CODEWITHSUNDEEP
c#pinvokemarshalling
lifey
lifey

Reputation: 87

Calling from C# to C function which accept a struct array allocated by caller

I have the following C struct

struct XYZ
{
void            *a;
char            fn[MAX_FN];     
unsigned long   l;          
unsigned long   o;  
};

And I want to call the following function from C#:

extern "C"  int     func(int handle, int *numEntries, XYZ *xyzTbl);

Where xyzTbl is an array of XYZ of size numEntires which is allocated by the caller

I have defined the following C# struct: 

[StructLayoutAttribute(Sequential, CharSet = CharSet.Ansi)]
public struct XYZ
{
   public System.IntPtr rva;
   [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 128)]
   public string fn;
   public uint l;
   public uint o;
}

and a method:

 [DllImport(@"xyzdll.dll", CallingConvention = CallingConvention.Cdecl)]
 public static extern Int32 func(Int32 handle, ref Int32 numntries,
     [MarshalAs(UnmanagedType.LPArray)] XYZ[] arr);

Then I try to call the function : 

XYZ xyz = new XYZ[numEntries];
for (...) xyz[i] = new XYZ();
func(handle,numEntries,xyz);

Of course it does not work. Can someone shed light on what I am doing wrong ?

Upvotes: 3

Views: 1861

Answers (4)

user541686
user541686

Reputation: 210352

I don't believe you can use LPArray when you have a managed structure. Just take that out, and use [In] and [Out] (if needed) instead.

IIRC, if you use LPArray, that would try to pass a pointer to the first element, which is illegal because the structure isn't blittable. You'd need to remove the [MarshalAs(...)] entirely.

Edit:

I don't remember, but you might need to initialize the string fields before passing them... I'll check that when I get the chance.

Upvotes: 0

Mark H
Mark H

Reputation: 13897

I would marshal this manually. First, make xyzTbl an IntPtr.

[DllImport(@"xyzdll.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern Int32 func(Int32 handle, ref Int32 numntries, IntPtr xyzTb);

Rather than allocating the XYZ array like you are doing - allocate enough unmanaged memory to store the table.

IntPtr unmanaged = 
    Marshal.AllocHGlobal(Marshal.SizeOf(typeof(XYZ)) * numEntries);

Call your func(handle, ref numEntries, unmanaged); The job then is to unmarshal the unmanaged memory back into managed types.

IntPtr[] entries = new IntPtr[numEntries];
List<XYZ> xyz = new List<XYZ>();
Marshal.Copy(unmanaged, entries, 0, numEntries);
foreach (IntPtr entry in entries)
    xyz.Add(Marshal.PtrToStructure(entry, typeof(XYZ)));

Marsha.FreeHGlobal(unmanaged);

Upvotes: 0

Ohad Schneider
Ohad Schneider

Reputation: 38106

[StructLayoutAttribute(Sequential, CharSet = CharSet.Ansi)]
public struct XYZ
{
   public System.IntPtr rva;
   [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 128)]
   public string fn;
   public uint l;
   public uint o;
}

Shouldn't those uint be ulong ? Also, MAX_FN is 128 right ?

XYZ xyz = new XYZ[numEntries];
for (...) xyz[i] = new XYZ();

XYZ is a value type (struct), so the second line here is redundant (structs are always initialized)

 [DllImport(@"xyzdll.dll", CallingConvention = CallingConvention.Cdecl)]
 public static extern Int32 func(Int32 handle, ref Int32 numntries,
 [MarshalAs(UnmanagedType.LPArray)] XYZ[] arr);

[MarshalAs(UnmanagedType.LPArray)] is redundant, the compiler will see it's a struct array.

Upvotes: 1

Nickolay Olshevsky
Nickolay Olshevsky

Reputation: 14160

Check this one: Marshal C++ struct array into C# , probably it would help.

Upvotes: 0

Related Questions