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

Question

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 ?

When calling func the second parameter needs a ref in front of it, since it is ref in the function declaration. — CodesInChaos
– CodesInChaos, Commented Feb 13, 2011 at 11:06

Ohad Schneider · Accepted Answer · 2011-01-02 13:27:23Z

1

[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.

edited Jan 2, 2011 at 13:27

answered Jan 2, 2011 at 13:11

Ohad Schneider

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Community · Accepted Answer · 2017-05-23 11:58:39Z

0

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

edited May 23, 2017 at 11:58

CommunityBot

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answered Jan 2, 2011 at 8:59

Nickolay Olshevsky

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2 Comments

lifey Over a year ago

Thanks Nickolay the post you are talking about is different. I need to allocate the struct array in C#....

Nickolay Olshevsky Over a year ago

Yes, actually. I also found msdn.microsoft.com/en-us/library/…, MSDN tells that you should (probably) also pass SizeConst parameter.

Mark H · Accepted Answer · 2011-01-02 14:19:07Z

0

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);

answered Jan 2, 2011 at 14:19

Mark H

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2 Comments

CodesInChaos Over a year ago

This kind of manual marshalling is only necessary if the unmanaged code keeps a reference to the memory passed in beyond the point where the called function returns.

CodesInChaos Over a year ago

And your finalization code isn't very robust either. But most likely that only matters in few applications. Typically those that require clean unloads of AppDomains.

user541686 · Accepted Answer · 2011-01-02 15:20:08Z

0

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.

edited Jan 2, 2011 at 15:20

answered Jan 2, 2011 at 8:27

user541686

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2 Comments

lifey Over a year ago

Hi, I tried your suggestion but it does not work. I think that Marshalling is required since C# and C++ represent arrays differently. Can I create C# array as contigious block of structs ?

user541686 Over a year ago

@lifey: Hm... what is the error you get? Knowing that would really help. And yes, marshaling does need to happen, it's just that I don't think you specify it explicitly as LPArray.

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