[Mono-devel-list] Calling Native C api - Marshalling issue.

[Zoltan Varga]

                                      Hi,

  classes are by default passed by-ref to unmanaged code, i.e. a pointer to the
data is passed to the C side. If you define your managed method as 'ref', then
the unmanaged code will receive a pointer-to-a-pointer to the data. Try
removing the 'ref', or defining CSMyData as a struct, which by default
marshalled
by-value.

             Zoltan


On 7/28/05, Yogendra Thakur <YogendraT at ami.com> wrote:
> 
> I am getting problem while executing the c library function from C#.
> 
> Following is the equivalent code .
> 
> C Code :
> -----------
>  //This is the structure in C
>  typedef struct MyDataType
>  {
>        u_int    data1;
>     u_int   data2;
>  } MyData;
> 
> //This is the C function which is called by passing pointer to above
> structure
>  function u_int DoSomeOperation(MyData* pData)
>  {
>   return pData->data1 +pData->data2;
>  }
> 
> 
>  C# Code  :
> ----------------
> Following code shows how I am executing that call.
> 
>  using System;
>  using System.Runtime.InteropServices;
>  namespace Test
>  {
>   //Define equivalent class to MyData structure
>   [StructLayout(LayoutKind.Sequential)]
>   public class CSMyData
>   {
>    public UInt32 data1;
>    public UInt32 data2;
>   }
> 
> 
> 
>   public class Test
>   {
> 
>    //Import the dll and define C function.
>    [DllImport ("mylibrary.dll", EntryPoint="DoSomeOperation")]
>    static extern Int32 OperateData( ref  CSMyData pData);
>    //Other way of defining
>    //static extern Int32 OperateData( [In,Out]  CSMyData pData);
> 
>    //main function for console application
>    static void Main()
>    {
>     UInt32 result = 0;
>     CSMyData inData = new CSMyData();
> 
>     inData.data1 = 0xFFFF;
>     inData.data2 = 0xFF;
> 
>     result = OperateData(ref inData);
> 
>     Console.WriteLine(" Addition = " result.ToString();
>    }
> 
>   }
> 
> }
> 
> 
> Issue:
> --------
> When I am printing value in the  C code side ,ie  in DoSomeOperation
> function , the values are corrupted.
> 
> Please let me know what part is wrong.
> 
> regards,
> Yogi
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