How to marshall array of structs in C#?

Question

I've the following structure in C#:

[StructLayoutAttribute(LayoutKind.Sequential)] public struct RECORD { public uint m1; public uint m2; public uint m3; }

I need too pass an array (fixed length) of these structs over to native Code, which writes some data to these structures. The array is allocated in C# and passed over to the C dll. I declared the imported function as:

[DllImport("marshall.dll", CallingConvention = CallingConvention.Cdecl)] private static extern void doIt(RECORD[] dataRecord);

but I don't get any data back. I already tried the PInvoke Interop Assistant. Should I use IntPtr here? Any ideas?

Edit:

Here is the C# code to call the native function:

RECORD[] rec = new RECORD[256]; doIt(rec); // values of rec are all zero here

Here is the C function:

int doIt(RECORD* rec) { // deref pointer and write some data }

Jon Skeet · Accepted Answer · 2009-04-08 11:31:26Z

14

I'm far from a P/Invoke expert, but I wonder if making it an in/out parameter might help:

DllImport("marshall.dll", CallingConvention = CallingConvention.Cdecl)] private static extern void doIt([In, Out] RECORD[] dataRecord);

I wouldn't have expected that to be necessary though, as with LayoutKind.Sequential I'd hope that your struct is already a blittable type, and that the array would be blittable too.

answered Apr 8, 2009 at 11:31

Jon Skeet

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3 Comments

SDD Over a year ago

That did the trick, thx. Are there other places where these keywords are used? I've never seen them before

SDD Over a year ago

From MSDN: [refering to arrays of blittable types] However, these types are actually marshaled as In parameters, and you must apply the InAttribute and OutAttribute attributes if you want to marshal the argument as an In/Out parameter.

Menno Deij - van Rijswijk Over a year ago

Thanks this worked for me - and ended a two hour struggle to get this done.

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