Passing a struct pointer as a parameter in C#

Question

Hello all, I have a function in C++ that I exported to a DLL. I contains a struct pointer as one of the parameters. I need to use this function in C#, so I used DLLImport for the function and recreated the struct in C# using StructLayout. I've tried passing in the parameter using ref as well as tried Marshaling it in using MarshalAs(UnmangedType.Struct) and Marshal.PtrToStructure. The parameter still isn't passing correctly.

Example:

[DllImport("testdll.dll")]
public static extern int getProduct(int num1, int num2, [MarshalAs(UnmanagedType.Struct)] ref test_packet tester);

One more tidbit of info, the struct contains a byte* var, which I think may be causing the problem in terms of passing the param as ref. Any ideas? Am I on the right track? Thanks for the help.

Thanks nobugz for the response. Here's a sample of the struct def:

//C# DEFINITION 

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)] 

public struct test_packet 
{ 

     public UInt32 var_alloc_size; 

    public byte* var; 

    [MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_TAG_LENGTH)] 
    public byte[] tag; 

} 

//ORIGINAL UNMANAGED STRUCT

typedef struct test_packet_tag 
{

    unsigned int var_alloc_size;

    unsigned char *var;

    unsigned char tag[MAX_TAG_LENGTH];
} test_packet;

Answer 1

Using "ref" is the correct way, get rid of the [MarshalAs] attribute. The real problem is almost certainly the structure declaration. You didn't post anything that would help us help you with that.

---

The DllImportAttribute.CharSet property is wrong, make it CharSet.Ansi. The "var" member is declared wrong, make it byte[]. Be sure to initialize it before the call:

 var product = new test_packet();
 product.var_alloc_size = 666;    // Adjust as needed
 product.var = new byte[product.var_alloc_size];
 int retval = getProduct(42, 43, ref product);

Best guess, hope it works.

Answer 2

Your original P/Invoke declaration should be okay, though you shouldn't need UnmanagedType.Struct there at all. The problem seems to be with your C# struct declaration. In particular, why is the order of the field declarations different from the C++ version?

Answer 3

Here's an example from my personal stuff. It can be really complicated. Note that its not easy to move arrays over as pointers, so you should look at how one does that in the c# side. This should hit a lot of the major data types. You must make sure your elements line up EXACTLY. Otherwise it will look like its working, but you'll get bad ptrs (at best). I had a great deal of trouble when moving this struct, and this was the only approach that worked. Good luck

Function sig -

  [DllImport("stochfitdll.dll", EntryPoint = "Init", ExactSpelling = false,  CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
    public static extern void Init([MarshalAs(UnmanagedType.LPStruct)] ModelSettings settings);

C++ side

#pragma pack(push, 8)
struct ReflSettings
{
    LPCWSTR Directory;
    double* Q;
    double* Refl;
    double* ReflError;
    double* QError;
    int QPoints;
    double SubSLD;
    double FilmSLD;
    double SupSLD;
    int Boxes;
    double FilmAbs;
    double SubAbs;
    double SupAbs;
    double Wavelength;
    BOOL UseSurfAbs;
    double Leftoffset;
    double QErr;
    BOOL Forcenorm;
    double Forcesig;
    BOOL Debug;
    BOOL XRonly;
    int Resolution;
    double Totallength;
    double FilmLength;
    BOOL Impnorm;
    int Objectivefunction;
    double Paramtemp;
    LPCWSTR Title;

 };
 #pragma pack(pop)

C# side -

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode, Pack = 8)]
public class ModelSettings:IDisposable
{
    #region Variables

    public string Directory;
    public IntPtr Q;
    public IntPtr Refl;
    public IntPtr ReflError;
    public IntPtr QError;
    public int QPoints;
    public double SubSLD;
      public double SurflayerSLD;
        public double SupSLD;
         public int Boxes;
         public double SurflayerAbs;
         public double SubAbs;
       public double SupAbs;
        public double Wavelength;
         public bool UseAbs;
         public double SupOffset;
     public double Percerror;
       public bool Forcenorm;
         public double Forcesig;
         public bool Debug;
         public bool ForceXR;
    public int Resolution;
    public double Totallength;
    public double Surflayerlength;
    public bool ImpNorm;
    public int FitFunc;
    public double ParamTemp;
    public string version = "0.0.0";

    [XmlIgnoreAttribute] private bool disposed = false;

#endregion

    public ModelSettings()
    { }

    ~ModelSettings()
    {
        Dispose(false);
    }


    #region Public Methods
    public void SetArrays(double[] iQ, double[] iR, double[] iRerr, double[] iQerr)
    {
        //Blank our arrays if they hold data
        if (Q == IntPtr.Zero)
            ReleaseMemory();

        int size = Marshal.SizeOf(iQ[0]) * iQ.Length;

            try
            {
                QPoints = iQ.Length;
                Q = Marshal.AllocHGlobal(size);
                Refl = Marshal.AllocHGlobal(size);
                ReflError = Marshal.AllocHGlobal(size);

                if (iQerr != null)
                    QError = Marshal.AllocHGlobal(size);
                else
                    QError = IntPtr.Zero;

                Marshal.Copy(iQ, 0, Q, iQ.Length);
                Marshal.Copy(iR, 0, Refl, iR.Length);
                Marshal.Copy(iRerr, 0, ReflError, iRerr.Length);

                if (iQerr != null)
                    Marshal.Copy(iQerr, 0, QError, iQerr.Length);
            }
            catch (Exception ex)
            {
               //error handling
            }
    }
    #endregion

    #region IDisposable Members

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    private void Dispose(bool disposing)
    {
        if (!this.disposed)
        {
            // Call the appropriate methods to clean up
            // unmanaged resources here.
            // If disposing is false,
            // only the following code is executed.
            ReleaseMemory();
            // Note disposing has been done.
            disposed = true;
        }
    }

    private void ReleaseMemory()
    {
        if (Q != IntPtr.Zero)
            {
                Marshal.FreeHGlobal(Q);
                Marshal.FreeHGlobal(Refl);
                Marshal.FreeHGlobal(ReflError);

                if (QError != IntPtr.Zero)
                    Marshal.FreeHGlobal(QError);
            }
    }

    #endregion
}