in order that marshaler be done perfectly.

    12. For more information on passing structures between managed and unmanaged code, pls refer to :

    "Passing Structures between Managed and Unmanaged Code"

    http://limbioliong.wordpress.com/2011/06/03/passing-structures-between-managed-and-unmanaged-code/

    - Bio.


    Please visit my blog : http://limbioliong.wordpress.com/

    Tuesday, March 27, 2012 7:34 AM
  • Thanks Bio, this was very informational and can be understood easily.

    - Rajiv

    Tuesday, March 27, 2012 11:54 AM